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A long-term study of a 40-year-old crude oil spill provides insights about petroleum hydrocarbon natural attenuation processes and rates. In the source zone, fermentation coupled to methanogenesis is the dominant natural source zone depletion (NSZD) process, and most of the carbon mass exits the surface as CO<sub>2</sub> efflux. Monitored natural attenuation (MNA) of the groundwater plume shows that benzene degradation is coupled to iron reduction and that the benzene plume is stable.  A plume of hydrocarbon oxidation products measured as nonvolatile dissolved organic carbon (NVDOC) expanded ~20 m in 20 years. Most of the NVDOC is biodegraded by 200 m from the source, but optical data suggest there are components that persists for 300 m. Biological effects screening indicates decreasing biological effects with distance from the source.
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Metals and metalloids are deposited into small arms range soils from bullets, casings, and primers. The mobility potential depends on the individual metal(loid), its speciation, local pH, redox conditions and soil characteristics. This article discusses the speciation, mobility, and toxicity of metal(loid)s relevant to small arms ranges and briefly discusses management strategies to mitigate risk.
 
 
 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
 
<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
  
 
'''Related Article(s):'''
 
'''Related Article(s):'''
*[[Natural Source Zone Depletion (NSZD)]]
 
*[[Monitored Natural Attenuation (MNA)]]
 
*[[Biodegradation - Hydrocarbons]]
 
  
  
'''CONTRIBUTOR(S):''' [[Dr. Barbara Bekins]]
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'''CONTRIBUTOR(S):''' [[Dr. Amanda Barker]]
  
  
 
'''Key Resource(s)''':
 
'''Key Resource(s)''':
*[https://doi.org/10.1111/j.1745-6584.2009.00654.x Crude oil at the Bemidji site: 25 years of monitoring, modeling, and understanding]<ref name= "Essaid2011">Essaid, H.I., Bekins, B.A., Herkelrath, W.N. and Delin, G.N., 2011. Crude oil at the Bemidji site: 25 years of monitoring, modeling, and understanding. Groundwater, 49(5), pp.706-726. [https://doi.org/10.1111/j.1745-6584.2009.00654.x doi: 10.1111/j.1745-6584.2009.00654.x]</ref>
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*[[media:Fabian-2005-Army-Small-Arms-Training-Range-BMP.pdf | Army Small Arms Training Range Environmental Best Practices (BMPs) Manual]]  
*[https://doi.org/10.1111/gwat.12419 Crude Oil Metabolites in Groundwater at Two Spill Sites]<ref name= "Bekins2016">Bekins, B.A., Cozzarelli, I.M., Erickson, M.L., Steenson, R.A. and Thorn, K.A., 2016. Crude oil metabolites in groundwater at two spill sites. Groundwater, 54(5), pp.681-691. [https://doi.org/10.1111/gwat.12419 doi: 10.1111/gwat.12419]</ref>
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*[[media: 2014-Mgmt_of_the_Environmental_Impact_of_Shooting_Ranges_The_Finished_Env..pdf| Management of the Environmental Impact of Shooting Ranges, the Finnish Environment]]  
  
 
==Introduction==
 
==Introduction==
The purpose of this article is to provide an overview of a petroleum release research site and to summarize key results relating to natural source zone depletion (NSZD) and to anaerobic degradation of hydrocarbon plumes. A final section briefly describes ongoing research projects at the site.  
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Military small arms training uses bullets made of metals and metalloids which can include lead, antimony, copper, nickel, zinc, chromium, tungsten, mercury, and arsenic. Some of these metal(loid)s are also key ingredients of small arms ammunition primers used to initiate bullet firing. Unlike organic compounds found in propellants and explosives, metal(loid)s do not degrade in the environment. As such, the pervasive use of metal(loid)s on military training grounds and shooting ranges has resulted in contamination of soils, sediments, surface water, and groundwater both local to specific sites and off-site. A simplified schematic showing potential pathways of metal(loid) release on shooting ranges is shown in Figure 1. Upon exposure to the atmosphere, initially zero-valent metal(loid)s are oxidized, form secondary mineral phases, and subsequently release oxidized species into soil solution. The formation of a weathering crust on the surface of fired bullets is shown in Figures 2 and 3. Mobilization of metal(loid)s in pore water and soil solution is controlled by a variety of factors, including pH, oxidation-reduction (redox) environment, presence of colloids and/or soil organic matter (SOM), soil type and hydraulic characteristics, temperature, and water infiltration<ref name= "Vantelon2005">Vantelon, D., Lanzirotti, A., Scheinost, A.C. and Kretzschmar, R., 2005. Spatial distribution and speciation of lead around corroding bullets in a shooting range soil studied by micro-X-ray fluorescence and absorption spectroscopy. Environmental Science & Technology, 39(13), pp.4808-4815. [https://doi.org/10.1021/es0482740 doi: 10.1021/es0482740]</ref><ref name= "Scheinost2006">Scheinost, A.C., Rossberg, A., Vantelon, D., Xifra, I., Kretzschmar, R., Leuz, A.K., Funke, H. and Johnson, C.A., 2006. Quantitative antimony speciation in shooting-range soils by EXAFS spectroscopy. Geochimica et Cosmochimica Acta, 70(13), pp.3299-3312. [https://doi.org/10.1016/j.gca.2006.03.020 doi: 10.1016/j.gca.2006.03.020]</ref><ref name= "Ackermann2009">Ackermann, S., Gieré, R., Newville, M. and Majzlan, J., 2009. Antimony sinks in the weathering crust of bullets from Swiss shooting ranges. Science of the Total Environment, 407(5), pp.1669-1682. [https://doi.org/10.1016/j.scitotenv.2008.10.059 doi: 10.1016/j.scitotenv.2008.10.059]</ref><ref name= "Sanderson2018">Sanderson, P., Qi, F., Seshadri, B., Wijayawardena, A. and Naidu, R., 2018. Contamination, fate and management of metals in shooting range soils—A Review. Current Pollution Reports, 4(2), pp.175-187. [https://doi.org/10.1007/s40726-018-0089-5 doi: 10.1007/s40726-018-0089-5]</ref>.
 
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[[File:Barker1w2 Fig1.png|thumb|500 px|left|Figure 1. Simplified schematic of shooting ranges with berm-style backstops that typically contain high loadings of metal(loid)s in the berm due to bullet fragmentation and also at the firing point as a result of residue and primer materials.]]
The Bemidji crude oil research site was established by the U. S. Geological Survey’s Toxics Substance Hydrology Program in 1983 at the location of an oil release which occurred in 1979 when a high-pressure pipeline ruptured. The goal of the research is to improve understanding of monitored natural attenuation (MNA) of petroleum hydrocarbon spills with transformative research on the source, transport and fates of the major contaminant classes. To date, 274 publications by interdisciplinary teams working at the site have covered physical, biological, and geochemical processes controlling these contaminants<ref name= "USGS2017">U.S. Geological Survey, 2017. Crude Oil Contamination in a Shallow Outwash Aquifer - Bemidji, Minnesota</ref>. Since 2009, the pipeline owner, Enbridge Energy LLC, has provided supplemental funding for site management and research seed money through a collaborative agreement between USGS, Minnesota Pollution Control Agency, and Beltrami County.  
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[[File:Barker1w2 Fig2.png|thumb|500 px|left|Figure 2. Chemical characterization of a 5.56mm corroding bullet that underwent 15 years of weathering in Alaskan soils using scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). Relative concentrations for iron (red), lead (green), and antimony (purple) are shown in the right x-ray map. Data was collected at the Advanced Instrumentation Laboratory (AIL), University of Alaska Fairbanks (UAF). Data collected by A.J. Barker]]
 
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[[File:Barker1w2 Fig3.png|thumb|right|Figure 3. Optical images of a new 5.56mm bullet (left) and a 5.56mm bullet that weathered for 15 years (right). The tip of the bullet fragmented and the weathering crust can be hundreds of microns thick and comprised of secondary mineral phases. Photo by A.J. Barker.]]
==Site Description==
 
[[File:Bekins1w2 Fig1.jpg|thumb|600 px|left|Figure 1. Map of the site located near Bemidji, MN, showing oil pipelines, location of 1979 rupture, oil bodies at the water table, area sprayed by oil, direction of flow, the 10 ppb BTEX contour and the down-gradient lake. Figure reprinted from McGuire, et al., 2018<ref name= "McGurie2018">McGuire, J.T., Cozzarelli, I.M., Bekins, B.A., Link, H. and Martinović-Weigelt, D., 2018. Toxicity Assessment of Groundwater Contaminated by Petroleum Hydrocarbons at a Well-Characterized, Aged, Crude Oil Release Site. Environmental Science & Technology, 52(21), pp.12172-12178. [https://doi.org/10.1021/acs.est.8b03657 doi: 10.1021/acs.est.8b03657]</ref>.]]
 
 
 
The site (Figure 1) is located 16 km northwest of Bemidji, MN. An oil release occurred on August 20, 1979, when a high-pressure pipeline ruptured along a seam, spilling 10,700 barrels (1.7 million liters) of
 
light, low sulfur crude. The immediate cleanup removed about 70% of the oil by pumping, excavation, land farming, and burning. An area of 6,500 m<sup>2</sup> that was sprayed by oil from the pipeline contains oil in the top few centimeters of soil. The remaining oil infiltrated into the aquifer forming three oil bodies at the water table designated as the north, middle and south pools. Additional remediation by a dual-pump recovery system from 1999 through 2003 removed an estimated 36-41% of the oil from the three oil pools<ref>Delin, G.N. and Herkelrath, W.N., 2014. Effects of a Dual‐Pump Crude‐Oil Recovery System, Bemidji, Minnesota, USA. Groundwater Monitoring & Remediation, 34(1), pp.57-67. [https://doi.org/10.1111/gwmr.12040 doi: 10.1111/gwmr.12040]</ref>.
 
 
 
Most studies of the Bemidji source zone and plume have been focused on the north pool. The oil body at the water table is 1-2 m thick and spans an area of 100 m × 25 m. Oil saturations are 10-70%, with the highest saturation in a coarse-grained layer below the water table<ref name= "Essaid2011"/>. Residual oil saturations of 15-30% are present from the land surface to the water table<ref>Dillard, L.A., Essaid, H.I. and Herkelrath, W.N., 1997. Multiphase flow modeling of a crude‐oil spill site with a bimodal permeability distribution. Water Resources Research, 33(7), pp.1617-1632. [https://doi.org/10.1029/97WR00857 doi: 10.1029/97WR00857]</ref>.  
 
 
 
The depth to the water table near the north pool is 6-8 m. The aquifer is a glacial outwash deposit of medium to coarse variably sorted sand with <0.1% organic carbon. It is underlain by poorly permeable till at 18-27 m depth. The range of estimated hydraulic conductivities is 0.56-7 x 10<sup>-5</sup> m/s<ref name= "Essaid2011"/>. The background groundwater is aerobic with dissolved oxygen of 9 mg/L<ref name= "Bennett1993">Bennett, P.C., Siegel, D.E., Baedecker, M.J. and Hult, M.F., 1993. Crude oil in a shallow sand and gravel aquifer—I. Hydrogeology and inorganic geochemistry. Applied Geochemistry, 8(6), pp.529-549. [https://doi.org/10.1016/0883-2927(93)90012-6 doi: 10.1016/0883-2927(93)90012-6]</ref>. Average pore water velocity estimates range from 0.004 to 0.056 m/day (0.5-67 ft/year, 0.1-20.5 m/year)<ref name= "Bennett1993"/>.
 
 
 
[[File:Bekins1w2 Fig2.jpg|thumb|400 px|right|Figure 2.  Conceptual cross section of the site illustrating microbial processes, modified from Ng et al., 2015 <ref name="Ng2015">Crystal Ng, G.H., Bekins, B.A., Cozzarelli, I.M., Baedecker, M.J., Bennett, P.C., Amos, R.T. and Herkelrath, W.N., 2015. Reactive transport modeling of geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN. Water Resources Research. 51(6), pp. 4156-4183. [http://dx.doi.org/10.1002/2015wr016964 doi:10.1002/2015WR016964]</ref> and Sihota et al., 2016<ref name= "Sihota2016">Sihota, N.J., Trost, J.J., Bekins, B.A., Berg, A., Delin, G.N., Mason, B., Warren, E. and Mayer, K.U., 2016. Seasonal variability in vadose zone biodegradation at a crude oil pipeline rupture site. Vadose Zone Journal, 15(5). [https://doi.org/10.2136/vzj2015.09.0125 doi: 10.2136/vzj2015.09.0125]</ref>.]]
 
==Natural Source Zone Depletion (NSZD)==
 
Natural Source Zone Depletion (NSZD) refers to the changes over time in the composition of spilled oil or fuel that occurs naturally by dissolution, volatilization and biodegradation<ref name= "Garg2017">Garg, S., Newell, C.J., Kulkarni, P.R., King, D.C., Adamson, D.T., Renno, M.I. and Sale, T., 2017. Overview of natural source zone depletion: processes, controlling factors, and composition change. Groundwater Monitoring & Remediation, 37(3), pp.62-81. [https://doi.org/10.1111/gwmr.12219 doi:10.1111/gwmr.12219]</ref>.  Figure 2 shows a conceptual model of NSZD processes at the Bemidji site. Biodegradation within the oil body occurs primarily by fermentation coupled to methanogenesis<ref name= "Bekins2005">Bekins, B.A., Hostettler, F.D., Herkelrath, W.N., Delin, G.N., Warren, E. and Essaid, H.I., 2005. Progression of methanogenic degradation of crude oil in the subsurface. Environmental Geosciences, 12(2), pp.139-152. [https://doi.org/10.1306/eg.11160404036 doi: 10.1306/eg.11160404036]</ref><ref>Beaver, C.L., Williams, A.E., Atekwana, E.A., Mewafy, F.M., Abdel Aal, G., Slater, L.D. and Rossbach, S., 2016. Microbial communities associated with zones of elevated magnetic susceptibility in hydrocarbon-contaminated sediments. Geomicrobiology Journal, 33(5), pp.441-452. [https://doi.org/10.1080/01490451.2015.1049676 doi: 10.1080/01490451.2015.1049676 ]</ref><ref>Bekins, B.A., Godsy, E.M. and Warren, E., 1999. Distribution of microbial physiologic types in an aquifer contaminated by crude oil. Microbial Ecology, 37(4), pp.263-275. [https://doi.org/10.1007/s002489900149 doi: 10.1007/s002489900149]</ref>. The biogenic methane produced in the oil body diffuses toward the land surface where it is oxidized in the middle of the vadose zone by methanotrophic bacteria<ref>Amos, R.T., Mayer, K.U., Bekins, B.A., Delin, G.N. and Williams, R.L., 2005. Use of dissolved and vapor‐phase gases to investigate methanogenic degradation of petroleum hydrocarbon contamination in the subsurface. Water Resources Research, 41(2). [https://doi.org/10.1029/2004WR003433 doi: 10.1029/2004WR003433]</ref>. The CO<sub>2</sub> produced by methane oxidation together with some produced in the source zone exits as surface efflux over the oil body<ref name= "Sihota2010">Sihota, N.J., Singurindy, O. and Mayer, K.U., 2010. CO2-efflux measurements for evaluating source zone natural attenuation rates in a petroleum hydrocarbon contaminated aquifer. Environmental Science & Technology, 45(2), pp.482-488. [https://doi.org/10.1021/es1032585 doi: 0.1021/es1032585]</ref>. Garg et al., 2017<ref name= "Garg2017"/> present a useful summary of the important NSZD results from the site. This brief section will focus on how NSZD has been measured at the north pool and what has been discovered. Estimates of NSZD at Bemidji have been based on a variety of indicators, including vadose zone gas concentrations, compositions of oil samples from wells, surface CO<sub>2</sub> efflux, temperature measurements, and modeling.
 
 
 
Gas concentration data from 1985, six years after the spill, indicated that volatilization accounted for 97% of NSZD and biodegradation only 3%. The volatile fraction was dominated by C3-C5 alkanes and was oxidized in the vadose zone before reaching the surface<ref>Hult, M.F., Grabbe, R.R. and Ragone, S.E., 1985. Distribution of gases and hydrocarbon vapors in the unsaturated zone. In US Geological Survey Program on Toxic Waste—Groundwater Contamination—Proceedings of the Second Technical Meeting (pp. 21-25). [[media:1985-Hult-Distribution_of_gases_and_hydrocarbon_vapors_in_the_usaturated_zone.pdf| Report.pdf]]</ref>. Total losses were estimated to be 10.5 kg/day. By 1997, volatilization losses were 10 times lower and although biodegradation losses increased, total losses had dropped to 2 kg/day. The total average mass loss (Table 1) was estimated from the gas data to be 0.6% per year over the period 1979-1997<ref name = "Chaplin2002"/>.
 
 
 
Analyses of oil samples collected in 2010 from 13 wells in the north oil pool showed that 31 years after the spill, ''n''-alkanes, toluene, and ''o''-xylene were the most depleted hydrocarbons, while ''n''-C<sub>10–24</sub> cyclohexanes, tri- and tetra- methylbenzenes, acyclic isoprenoids, and naphthalenes were the least depleted. Benzene was still present at every sampling location. The technique of normalizing composition to a conserved compound yielded estimates for spatially variable mass losses ranging from 18% to 30% or an overall average rate of 0.8% per year (Table 1)<ref name= "Baedecker2011">Baedecker, M.J., Eganhouse, R.P., Bekins, B.A. and Delin, G.N., 2011. Loss of volatile hydrocarbons from an LNAPL oil source. Journal of Contaminant Hydrology, 126(3-4), pp.140-152. [https://doi.org/10.1016/j.jconhyd.2011.06.006 doi: 10.1016/j.jconhyd.2011.06.006]</ref>.
 
 
 
{| class="wikitable" style="text-align:center;"
 
|+ Table 1. Oil Loss Measurements*
 
|-
 
! rowspan="2" | Reference
 
! rowspan="2" | Method
 
! rowspan="2" | Time Interval
 
! Equivalent CO<sub>2</sub> flux
 
! colspan="2" | Total Oil Loss Rate
 
! Annual Loss
 
! rowspan="2" | Comments
 
|-
 
| &mu;mol/m<sup>2</sup>-s || kg/m<sup>2</sup>-d
 
| style="width: 70px;" | gal/acre-y || %/y
 
|-
 
| style="width:180px;" | Chaplin et al. (2002)<ref name = "Chaplin2002">Chaplin, B.P., Delin, G.N., Baker, R.J. and Lahvis, M.A., 2002. Long-term evolution of biodegradation and volatilization rates in a crude oil-contaminated aquifer. Bioremediation Journal  6(3), pp. 237-255 [https://doi.org/10.1080/10889860290777594 doi: 10.1080/10889860290777594]</ref> || Gas data and 1D model
 
| style="width:110px;" | 1979-1997 || 0.75 || '''9.33×10<sup>-4</sup>''' || 429 || 0.63 || Published estimate of 0.2% was based on total oil rather than N. Pool; Loss given is average of 3 oil sites in their Table 4.
 
|-
 
| Baedecker et al. (2018)<ref name= "Baedecker2018">Baedecker, M.J., Eganhouse, R.P., Qi, H., Cozzarelli, I.M., Trost, J.J. and Bekins, B.A., 2018. Weathering of oil in a surficial aquifer. Groundwater, 56(5), pp.797-809. [https://doi.org/10.1111/gwat.12619 doi: 10.1111/gwat.12619]</ref> || Oil composition || 1979-2010 || 0.93 || 1.16×10<sup>-3</sup> || 533 || '''0.79''' || Average using four conservative compounds. Computed from published values as (7×30.1+6×17.8)/13÷31 years.
 
|-
 
| Sihota et al. (2016)<ref name= "Sihota2016"/> || Surface CO<sub>2</sub> efflux || 2011-2013 || '''1.1''' || 1.37×10<sup>-3</sup> || 627 || 0.93 || Value given is annual average efflux June 2011-July 2013 from their Table 1.
 
|-
 
| Molins et al. (2010)<ref name= "Molins2010">Molins, S., Mayer, K.U., Amos, R.T. and Bekins, B.A., 2010. Vadose zone attenuation of organic compounds at a crude oil spill site—Interactions between biogeochemical reactions and multicomponent gas transport. Journal of Contaminant Hydrology, 112(1-4), pp.15-29. [https://doi.org/10.1016/j.jconhyd.2009.09.002 doi: 10.1016/j.jconhyd.2009.09.002]</ref> || Gas and water data with 2D model || 1979-2007 || '''2.15''' || 2.67×10<sup>-3</sup> || 1,226 || 1.81 || Value given is total flux.  Model lower boundary methane flux of 1.3 mol/m<sup>2</sup>-d was 0.7 of total.  Published value based on only VZ degradation was 1% y<sup>-1</sup>.
 
|-
 
| Ng et al. (2015)<ref name="Ng2015"/> || 2D model using oil, gas, and water data  || 1979-2008 || 1.46 || '''1.81×10<sup>-3</sup>''' ||831 || 1.23 || Oil loss rate computed from losses given in their Figure 7, masses in Table 3, and 82 m length of oil body from Figure 2.
 
|-
 
| colspan="8" | * Note:  The values in '''bold''' are taken directly from the listed reference. The other columns are computed from this column using the following values: oil specific gravity = 0.85 <ref>Landon, M.K., 1993. Investigation of mass loss based on evolution of composition and physical properties of spilled crude oil contaminating a shallow outwash aquifer (Doctoral dissertation, University of Minnesota)</ref>, carbon fraction of oil = 0.835<ref name= "Baedecker2018"/>, north pool oil body area = 2,323 m<sup>2</sup> and total mass of oil 124,950 kg<ref>Herkelrath, W.N., 1999, March. Impacts of remediation at the Bemidji oil spill site. In US Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina (Vol. 3, pp. 195-200). [[media:1999_-_Herkelrath-Impacts_of_remediation_at_the_Bemidji_oil_spill_site.pdf| Report.pdf]]</ref>.
 
|}
 
 
 
The CO<sub>2</sub> efflux technique involves measuring the flux of CO<sub>2</sub> leaving the surface above the source zone and comparing it to an average rate from natural soil respiration in uncontaminated locations. The difference in efflux between the source zone and background is assumed to reflect the amount generated by biodegradation of the source, but the estimate can be complicated by spatial variation in background rates<ref name= "Sihota2010"/>.  A refinement of the technique is based on the principle that degradation of oil produces ancient carbon, and analyzing for <sup>14</sup>C content of CO<sub>2</sub> provides more precise estimates of the efflux attributable to contaminant degradation<ref>Sihota, N.J. and Mayer, K.U., 2012. Characterizing vadose zone hydrocarbon biodegradation using carbon dioxide effluxes, isotopes, and reactive transport modeling. Vadose Zone Journal, 11(4). [https://doi.org/10.2136/vzj2011.0204 doi: 10.2136/vzj2011.0204]</ref>.  Monthly data from the site shows that surface efflux and subsurface gas concentrations vary seasonally, with 2-3 times higher efflux values in the summer and fall compared to the spring and winter<ref name= "Sihota2016"/>.  The CO<sub>2</sub> effluxes are typically expressed in units of micromoles of CO<sub>2</sub> per meter-squared per second (µmol m<sup> 2 </sup>s<sup> 1</sup>). The average annual rate measured for the north pool from June 2011 until July 2013 was 1.1 ± 0.4 µmol m<sup> 2</sup> s<sup> 1</sup>. This corresponds to an average oil biodegradation rate of 0.9% per year (Table 1).
 
 
 
The temperature method for evaluating NSZD involves measuring vertical profiles of vadose zone temperatures above the source zone and at a background site subject to the same external heating.  The source of the heat at the Bemidji site is the combined effect of methane oxidation in the vadose zone above the oil body and the active oil pipelines passing through the site. Temperature increases are used to estimate NSZD rates by converting the excess heat to reaction rates using the reaction enthalpy. Estimates using average annual temperature increases were comparable to NSZD rates estimated from the average annual surface efflux of CO<sub>2.</sub>.  However, modeling indicates that half of the excess heat measured in the aquifer came from the pipelines, underscoring the importance of subtracting temperature effects of nearby infrastructure before using excess temperatures to estimate NSZD rates<ref>Warren, E. and Bekins, B.A., 2018. Relative contributions of microbial and infrastructure heat at a crude oil-contaminated site. Journal of Contaminant Hydrology, 211, pp.94-103. [https://doi.org/10.1016/j.jconhyd.2018.03.011 doi: 10.1016/j.jconhyd.2018.03.011]</ref>.  The method has been carefully tested at a refined petroleum products terminal, where researchers found 8% higher estimated NSZD rates using CO<sub>2</sub> flux measurements compared to temperature based estimates.
 
 
 
Two comprehensive modeling studies have focused on NSZD at the site<ref name= "Molins2010"/>.  Molins et al., 2010<ref name= "Molins2010"/> modeled the NSZD processes of volatilization, biodegradation and dissolution in the vadose zone. They used gas and aqueous concentration data with estimates of vadose zone transport properties, but did not have surface efflux or oil composition data to constrain fluxes. Their calibrated methane efflux was 2.15 µmol m<sup> 2</sup> s<sup> 1</sup>, of which 30% came from inside the model and 70% from the smear zone. Within the model domain the modeled loss was 1% per year during the period 1979 – 2007. Including the flux and oil outside the model domain gives a loss of 1.8% per year (Table 1). An important conclusion of this paper is that soil diffusion properties are poorly constrained, and surface efflux measurements might be an effective strategy to constrain gas flux. The model by Ng et al., 2015<ref name="Ng2015"/> included biodegradation and dissolution constrained by surface efflux and oil composition data but not volatilization. Their overall loss was 1.8x10<sup>-3</sup> kg/m<sup>2</sup>-d during 1979 – 2008 or 1.2% per year (Table 1). A major finding of this study is that 85% of the carbon loss from the oil occurs through CO<sub>2</sub> gas efflux from the surface over the source zone<ref name= "Ng2014">Ng, G.H.C., Bekins, B.A., Cozzarelli, I.M., Baedecker, M.J., Bennett, P.C. and Amos, R.T., 2014. A mass balance approach to investigating geochemical controls on secondary water quality impacts at a crude oil spill site near Bemidji, MN. Journal of Contaminant Hydrology, 164, pp.1-15.[http://dx.doi.org/10.1016/j.jconhyd.2014.04.006 doi: 10.1016/j.jconhyd.2014.04.006]</ref>.
 
 
 
[[File:Bekins1w2 Fig3.PNG|thumb|left|500 px|Figure 3. Concentrations integrated over 2D cross-sectional area for modeled five oil phase components (B: BEX, T: toluene, N: NVDOC, S: short chain n-alkanes, L: long chain n-alkanes), at the initial time of the spill, at the final simulation time (10,000 days), and for the degraded amount over the simulation period.  For each component, the percentage of its initial amount that is degraded at the end of the simulation period is printed in parentheses. Figure reprinted from Ng et al., 2015<ref name="Ng2015"/>.]]
 
 
 
NSZD rates estimated with CO<sub>2</sub> efflux and temperature increases are sometimes used to estimate the volume of oil or mass of total petroleum hydrocarbons (TPH) degraded per year. Conversion to volume of LNAPL requires knowing the mass fraction of C and specific gravity of the LNAPL, while conversion to mass of TPH is sometimes based on a model hydrocarbon such as decane (C<sub>10</sub>H<sub>22</sub>)<ref name= "Sihota2010"/>. However, the CO<sub>2</sub> efflux and heat are generated by the biodegradation of a subset of the compounds in the oil. Baedecker et al., 2011<ref name= "Baedecker2011"/><ref name= "Baedecker2018"/> showed that some compounds degrade much faster than others. Figure 3 shows model estimates for the amount of degradation by 2008 for each major class of oil compounds<ref name="Ng2015"/>. Biodegradation of the ''n''-alkane fraction contributes most of the CO<sub>2</sub> efflux<ref name="Ng2015"/> and associated heat, which may have limited relevance for risk evaluations.
 
  
The differences between the loss rates of various compound classes are due to several factors.  For example, the ''n''-alkanes degrade in place under methanogenic conditions<ref>Anderson, R.T. and Lovley, D.R., 2000. Biogeochemistry: hexadecane decay by methanogenesis. Nature, 404(6779), pp.722-823 [https://doi.org/10.1038/35008145 doi: 10.1038/35008145]</ref> without migrating in the aqueous phase<ref name="Ng2015"/>.  Losses are fastest below a local topographic depression where focused recharge transports nitrate to the oil body<ref name= "Bekins2005"/><ref name= "Baedecker2011"/><ref name= "Baedecker2018"/>. However, the alkylbenzenes dissolve in the groundwater before degrading. Thus, their loss rates vary with effective solubility and susceptibility to methanogenic degradation. For example, benzene and ethylbenzene do not degrade under methanogenic conditions at this site, and their losses are controlled by solubility and oil pore-space saturation<ref>Bekins B., Baedecker, M.J., Egahouse, R.P., Herkelrath, W.N., 2011. Long-term natural attenuation of crude oil in the subsurface. In: GQ10: Groundwater quality management in a rapidly changing world: Proceedings of the 7th International Groundwater Quality Conference, Schirmer, M.; Hoehn, E.; Vogt, T., Editors. IAH: Zurich, Switzerland. [[media:2011-Bekins-Long-term_natural_attenuation_of_crude_oil_in_the_subsurface.pdf| Report.pdf]]</ref>.  Although benzene degradation under methanogenic conditions has been demonstrated in the lab<ref>Edwards, E.A. and Grbić-Galić, D., 1992. Complete mineralization of benzene by aquifer microorganisms under strictly anaerobic conditions. Appl. Environ. Microbiol., 58(8), pp.2663-2666. ISSN: 0099-2240; Online ISSN: 1098-5336. </ref>, a literature review concluded it is unreliable in the field<ref>National Research Council (NRC). 2000. Natural Attenuation for Groundwater Remediation. Washington, DC: The National Academies Press. [https://doi.org/10.17226/9792 doi: 10.17226/9792]</ref>.  The higher effective solubility of benzene has resulted in greater cumulative losses than for ethylbenzene<ref name= "Baedecker2011"/>. For alkylbenzenes that degrade under methanogenic conditions, losses are accelerated by methanogenic degradation in the aqueous phase adjacent to the oil, which lowers concentrations driving further dissolution<ref name= "Essaid2003">Essaid, H.I., Cozzarelli, I.M., Eganhouse, R.P., Herkelrath, W.N., Bekins, B.A. and Delin, G.N., 2003. Inverse modeling of BTEX dissolution and biodegradation at the Bemidji, MN crude-oil spill site. Journal of Contaminant Hydrology, 67(1-4), pp.269-299. [https://doi.org/10.1016/S0169-7722(03)00034-2 doi: 10.1016/S0169-7722(03)00034-2]</ref>.
+
==Metal(loid)s==
  
==Natural Attenuation of the Groundwater Plume==
+
===Lead (Pb)===
A groundwater contaminant plume extends 335 m downgradient from the north oil pool to the shore of a small unnamed lake<ref>Mason, B. E., 2014. Groundwater and Surface Water Interactions Down-Gradient from a Decades-Old Crude Oil Spill, Beltrami County Minnesota. MS Thesis, University of Minnesota, Minneapolis, MN.</ref><ref name= "Podgorski2018">Podgorski, D.C., Zito, P., McGuire, J.T., Martinovic-Weigelt, D., Cozzarelli, I.M., Bekins, B.A. and Spencer, R.G., 2018. Examining Natural Attenuation and Acute Toxicity of Petroleum-Derived Dissolved Organic Matter with Optical Spectroscopy. Environmental Science & Technology, 52(11), pp.6157-6166. [https://doi.org/10.1021/acs.est.8b00016 doi: 10.1021/acs.est.8b00016]</ref>.  Essaid et al., 2011<ref name= "Essaid2011"/> present a comprehensive summary of processes in the plume. This brief review will focus on the key biodegradation processes, the importance of anaerobic biodegradation, and recent work on partial transformation products of hydrocarbons found in the plume.
+
[https://en.wikipedia.org/wiki/Lead Lead] (Pb) and antimony alloy bullet cores are commonly used in small arms ammunition to provide the high density needed to penetrate surfaces. Pb is estimated to comprise approximately 90-99% of the bullet core with steel, copper, and zinc making up the majority of the remaining mass for the slug and casing<ref name= "Vantelon2005"/><ref name= "Sanderson2018"/>. Pb is also used in the compounds lead styphnate and lead azide as primers for ammunition. As a result, Pb can be found on training ranges in soils/structures where the bullet fragments and also at the firing point<ref name= "Pichtel2012">Pichtel, J., 2012. Distribution and fate of military explosives and propellants in soil: a review. Applied and Environmental Soil Science, 2012. [http://dx.doi.org/10.1155/2012/617236 doi: 10.1155/2012/617236]</ref>. Concentrations of Pb in soils at shooting ranges can vary widely depending on the number of rounds fired at the site, but concentrations often range from 55 to 80,000 mg/kg<ref name= "Vantelon2005"/><ref name= "Dermatas2006">Dermatas, D., Cao, X., Tsaneva, V., Shen, G. and Grubb, D.G., 2006. Fate and behavior of metal (loid) contaminants in an organic matter-rich shooting range soil: Implications for remediation. Water, Air, & Soil Pollution: Focus, 6(1-2), pp.143-155. [https://doi.org/10.1007/s11267-005-9003-4 doi: 10.1007/s11267-005-9003-4]</ref><ref name= "Sanderson2010">Sanderson, P., Bolan, N., Bowman, M. and Naidu, R., 2010. Distribution and availability of metal contaminants in shooting range soils around Australia. In Proceedings of the 19th World Congress of Soil Science: Soil solutions for a changing world, Brisbane, Australia, 1-6 August 2010. Symposium 3.5. 1 Heavy metal contaminated soils (pp. 65-67). International Union of Soil Sciences (IUSS), c/o Institut für Bodenforschung, Universität für Bodenkultur.</ref><ref name= "Seijo2016">Rodríguez‐Seijo, A., Alfaya, M.C., Andrade, M.L. and Vega, F.A., 2016. Copper, chromium, nickel, lead and zinc levels and pollution degree in firing range soils. Land Degradation & Development, 27(7), pp.1721-1730. [https://doi.org/10.1002/ldr.2497 doi: 10.1002/ldr.2497]</ref>. The mobility of Pb is highly dependent on local pH. Acidic conditions contribute to the mobilization of Pb species in the environment whereas basic conditions tend to promote precipitation and sorption reactions<ref name= "CAO2003">Cao, X., Ma, L.Q., Chen, M., Hardison, D.W. and Harris, W.G., 2003. Weathering of lead bullets and their environmental effects at outdoor shooting ranges. Journal of Environmental Quality, 32(2), pp.526-534. [https://doi.org/10.2134/jeq2003.5260 doi:10.2134/jeq2003.5260]</ref><ref>Knechtenhofer, L.A., Xifra, I.O., Scheinost, A.C., Flühler, H. and Kretzschmar, R., 2003. Fate of heavy metals in a strongly acidic shooting‐range soil: small‐scale metal distribution and its relation to preferential water flow. Journal of Plant Nutrition and Soil Science, 166(1), pp.84-92. [https://doi.org/10.1002/jpln.200390017 doi: 10.1002/jpln.200390017]</ref>. The formation of secondary mineral phases is a common observation, including [https://en.wikipedia.org/wiki/White_lead hydrocerussite] [Pb<sub>3</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>2</sub>], [https://en.wikipedia.org/wiki/Cerussite cerussite] [PbCO<sub>3</sub>], [https://en.wikipedia.org/wiki/Litharge litharge] and [https://en.wikipedia.org/wiki/Massicot massicot] [PbO polymorphs], but Pb also binds with sulfur and phosphorus depending on mineralogical conditions at a given site<ref name= "CAO2003"/><ref name= "Vantelon2005"/><ref>Scheetz, C.D. and Rimstidt, J.D., 2009. Dissolution, transport, and fate of lead on a shooting range in the Jefferson National Forest near Blacksburg, VA, USA. Environmental Geology, 58(3), pp.655-665. [https://doi.org/10.1007/s00254-008-1540-5 doi: 10.1007/s00254-008-1540-5]</ref><ref>Li, J., Wang, Q., Oremland, R.S., Kulp, T.R., Rensing, C. and Wang, G., 2016. Microbial antimony biogeochemistry: enzymes, regulation, and related metabolic pathways. Appl. Environ. Microbiol., 82(18), pp.5482-5495. [https://doi.org/10.1128/aem.01375-16 doi: 10.1128/aem.01375-16]</ref>. Additionally, Pb has been known to bind with soil organic matter, clay minerals, and iron/aluminum/manganese - oxides<ref>Manceau, A., Boisset, M.C., Sarret, G., Hazemann, J.L., Mench, M., Cambier, P. and Prost, R., 1996. Direct determination of lead speciation in contaminated soils by EXAFS spectroscopy. Environmental Science & Technology, 30(5), pp.1540-1552. [https://doi.org/10.1021/es9505154 doi: 10.1021/es9505154]</ref><ref>Mozafar, A., Ruh, R., Klingel, P., Gamper, H., Egli, S. and Frossard, E., 2002. Effect of heavy metal contaminated shooting range soils on mycorrhizal colonization of roots and metal uptake by leek. Environmental Monitoring and Assessment, 79(2), pp.177-191. [https://doi.org/10.1023/A:1020202801163 doi: 10.1023/A:1020202801163]</ref>. Pb is found in the near-surface environment in one oxidation state, Pb<sup>2+</sup>. Its toxicity is manifested primarily by effects to the central nervous system and brain function, but it can also cause damage to reproductive processes, auditory function, and vision<ref>Mason B. (1952) Principles of geochemistry, 276 p. New York, John Wiley and Sons. ISBN: 0471575224</ref>.
[[File:Bekins1w2 Fig4.PNG|500px|thumb|left|Figure 4. Redox zones of the Bemidji plume from Cozzarelli, et al., 2016<ref name= "Cozzarelli2016">Cozzarelli, I.M., Schreiber, M.E., Erickson, M.L. and Ziegler, B.A., 2016. Arsenic cycling in hydrocarbon plumes: secondary effects of natural attenuation. Groundwater, 54(1), pp.35-45. [https://doi.org/10.1111/gwat.12316 doi: 10.1111/gwat.12316]</ref>. The zones are defined by dissolved oxygen (DO) and iron content according to: (1) Methanogenic – DO<100 µg/L, Fe>25 mg/L; (2) Iron-reducing – DO<100 µg/L, Fe 1-25 mg/L; (3) Sub-oxic transition – DO 100-1,000 µg/L; (4) Oxic – DO>1,000 µg/L, Fe<0.5 mg/L; and (5) Sub-oxic – DO 100-1,000 µg/L, Fe 0.5-1.0 mg/L.]]
 
  
Figure 4 shows a simplified schematic of redox zones for a cross section of the north pool plume from Cozzarelli, et al., 2016<ref name= "Cozzarelli2016"/>. Although the background groundwater is aerobic with 8-10 mg/L dissolved oxygen, the plume core is anaerobic.  The anaerobic conditions are maintained by the ongoing supply of 40-50 mg/L total dissolved organic carbon entering the groundwater from the oil<ref name= "Eganhouse1993">Eganhouse, R.P., Baedecker, M.J., Cozzarelli, I.M., Aiken, G.R., Thorn, K.A. and Dorsey, T.F., 1993. Crude oil in a shallow sand and gravel aquifer—II. Organic geochemistry. Applied Geochemistry, 8(6), pp.551-567. [https://doi.org/10.1016/0883-2927(93)90013-7 doi: 10.1016/0883-2927(93)90013-7]</ref>, together with limited mixing with aerobic water on the plume fringes<ref>Gutierrez-Neri, M., Ham, P.A.S., Schotting, R.J. and Lerner, D.N., 2009. Analytical modelling of fringe and core biodegradation in groundwater plumes. Journal of Contaminant Hydrology, 107(1-2), pp.1-9. [https://doi.org/10.1016/j.jconhyd.2009.02.007 doi: 10.1016/j.jconhyd.2009.02.007]</ref>. The dominant redox processes in the plume core are iron reduction and fermentation coupled to methanogenesis; with nitrate and sulfate reduction being relatively unimportant<ref>Baedecker, M.J., Cozzarelli, I.M., Eganhouse, R.P., Siegel, D.I. and Bennett, P.C., 1993. Crude oil in a shallow sand and gravel aquifer-III. Biogeochemical reactions and mass balance modeling in anoxic groundwater. Applied Geochemistry, 8(6), pp.569-586. [https://doi.org/10.1016/0883-2927(93)90014-8 doi: 10.1016/0883-2927(93)90014-8]</ref>.  During iron reduction, the Fe(III) in poorly crystalline iron oxyhydroxide coatings on the aquifer sediments is reduced to Fe(II)<ref>Lovley, D.R., Baedecker, M.J., Lonergan, D.J., Cozzarelli, I.M., Phillips, E.J. and Siegel, D.I., 1989. Oxidation of aromatic contaminants coupled to microbial iron reduction. Nature, 339(6222), p.297. [https://doi.org/10.1038/339297a0 doi: 10.1038/339297a0]</ref> of which 99% is locally redeposited onto the aquifer sediments<ref name="Ng2014"/>. Over the last 40 years the sediment Fe(III) coatings have been almost completely consumed near the source zone and in the center of the plume<ref name= "Amos2012">Amos, R.T., Bekins, B.A., Cozzarelli, I.M., Voytek, M.A., Kirshtein, J.D., Jones, E.J.P. and Blowes, D.W., 2012. Evidence for iron‐mediated anaerobic methane oxidation in a crude oil‐contaminated aquifer. Geobiology, 10(6), pp.506-517. [https://doi.org/10.1111/j.1472-4669.2012.00341.x doi: 10.1111/j.1472-4669.2012.00341.x]</ref>. Accordingly, the methanogenic zone has expanded and the iron-reducing zone has moved approximately 60 m downgradient in the 10-year interval 1996-2006<ref name="Ng2015"/><ref name= "Amos2012"/><ref>Bekins, B.A., Cozzarelli, I.M., Godsy, E.M., Warren, E., Essaid, H.I. and Tuccillo, M.E., 2001. Progression of natural attenuation processes at a crude oil spill site: II. Controls on spatial distribution of microbial populations. Journal of Contaminant Hydrology, 53(3-4), pp.387-406. [https://doi.org/10.1016/S0169-7722(01)00175-9 doi: 10.1016/S0169-7722(01)00175-9]</ref>.  Aerobic degradation occurs on the plume fringes<ref>Amos, R.T. and Mayer, K.U., 2006. Investigating the role of gas bubble formation and entrapment in contaminated aquifers: Reactive transport modelling. Journal of Contaminant Hydrology, 87(1-2), pp.123-154. [https://doi.org/10.1016/j.jconhyd.2006.04.008 doi: 10.1016/j.jconhyd.2006.04.008]</ref>, but a high resolution push-probe survey shows it can account for at most 15% of the reduced carbon oxidation<ref name= "Amos2012"/>. A modeling study obtained similar estimates for proportions of aerobic versus anaerobic degradation of 17% and 83% respectively<ref name= "Essaid2003"/>.
+
===Antimony===
 +
[https://en.wikipedia.org/wiki/Antimony Antimony] (Sb) is a hard, toxic, metalloid that is added to Pb in molten form to make the Pb-alloyed core of a bullet. Sb is thought to behave similarly in the environment to its group VB neighbor Arsenic (As), and, like As, it is toxic and a suspected carcinogen<ref>World Health Organization, 2003. Background document for development of WHO Guidelines for Drinking-water Quality. World Health Organization, 20, pp.4-6.</ref>. Sb is added to bullets as a hardening agent because Pb is soft and would not have the same impact capacity without it<ref name= "Scheinost2006"/><ref name= "Sanderson2010"/>. Bullet cores typically contain no more than 2% Sb and, correspondingly, concentrations in soils are typically lower than for Pb. Sb is also often found in the primers of small arms ammunition as Sb sulfide. Although Sb is present in bullets approximately two orders of magnitude less than Pb, in some aqueous systems Sb has been found to be more mobile than Pb<ref name= "Johnson2005">Johnson, C.A., Moench, H., Wersin, P., Kugler, P. and Wenger, C., 2005. Solubility of antimony and other elements in samples taken from shooting ranges. Journal of Environmental Quality, 34(1), pp.248-254. [https://doi.org/10.2134/jeq2005.0248  doi:10.2134/jeq2005.0248]</ref><ref>Sanderson, P., Naidu, R. and Bolan, N., 2015. Effectiveness of chemical amendments for stabilisation of lead and antimony in risk-based land management of soils of shooting ranges. Environmental Science and Pollution Research, 22(12), pp.8942-8956. [https://doi.org/10.1007/s11356-013-1918-0 doi: 10.1007/s11356-013-1918-0]</ref><ref name= "doherty2017">Doherty, S.J., Tighe, M.K. and Wilson, S.C., 2017. Evaluation of amendments to reduce arsenic and antimony leaching from co-contaminated soils. Chemosphere, 174, pp.208-217. [https://doi.org/10.1016/j.chemosphere.2017.01.100 doi: 10.1016/j.chemosphere.2017.01.100]</ref>. However, in more acidic soils Pb often exceeds measured Sb aqueous concentrations due to the high solubility of Pb at low pH values<ref>Okkenhaug, G., Gebhardt, K.A.G., Amstaetter, K., Bue, H.L., Herzel, H., Mariussen, E., Almås, Å.R., Cornelissen, G., Breedveld, G.D., Rasmussen, G. and Mulder, J., 2016. Antimony (Sb) and lead (Pb) in contaminated shooting range soils: Sb and Pb mobility and immobilization by iron-based sorbents, a field study. Journal of Hazardous Materials, 307, pp.336-343. [https://doi.org/10.1016/j.jhazmat.2016.01.005 doi: 10.1016/j.jhazmat.2016.01.005]</ref>. The mobility of Sb on ranges is highly dependent on its speciation, which in near-surface environments is Sb(III) or Sb(V). The trivalent form has been shown to be less mobile than the pentavalent form, but the trivalent form is considerably more toxic, particularly Sb trioxide<ref name= "Scheinost2006"/><ref name = "Wilson2010">Wilson, S.C., Lockwood, P.V., Ashley, P.M. and Tighe, M., 2010. The chemistry and behaviour of antimony in the soil environment with comparisons to arsenic: a critical review. Environmental Pollution, 158(5), pp.1169-1181.
 +
[http://dx.doi.org/10.1016/j.envpol.2009.10.045 doi: 10.1016/j.envpol.2009.10.045]</ref>. Sb(V) is more mobile at higher pHs<ref>Hu, X., Guo, X., He, M. and Li, S., 2016. pH-dependent release characteristics of antimony and arsenic from typical antimony-bearing ores. Journal of Environmental Sciences, 44, pp.171-179. [https://doi.org/10.1016/j.jes.2016.01.003 doi: 10.1016/j.jes.2016.01.003]</ref>, while Sb(III) in solution is independent of pH<ref name= "Johnson2005"/>. Certain environmental factors influence Sb speciation, including pH, redox potential, biological activity, and co-contamination<ref>Filella, M., Belzile, N. and Chen, Y.W., 2002. Antimony in the environment: a review focused on natural waters: I. Occurrence. Earth-Science Reviews, 57(1), pp.125-176. [http://dx.doi.org/10.1016/s0012-8252(01)00070-8 doi:10.1016/S0012-8252(01)00070-8]</ref><ref name = "Wilson2010"/><ref name = "Ilgen2014">Ilgen, A.G., Majs, F., Barker, A.J., Douglas, T.A. and Trainor, T.P., 2014. Oxidation and mobilization of metallic antimony in aqueous systems with simulated groundwater. Geochimica et Cosmochimica Acta, 132, pp.16-30. [http://dx.doi.org/10.1016/j.gca.2014.01.019  doi:10.1016/j.gca.2014.01.019]</ref><ref name= "doherty2017"/>. Sb oxidation is rapid<ref name = "Ilgen2014"/>, forming secondary products both in soils and within the weathering crust surrounding bullets<ref name= "Ackermann2009"/>. Iron (Fe) is a natural sink for Sb, forming primarily inner sphere sorption complexes (both corner and edge-sharing) with Fe(III) oxides found in soils<ref name= "Scheinost2006"/><ref name= "Ackermann2009"/><ref>Ritchie, V.J., Ilgen, A.G., Mueller, S.H., Trainor, T.P. and Goldfarb, R.J., 2013. Mobility and chemical fate of antimony and arsenic in historic mining environments of the Kantishna Hills district, Denali National Park and Preserve, Alaska. Chemical Geology, 335, pp.172-188. [https://doi.org/10.1016/j.chemgeo.2012.10.016 doi: 10.1016/j.chemgeo.2012.10.016]</ref>.  
 +
</br>
 +
</br>
 +
===Copper===
 +
[https://en.wikipedia.org/wiki/Copper Copper] (Cu) is an essential nutrient for the body and has a relatively high maximum contaminant level (MCL) in drinking water set by the EPA (1.3 mg/kg) compared to other metal(loid)s that comprise bullets. However, toxicity from elevated Cu concentrations can arise and cause gastrointestinal distress (short-term) and liver and/or kidney damage (long-term)<ref>U. S. Environmental Protection Agency (USEPA), 2009. National Primary Drinking Water Regulations. EPA 816-F-09-004. [[media:2009-USEPA-national_Primary_Drinking_Water_Regulations.pdf| Report.pdf]]</ref>. The outer casing and jacket of a bullet is comprised primarily of Cu. When a bullet is fired, the outer Cu casing remains at the firing point along with the primer and propellant residues while the Cu jacket impacts soil and either deforms or fragments. New 5.56-mm and 7.62-mm “green” bullets contain solid Cu cores in place of Pb/Sb alloys. Copper concentrations in shooting range soils typically range from 19 to upwards of 6,000 mg/kg<ref>Robinson, B.H., Bischofberger, S., Stoll, A., Schroer, D., Furrer, G., Roulier, S., Gruenwald, A., Attinger, W. and Schulin, R., 2008. Plant uptake of trace elements on a Swiss military shooting range: Uptake pathways and land management implications. Environmental Pollution, 153(3), pp.668-676. [https://doi.org/10.1016/j.envpol.2007.08.034 doi: 10.1016/j.envpol.2007.08.034]</ref><ref name= "Seijo2016"/>. Similar to Pb, Cu mobility is highly dependent on pH<ref>Sherene, T., 2010. Mobility and transport of heavy metals in a polluted soil environment. In Biological forum - An International Journal (Vol. 2, No. 2, pp. 112-121). [[media:2010-Sherene-Mobility_and_transport_of_heavy_metals_i.pdf| Report.pdf]]</ref>, and it has been shown to easily complex with organic matter<ref>Huang, J., Huang, R., Jiao, J.J. and Chen, K., 2007. Speciation and mobility of heavy metals in mud in coastal reclamation areas in Shenzhen, China. Environmental Geology, 53(1), pp.221-228. [https://doi.org/10.1007/s00254-007-0636-7 doi: 10.1007/s00254-007-0636-7]</ref><ref>Ashraf, M.A., Maah, M.J. and Yusoff, I., 2012. Chemical speciation and potential mobility of heavy metals in the soil of former tin mining catchment. The Scientific World Journal, 2012. ISBN: 0471575224</ref>.  
  
The BTEX compounds have varying fates in the plume. Toluene and ''o''-xylene degrade under methanogenic conditions near the oil body<ref>Eganhouse, R.P., Dorsey, T.F., Phinney, C.S. and Westcott, A.M., 1996. Processes affecting the fate of monoaromatic hydrocarbons in an aquifer contaminated by crude oil. Environmental Science & Technology, 30(11), pp.3304-3312. [https://doi.org/10.1021/es960073b doi: 10.1021/es960073b]</ref> while benzene and ethylbenzene do not degrade until reaching the iron reducing zone<ref>Anderson, R.T., Rooney-Varga, J.N., Gaw, C.V. and Lovley, D.R., 1998. Anaerobic benzene oxidation in the Fe (III) reduction zone of petroleum-contaminated aquifers. Environmental Science & Technology, 32(9), pp.1222-1229. [https://doi.org/10.1021/es9704949 doi: 10.1021/es9704949]</ref>. Since 1995, the position of the 50 µg/L benzene contour has remained stable at about 125 m downgradient and benzene concentrations near the source zone have dropped from almost 7 mg/L to 4 mg/L as the oil body concentrations of benzene have decreased<ref name= "Bekins2016"/>. These data indicate that natural attenuation under mainly iron-reducing conditions is effectively limiting the advance of the benzene plume.
+
===Nickel===
 +
[https://en.wikipedia.org/wiki/Nickel Nickel] (Ni) toxicity is typically a concern to humans due to its prevalence as an allergen and the ability of Ni to interfere with other essential metals in cellular processes<ref>Coogan, T.P., Latta, D.M., Snow, E.T., Costa, M. and Lawrence, A., 1989. Toxicity and carcinogenicity of nickel compounds. CRC Critical reviews in toxicology, 19(4), pp.341-384. [https://doi.org/10.3109/10408448909029327 doi: 10.3109/10408448909029327]</ref><ref> Sharma, A.D., 2013. Low nickel diet in dermatology. Indian Journal of Dermatology, 58(3), p.240. [https://doi.org/10.4103/0019-5154.110846 doi: 10.4103/0019-5154.110846]</ref>. Trace amounts of Ni are found in the casing and jacket of a bullet, used to make the alloy with Cu. Ni is a biologically important micronutrient and an essential trace element for plants, microbes, animals, and humans<ref>Mason B. (1952) Principles of geochemistry, 276 p. New York, John Wiley and Sons. ISBN: 0471575224</ref><ref name = "Coleman2004">Coleman D.C., Crossley Jr. D.A. and Hendrix P.F., 2004. Fundamentals of Soil Ecology, 2nd ed. Elsevier Inc. New York City, NY.  ISBN: 9780121797263/e: 9780080472812</ref><ref>Nieminen, T.M., Ukonmaanaho, L., Rausch, N. and Shotyk, W., 2007. Biogeochemistry of nickel and its release into the environment. Metal Ions in Life Sciences, nickel and its surprising impact in nature. 2nd Volume. Chichester: John Wiley & Sons, pp.1-30.  ISSN: 1559-0836. </ref>. Ni concentrations in environmental systems tend to be conservative and uniformly distributed<ref>Iyaka, Y.A., 2011. Nickel in soils: a review of its distribution and impacts. Scientific Research and Essays, 6(33), pp.6774-6777. [[media:2011-Iyaka-Nickel_in_soils._A_review_of_its_distribution_and_impacts.pdf| Report.pdf]]</ref>. In shooting ranges, typical concentrations of Ni are lower compared to the other metal(loid)s like Pb, Sb, Cu, and Zn<ref>Mariussen, E., Johnsen, I.V. and Strømseng, A.E., 2017. Distribution and mobility of lead (Pb), copper (Cu), zinc (Zn), and antimony (Sb) from ammunition residues on shooting ranges for small arms located on mires. Environmental Science and Pollution Research, 24(11), pp.10182-10196. [https://doi.org/10.1007/s11356-017-8647-8 doi: 10.1007/s11356-017-8647-8]</ref>. Ni tends to be relatively immobile in soil solution under neutral and alkaline conditions, but can become highly mobilized under acidic conditions (similar to the other cationic metals; Pb, Cu, and Zn). Similar to other metal(loid)s, key environmental processes that affect Ni cycling in geomedia include adsorption, (co)precipitation, and complexation<ref>Harasim, P. and Filipek, T., 2015. Nickel in the environment. Journal of Elementology, 20(2). [https://doi.org/10.5601/jelem.2014.19.3.651 doi: 10.5601/jelem.2014.19.3.651]</ref>.  
  
[[File:Bekins1w2 Fig5.jpg|400px|thumb|right|Figure 5. Concentrations of nonvolatile dissolved organic carbon (NVDOC) and total petroleum hydrocarbons in the diesel range (TPHd) measured along the centerline of the north pool plume in August 2016. Well locations and oil spill source location shown in Figure 1 of this article. Figure reprinted from McGuire, et al., 2018<ref name= "McGurie2018"/>.]]
+
===Zinc===
 +
[https://en.wikipedia.org/wiki/Zinc Zinc] (Zn) is an essential micronutrient for sustained existence<ref name = "Coleman2004"/>. Similar to Ni, trace amounts of Zn are found in the casing and jacket of a bullet, used to make the alloy with Cu. Zn has been found to become mobilized at acidic pHs, similar to other cationic metals (Pb, Cu, and Ni) and often associates with clay minerals, soil organic matter, and the roots and shoots of plants, if available <ref>Haslett, B.S., Reid, R.J. and Rengel, Z., 2001. Zinc mobility in wheat: uptake and distribution of zinc applied to leaves or roots. Annals of Botany, 87(3), pp.379-386. [https://doi.org/10.1006/anbo.2000.1349 doi: 10.1006/anbo.2000.1349]</ref><ref>Rutkowska, B., Szulc, W., Bomze, K., Gozdowski, D. and Spychaj-Fabisiak, E., 2015. Soil factors affecting solubility and mobility of zinc in contaminated soils. International Journal of Environmental Science and Technology, 12(5), pp.1687-1694. [https://doi.org/10.1007/s13762-014-0546-7 doi: 10.1007/s13762-014-0546-7]</ref>.  
  
About 60-80% of the reduced carbon in the north pool plume consists of nonvolatile dissolved organic carbon (NVDOC)<ref name= "Eganhouse1993"/><ref name= "Bekins2016"/>. Comparing concentrations in the same wells of NVDOC to total petroleum hydrocarbons in the diesel range (TPHd) shows that the TPHd values are less than one-third of the NVDOC values (Figure 5). The difference is due to losses of polar compounds during extraction into an organic solvent and retention on a gas chromatograph column. Concentrations of NVDOC were measured on filtered samples by acidifying and purging to removed inorganic carbon followed by combustion<ref name= "Bekins2016"/>. Often the term DOC is used for this analysis for surface water samples, but for hydrocarbon-contaminated sites the modifier nonvolatile clarifies that the volatile hydrocarbons present in the water samples have been removed during the purging process.  Thus, NVDOC represents the total dissolved organic carbon (TDOC) minus the volatile organic carbon (VOC)<ref name= "Eganhouse1993"/>. The NVDOC in the contaminant plume is mainly composed of partial transformation products of the hydrocarbons in the oil<ref>Thorn, K.A. and Aiken, G.R., 1998. Biodegradation of crude oil into nonvolatile organic acids in a contaminated aquifer near Bemidji, Minnesota. Organic Geochemistry, 29(4), pp.909-931. [https://doi.org/10.1016/S0146-6380(98)00167-3 doi: 10.1016/S0146-6380(98)00167-3]</ref>.
+
===Chromium===
 +
[https://en.wikipedia.org/wiki/Chromium Chromium (Cr)] in the +3 valence state (Cr(III)) is an essential micronutrient, but has been found to be toxic in large doses. Chromium in the +6 valence state (Cr(VI)) is classified as a human carcinogen<ref>Wilbur, S.B., 1998. Toxicological profile for chromium. US Department of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry.</ref><ref>U.S. Environmental Protection Agency (USEPA), 1999. Integrated Risk Information System (IRIS) on chromium VI. National Center for Environmental Assessment. Office of Research and Development, Washington.</ref>. Depending on the quality of the source of metals used for the production of bullets, Cr may be found as an impurity in steel<ref name= "Levonmaki2002">Levonmäki, M. and Kairesalo, T., 2002. Do steel shots raise a chromium problem on shooting range areas. ISSF News, 2, pp.9-10. </ref><ref>Sorvari, J., Antikainen, R. and Pyy, O., 2006. Environmental contamination at Finnish shooting ranges—the scope of the problem and management options. Science of the Total Environment, 366(1), pp.21-31. [https://doi.org/10.1016/j.scitotenv.2005.12.019 doi: 10.1016/j.scitotenv.2005.12.019]</ref><ref name= "Seijo2016"/>. One study on Cr presence at shooting range soils estimated the Cr content of steel to be up to 27%, which is a much higher estimate than other studies have reported<ref name= "Levonmaki2002"/>. Typically, Cr concentrations in shooting range soils are reported comparable to background concentrations in local soils, indicating minimal environmental impact from training<ref name= "Seijo2016"/>. Cr is primarily found in the near-surface environment as Cr(III) and Cr(VI). In its hexavalent oxidation state Cr is highly mobile, while Cr(III) is more likely to participate in precipitation and sorption reactions, and therefore mobility in aqueous environments and sediments is limited<ref>Gorny, J., Billon, G., Noiriel, C., Dumoulin, D., Lesven, L. and Madé, B., 2016. Chromium behavior in aquatic environments: a review. Environmental Reviews, 24(4), pp.503-516. [https://doi.org/10.1139/er-2016-0012 doi: 10.1139/er-2016-0012]</ref>.
  
Concentrations of NVDOC in the aquifer increase from <2 mg/L in the clean background to 30-50 mg/L under the north oil pool. The pool of compounds comprising the NVDOC is progressively oxidized as it migrates downgradient in the aquifer leading to a 90% decrease in concentration by 150-200 m downgradient. However, the position of the 5 mg/L NVDOC contour expanded downgradient from 125 m in 1988 to over 200 m in 2010<ref name= "Bekins2016"/>.  During biodegradation, there is change in character from aliphatic, low oxygen-to-carbon ratio (O/C) compounds under the oil to more aromatic, high O/C compounds downgradient<ref name= "Podgorski2018"/>.  Ultrahigh resolution mass spectrometry analyses show that over 10,000 compounds are present in the NVDOC. Thus, assessing whether the mixture poses a human health or environmental hazard is hampered by the high number of individual compounds and low concentrations of any single compound.  An assessment of the entire mixture of nonvolatile compounds extracted from wells located within 100 m of the north oil pool showed increased activity of toxicologically relevant transcriptional pathways and human nuclear receptors in human liver cells<ref name= "McGurie2018"/>.  The measured effects decrease as NVDOC concentrations and properties change as a result of continued biotransformation during migration away from the source zone.  As stated by McGuire, et al., 2018: “effects observed at the molecular level do not necessarily indicate adverse outcomes at the organismal level”<ref name= "McGurie2018"/>.  The potential health effects of polar hydrocarbon oxidation products is an active and controversial area of research (see for example the publication by Zemo et al., 2013<ref>Zemo, D.A., O'Reilly, K.T., Mohler, R.E., Tiwary, A.K., Magaw, R.I. and Synowiec, K.A., 2013. Nature and estimated human toxicity of polar metabolite mixtures in groundwater quantified as TPHd/DRO at biodegrading fuel release sites. Groundwater Monitoring & Remediation, 33(4), pp.44-56. [https://doi.org/10.1111/gwmr.12030 doi: 10.1111/gwmr.12030]</ref> and comments on that publication by Brewer and Blumberg<ref>Brewer, R.C. and Hellmann‐Blumberg, U., 2014. Nature and estimated human toxicity of polar metabolite mixtures in groundwater quantified as TPHd/DRO at biodegrading fuel release sites. Groundwater Monitoring & Remediation, 34(2), pp.24-25. [https://doi.org/10.1111/gwmr.12053 doi: 10.1111/gwmr.12053]</ref>. For additional discussion see the ITRC TPH guidance document available online<ref>Interstate Technology Regulatory Council (ITRC), 2018. TPH Risk Evaluation at Petroleum-Contaminated Sites</ref>.
+
===Tungsten===
 +
Exposure to high [https://en.wikipedia.org/wiki/Tungsten tungsten] (W) concentrations has been linked to altered thyroid function, heart disease, and potentially childhood leukemia and lung cancer<ref name = "Datta2017">Datta, S., Vero, S.E., Hettiarachchi, G.M. and Johannesson, K., 2017. Tungsten contamination of soils and sediments: current state of science. Current Pollution Reports, 3(1), pp.55-64. [https://doi.org/10.1007/s40726-016-0046-0 doi: 0.1007/s40726-016-0046-0]</ref>. Historically, W and nylon have been used as an alternative for Pb in bullets<ref name = "Datta2017"/>. The tungsten-nylon bullets were termed ‘green bullets’, but did not stay in production for longer than a decade as subsequent research eventually revealed W to be both toxic and highly mobile<ref>Agency for Toxic Substances and Disease Registry (ATSDR), 2005. Toxicological Profile for Tungsten. U.S. Public Health Service, U.S. Department of Health and Human Services, Atlanta, GA.</ref>. Concentrations of W in shooting range soils where green bullets have been spent ranged from below detection to greater than 2,000 mg/kg<ref>Clausen, J.L. and Korte, N., 2009. Environmental fate of tungsten from military use. Science of the total environment, 407(8), pp.2887-2893.
  
==Ongoing Studies==
+
[https://doi.org/10.1016/j.scitotenv.2009.01.029 doi: 10.1016/j.scitotenv.2009.01.029]</ref>. W has been shown to be relatively mobile in the environment, with its mobility dependent on speciation. However, there is evidence to suggest W mobility decreases as soil ages, as long as the source is limited<ref>Bednar, A.J., Jones, W.T., Boyd, R.E., Ringelberg, D.B. and Larson, S.L., 2008. Geochemical parameters influencing tungsten mobility in soils. Journal of environmental quality, 37(1), pp.229-233. [https://doi.org/10.2134/jeq2007.0305 doi:10.2134/jeq2007.0305]</ref>. W metal is not stable in the environment and instead primarily exists as W(VI) and prefers to form a variety of polyatomic anions (like WO<sub>4</sub><sup>2-</sup> and H<sub>2</sub>W<sub>12</sub>O<sub>40</sub><sup>6-</sup>, etc.) and polyoxometalates in solution<ref>Bostick, B.C., Sun, J., Landis, J.D. and Clausen, J.L., 2018. Tungsten Speciation and Solubility in Munitions-Impacted Soils. Environmental science & technology, 52(3), pp.1045-1053. [https://doi.org/10.1021/acs.est.7b05406 doi: 10.1021/acs.est.7b05406]</ref>.  
The site continues to be an active research facility for understanding natural attenuation processes as well as developing methods for measuring remediation timeframes and plume toxicity. Ongoing process studies cover the properties of hydrophobic soils in the spray zone, causes of variations in NSZD rates, natural attenuation of secondary water quality impacts from iron and arsenic in the plume, and the toxicity and composition changes of the NVDOC. Methods studies cover remediation of hydrophobic soils, use of electrical and magnetic signals to monitor biodegradation rates, and geophysical techniques for mapping plume discharge to the lake. Outside researchers are welcome at the site and should check the [https://mn.water.usgs.gov/projects/bemidji/policy.html policy webpage] for more information.
 
  
==Summary==
+
===Mercury===
A USGS research study of an August 1979 crude oil pipeline spill near Bemidji, Minnesota, continues to provide valuable results almost 40 years after the spill<ref name= "USGS2017"/>. Analyses of oil samples indicate that 18-31% of the oil mass in the source zone had been lost by 2010 to natural processes of volatilization, dissolution, and biodegradation. Methods developed at the site for estimating NSZD rates include surface carbon dioxide efflux measurement and monitoring of increased subsurface temperatures.  
+
A compound containing [https://en.wikipedia.org/wiki/Mercury_(element) mercury] (Hg) (mercury fulminate, Hg(CNO)<sub>2</sub>) was previously used as a primer for ammunition for approximately 100 years before it was widely replaced by lead azide, silver azide, or lead styphnate which have greater stability<ref>Beck, W., Evers, J., Göbel, M., Oehlinger, G. and Klapötke, T.M., 2007. The crystal and molecular structure of mercury fulminate (Knallquecksilber). Zeitschrift für anorganische und allgemeine Chemie, 633(9), pp.1417-1422. [https://doi.org/10.1002/zaac.200700176 doi: 10.1002/zaac.200700176]</ref><ref name= "Pichtel2012"/>. Despite the minimal to negligible use in recent history, there is some evidence to suggest that legacy shooting ranges may have persistent Hg contamination in soil<ref name= "Stauffer2017">Stauffer, M., Pignolet, A. and Alvarado, J.C., 2017. Persistent mercury contamination in shooting range soils: the legacy from former primers. Bulletin of environmental contamination and toxicology, 98(1), pp.14-21. [https://doi.org/10.1007/s00128-016-1976-3 doi: 0.1007/s00128-016-1976-3]</ref><ref name = "Bigham2017">Bigham, G.N., Murray, K.J., Masue‐Slowey, Y. and Henry, E.A., 2017. Biogeochemical controls on methylmercury in soils and sediments: Implications for site management. Integrated environmental assessment and management, 13(2), pp.249-263. [https://doi.org/10.1002/ieam.1822 doi: 10.1002/ieam.1822]</ref>. Additionally, marine environments near legacy military training grounds may contain elevated concentrations of Hg from the dumping of ammunition after World War II, including the Baltic Sea<ref>Gębka, K., Bełdowski, J. and Bełdowska, M., 2016. The impact of military activities on the concentration of mercury in soils of military training grounds and marine sediments. Environmental Science and Pollution Research, 23(22), pp.23103-23113. [https://doi.org/10.1007/s11356-016-7436-0 doi: 10.1007/s11356-016-7436-0]</ref>. Similar to other primer metal(loid)s, Hg in firing range soils should primarily be located near the firing point instead of traveling with the bullet. An old Swiss shooting range was found to have slightly elevated total Hg concentrations above background values (44-315 μg/kg) with maximum values ranging from 906 to 1,517 μg/kg in soils at the firing point, with concentrations decreasing with increasing distance from the firing point<ref name= "Stauffer2017"/>. While the concentrations of Hg found at shooting ranges are significantly lower than concentrations reported for Pb and Sb, it still remains a health and environmental hazard due to its harmful effects and toxicity even at low concentrations<ref name = "Bigham2017"/>.
  
In the groundwater plume, the most important natural attenuation processes are anaerobic biodegradation coupled to iron reduction, followed by fermentation and methanogenesis. The downgradient front of the benzene plume has been stable since 1995 and peak groundwater benzene concentrations are dropping as benzene concentrations decrease in the oil. Most of the dissolved organic carbon transported from the source zone in the groundwater plume consists of partial transformation products of the oil compounds (also known as polar metabolites). Biotransformation in the aquifer decreases concentrations of these compounds, but a refractory fraction of about 10% discharges to a small lake 335 m from the source.  
+
===Arsenic===
 +
[[File:Barker1w2 Fig4.png|thumb|Figure 4. Soil berms with high amounts of soil organic matter and clay minerals as shown in this picture tend to retain concentrations of metal(loid)s as a result of large surface area and active binding sites. Image shows constructed shooting range berms in Delta Junction, Alaska on the Donnelly Training Area. Photo by A.J. Barker.]]
 +
[https://en.wikipedia.org/wiki/Arsenic Arsenic] (As) is a highly toxic and carcinogenic metalloid that affects nearly all organ systems<ref>Ratnaike, R.N., 2003. Acute and chronic arsenic toxicity. Postgraduate medical journal, 79(933), pp.391-396. [[media:2003-Ratnaike-Acute_and_chronic_arsenic_toxicity.pdf| Report.pdf]]</ref>. Arsenic has been found at shooting ranges to a much lesser extent than other metal(loid)s, as it is associated with Pb as an impurity. While As is a well-known contaminant in other systems, at shooting ranges it is often not measured or measured at concentrations close to background concentrations in nearby soils<ref name= "Dermatas2006"/><ref>Bannon, D.I., Drexler, J.W., Fent, G.M., Casteel, S.W., Hunter, P.J., Brattin, W.J. and Major, M.A., 2009. Evaluation of small arms range soils for metal contamination and lead bioavailability. Environmental science & technology, 43(24), pp.9071-9076. [https://doi.org/10.1021/es901834h doi: 10.1021/es901834h]</ref>. Arsenic exhibits multiple oxidation states in the near-surface environment, but is most often found as arsenite (As(III)) and/or arsenate (As(V)). Major environmental factors that affect As mobility are pH, redox condition, and soil type<ref>Bissen, M. and Frimmel, F.H., 2003. Arsenic - a review. Part I: occurrence, toxicity, speciation, mobility. Acta hydrochimica et hydrobiologica, 31(1), pp.9-18. [https://doi.org/10.1002/aheh.200390025 doi: 10.1002/aheh.200390025]</ref>.
 +
 +
==Management Practices==
 +
Range design is an important factor in containing metal(loid)s and preventing their migration into the subsurface. Target berms may be comprised of a natural hillside or slope, or constructed with off-site source material that is shaped into an artificial barrier (as shown in Figure 4). Ensuring berm construction includes a berm face that is steep enough to prevent bullet skipping, but stable enough to minimize soil erosion, can minimize contaminated sediment losses. Siting the berm with respect to hydrologic pathways to waterbodies, aquifers, and potential ecological receptors is crucial. Additionally, it is important to decide what type of soil will comprise the berm or hillside (i.e. sand, organic-rich material, etc.) and determining soil texture and major mineral and element fractions. These soil composition variables will affect the overall speciation and mobility of the target contaminant species in bullets (Pb, Sb, etc.).
 +
Bullet catchers (see Additional Resources) are designed to catch bullets and contain bullet-related metal(loid)s for later collection and disposal.  These systems are designed for long-term use on range.  The bullet-related metal(loid)s are stored internally in a protective liner to minimize soil and water interactions with the fired bullet rounds.  
  
Ongoing studies are assessing the toxicity of the partial transformation products, testing remedial strategies for hydrophobic oil-coated soils, and developing geophysical techniques for monitoring NSZD and surface water discharge of the groundwater plume.  
+
Additionally, soil amendments such as phosphate and lime can alter soil pH to inhibit mobility of some metals (mainly cationic metals like Pb, Cu, Ni, and Zn), and other amendments such as Fe/Mn hydro(oxides) can facilitate sorption reactions with metal(loid)s, such as Sb and As. Although costly, spent bullets and large bullet fragments can also be removed from soils.
  
 
==References==
 
==References==
Line 108: Line 63:
  
 
==See Also==
 
==See Also==
*[https://mn.water.usgs.gov/projects/bemidji/index.html Bemidji Crude – Oil Research Project]
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*[https://www.stappebc.com/environmental-bullet-catcher/about/ Environmental Bullet Catcher]
*[https://www.usgs.gov/news/crude-oil-byproducts-groundwater-plumes Crude Oil Byproducts in Groundwater Plumes]
 

Revision as of 20:07, 18 July 2019

Metals and metalloids are deposited into small arms range soils from bullets, casings, and primers. The mobility potential depends on the individual metal(loid), its speciation, local pH, redox conditions and soil characteristics. This article discusses the speciation, mobility, and toxicity of metal(loid)s relevant to small arms ranges and briefly discusses management strategies to mitigate risk.

Related Article(s):


CONTRIBUTOR(S): Dr. Amanda Barker


Key Resource(s):

Introduction

Military small arms training uses bullets made of metals and metalloids which can include lead, antimony, copper, nickel, zinc, chromium, tungsten, mercury, and arsenic. Some of these metal(loid)s are also key ingredients of small arms ammunition primers used to initiate bullet firing. Unlike organic compounds found in propellants and explosives, metal(loid)s do not degrade in the environment. As such, the pervasive use of metal(loid)s on military training grounds and shooting ranges has resulted in contamination of soils, sediments, surface water, and groundwater both local to specific sites and off-site. A simplified schematic showing potential pathways of metal(loid) release on shooting ranges is shown in Figure 1. Upon exposure to the atmosphere, initially zero-valent metal(loid)s are oxidized, form secondary mineral phases, and subsequently release oxidized species into soil solution. The formation of a weathering crust on the surface of fired bullets is shown in Figures 2 and 3. Mobilization of metal(loid)s in pore water and soil solution is controlled by a variety of factors, including pH, oxidation-reduction (redox) environment, presence of colloids and/or soil organic matter (SOM), soil type and hydraulic characteristics, temperature, and water infiltration[1][2][3][4].

Figure 1. Simplified schematic of shooting ranges with berm-style backstops that typically contain high loadings of metal(loid)s in the berm due to bullet fragmentation and also at the firing point as a result of residue and primer materials.
Figure 2. Chemical characterization of a 5.56mm corroding bullet that underwent 15 years of weathering in Alaskan soils using scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). Relative concentrations for iron (red), lead (green), and antimony (purple) are shown in the right x-ray map. Data was collected at the Advanced Instrumentation Laboratory (AIL), University of Alaska Fairbanks (UAF). Data collected by A.J. Barker
Figure 3. Optical images of a new 5.56mm bullet (left) and a 5.56mm bullet that weathered for 15 years (right). The tip of the bullet fragmented and the weathering crust can be hundreds of microns thick and comprised of secondary mineral phases. Photo by A.J. Barker.

Metal(loid)s

Lead (Pb)

Lead (Pb) and antimony alloy bullet cores are commonly used in small arms ammunition to provide the high density needed to penetrate surfaces. Pb is estimated to comprise approximately 90-99% of the bullet core with steel, copper, and zinc making up the majority of the remaining mass for the slug and casing[1][4]. Pb is also used in the compounds lead styphnate and lead azide as primers for ammunition. As a result, Pb can be found on training ranges in soils/structures where the bullet fragments and also at the firing point[5]. Concentrations of Pb in soils at shooting ranges can vary widely depending on the number of rounds fired at the site, but concentrations often range from 55 to 80,000 mg/kg[1][6][7][8]. The mobility of Pb is highly dependent on local pH. Acidic conditions contribute to the mobilization of Pb species in the environment whereas basic conditions tend to promote precipitation and sorption reactions[9][10]. The formation of secondary mineral phases is a common observation, including hydrocerussite [Pb3(CO3)2(OH)2], cerussite [PbCO3], litharge and massicot [PbO polymorphs], but Pb also binds with sulfur and phosphorus depending on mineralogical conditions at a given site[9][1][11][12]. Additionally, Pb has been known to bind with soil organic matter, clay minerals, and iron/aluminum/manganese - oxides[13][14]. Pb is found in the near-surface environment in one oxidation state, Pb2+. Its toxicity is manifested primarily by effects to the central nervous system and brain function, but it can also cause damage to reproductive processes, auditory function, and vision[15].

Antimony

Antimony (Sb) is a hard, toxic, metalloid that is added to Pb in molten form to make the Pb-alloyed core of a bullet. Sb is thought to behave similarly in the environment to its group VB neighbor Arsenic (As), and, like As, it is toxic and a suspected carcinogen[16]. Sb is added to bullets as a hardening agent because Pb is soft and would not have the same impact capacity without it[2][7]. Bullet cores typically contain no more than 2% Sb and, correspondingly, concentrations in soils are typically lower than for Pb. Sb is also often found in the primers of small arms ammunition as Sb sulfide. Although Sb is present in bullets approximately two orders of magnitude less than Pb, in some aqueous systems Sb has been found to be more mobile than Pb[17][18][19]. However, in more acidic soils Pb often exceeds measured Sb aqueous concentrations due to the high solubility of Pb at low pH values[20]. The mobility of Sb on ranges is highly dependent on its speciation, which in near-surface environments is Sb(III) or Sb(V). The trivalent form has been shown to be less mobile than the pentavalent form, but the trivalent form is considerably more toxic, particularly Sb trioxide[2][21]. Sb(V) is more mobile at higher pHs[22], while Sb(III) in solution is independent of pH[17]. Certain environmental factors influence Sb speciation, including pH, redox potential, biological activity, and co-contamination[23][21][24][19]. Sb oxidation is rapid[24], forming secondary products both in soils and within the weathering crust surrounding bullets[3]. Iron (Fe) is a natural sink for Sb, forming primarily inner sphere sorption complexes (both corner and edge-sharing) with Fe(III) oxides found in soils[2][3][25].

Copper

Copper (Cu) is an essential nutrient for the body and has a relatively high maximum contaminant level (MCL) in drinking water set by the EPA (1.3 mg/kg) compared to other metal(loid)s that comprise bullets. However, toxicity from elevated Cu concentrations can arise and cause gastrointestinal distress (short-term) and liver and/or kidney damage (long-term)[26]. The outer casing and jacket of a bullet is comprised primarily of Cu. When a bullet is fired, the outer Cu casing remains at the firing point along with the primer and propellant residues while the Cu jacket impacts soil and either deforms or fragments. New 5.56-mm and 7.62-mm “green” bullets contain solid Cu cores in place of Pb/Sb alloys. Copper concentrations in shooting range soils typically range from 19 to upwards of 6,000 mg/kg[27][8]. Similar to Pb, Cu mobility is highly dependent on pH[28], and it has been shown to easily complex with organic matter[29][30].

Nickel

Nickel (Ni) toxicity is typically a concern to humans due to its prevalence as an allergen and the ability of Ni to interfere with other essential metals in cellular processes[31][32]. Trace amounts of Ni are found in the casing and jacket of a bullet, used to make the alloy with Cu. Ni is a biologically important micronutrient and an essential trace element for plants, microbes, animals, and humans[33][34][35]. Ni concentrations in environmental systems tend to be conservative and uniformly distributed[36]. In shooting ranges, typical concentrations of Ni are lower compared to the other metal(loid)s like Pb, Sb, Cu, and Zn[37]. Ni tends to be relatively immobile in soil solution under neutral and alkaline conditions, but can become highly mobilized under acidic conditions (similar to the other cationic metals; Pb, Cu, and Zn). Similar to other metal(loid)s, key environmental processes that affect Ni cycling in geomedia include adsorption, (co)precipitation, and complexation[38].

Zinc

Zinc (Zn) is an essential micronutrient for sustained existence[34]. Similar to Ni, trace amounts of Zn are found in the casing and jacket of a bullet, used to make the alloy with Cu. Zn has been found to become mobilized at acidic pHs, similar to other cationic metals (Pb, Cu, and Ni) and often associates with clay minerals, soil organic matter, and the roots and shoots of plants, if available [39][40].

Chromium

Chromium (Cr) in the +3 valence state (Cr(III)) is an essential micronutrient, but has been found to be toxic in large doses. Chromium in the +6 valence state (Cr(VI)) is classified as a human carcinogen[41][42]. Depending on the quality of the source of metals used for the production of bullets, Cr may be found as an impurity in steel[43][44][8]. One study on Cr presence at shooting range soils estimated the Cr content of steel to be up to 27%, which is a much higher estimate than other studies have reported[43]. Typically, Cr concentrations in shooting range soils are reported comparable to background concentrations in local soils, indicating minimal environmental impact from training[8]. Cr is primarily found in the near-surface environment as Cr(III) and Cr(VI). In its hexavalent oxidation state Cr is highly mobile, while Cr(III) is more likely to participate in precipitation and sorption reactions, and therefore mobility in aqueous environments and sediments is limited[45].

Tungsten

Exposure to high tungsten (W) concentrations has been linked to altered thyroid function, heart disease, and potentially childhood leukemia and lung cancer[46]. Historically, W and nylon have been used as an alternative for Pb in bullets[46]. The tungsten-nylon bullets were termed ‘green bullets’, but did not stay in production for longer than a decade as subsequent research eventually revealed W to be both toxic and highly mobile[47]. Concentrations of W in shooting range soils where green bullets have been spent ranged from below detection to greater than 2,000 mg/kg[48]. W has been shown to be relatively mobile in the environment, with its mobility dependent on speciation. However, there is evidence to suggest W mobility decreases as soil ages, as long as the source is limited[49]. W metal is not stable in the environment and instead primarily exists as W(VI) and prefers to form a variety of polyatomic anions (like WO42- and H2W12O406-, etc.) and polyoxometalates in solution[50].

Mercury

A compound containing mercury (Hg) (mercury fulminate, Hg(CNO)2) was previously used as a primer for ammunition for approximately 100 years before it was widely replaced by lead azide, silver azide, or lead styphnate which have greater stability[51][5]. Despite the minimal to negligible use in recent history, there is some evidence to suggest that legacy shooting ranges may have persistent Hg contamination in soil[52][53]. Additionally, marine environments near legacy military training grounds may contain elevated concentrations of Hg from the dumping of ammunition after World War II, including the Baltic Sea[54]. Similar to other primer metal(loid)s, Hg in firing range soils should primarily be located near the firing point instead of traveling with the bullet. An old Swiss shooting range was found to have slightly elevated total Hg concentrations above background values (44-315 μg/kg) with maximum values ranging from 906 to 1,517 μg/kg in soils at the firing point, with concentrations decreasing with increasing distance from the firing point[52]. While the concentrations of Hg found at shooting ranges are significantly lower than concentrations reported for Pb and Sb, it still remains a health and environmental hazard due to its harmful effects and toxicity even at low concentrations[53].

Arsenic

Figure 4. Soil berms with high amounts of soil organic matter and clay minerals as shown in this picture tend to retain concentrations of metal(loid)s as a result of large surface area and active binding sites. Image shows constructed shooting range berms in Delta Junction, Alaska on the Donnelly Training Area. Photo by A.J. Barker.

Arsenic (As) is a highly toxic and carcinogenic metalloid that affects nearly all organ systems[55]. Arsenic has been found at shooting ranges to a much lesser extent than other metal(loid)s, as it is associated with Pb as an impurity. While As is a well-known contaminant in other systems, at shooting ranges it is often not measured or measured at concentrations close to background concentrations in nearby soils[6][56]. Arsenic exhibits multiple oxidation states in the near-surface environment, but is most often found as arsenite (As(III)) and/or arsenate (As(V)). Major environmental factors that affect As mobility are pH, redox condition, and soil type[57].

Management Practices

Range design is an important factor in containing metal(loid)s and preventing their migration into the subsurface. Target berms may be comprised of a natural hillside or slope, or constructed with off-site source material that is shaped into an artificial barrier (as shown in Figure 4). Ensuring berm construction includes a berm face that is steep enough to prevent bullet skipping, but stable enough to minimize soil erosion, can minimize contaminated sediment losses. Siting the berm with respect to hydrologic pathways to waterbodies, aquifers, and potential ecological receptors is crucial. Additionally, it is important to decide what type of soil will comprise the berm or hillside (i.e. sand, organic-rich material, etc.) and determining soil texture and major mineral and element fractions. These soil composition variables will affect the overall speciation and mobility of the target contaminant species in bullets (Pb, Sb, etc.). Bullet catchers (see Additional Resources) are designed to catch bullets and contain bullet-related metal(loid)s for later collection and disposal. These systems are designed for long-term use on range. The bullet-related metal(loid)s are stored internally in a protective liner to minimize soil and water interactions with the fired bullet rounds.

Additionally, soil amendments such as phosphate and lime can alter soil pH to inhibit mobility of some metals (mainly cationic metals like Pb, Cu, Ni, and Zn), and other amendments such as Fe/Mn hydro(oxides) can facilitate sorption reactions with metal(loid)s, such as Sb and As. Although costly, spent bullets and large bullet fragments can also be removed from soils.

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See Also