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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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The heterogeneous distribution of munitions constituents, released as particles from munitions firing and detonations on military training ranges, presents challenges for representative soil sample collection and for defensible decision making. Military range characterization studies and the development of the incremental sampling methodology (ISM) have enabled the development of recommended methods for soil sampling that produce representative and reproducible concentration data for munitions constituents. This article provides a broad overview of recommended soil sampling and processing practices for analysis of munitions constituents on military ranges.  
 
 
 
<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):'''
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'''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. Samuel Beal]]
  
  
'''Key Resource(s)''':
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'''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:Taylor-2011 ERDC-CRREL TR-11-15.pdf| Guidance for Soil Sampling of Energetics and Metals]]<ref name= "Taylor2011">Taylor, S., Jenkins, T.F., Bigl, S., Hewitt, A.D., Walsh, M.E. and Walsh, M.R., 2011. Guidance for Soil Sampling for Energetics and Metals (No. ERDC/CRREL-TR-11-15). [[media:Taylor-2011 ERDC-CRREL TR-11-15.pdf| Report.pdf]]</ref>
*[https://doi.org/10.1111/gwat.12419 doi: 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:Hewitt-2009 ERDC-CRREL TR-09-6.pdf| Report.pdf | Validation of Sampling Protocol and the Promulgation of Method Modifications for the Characterization of Energetic Residues on Military Testing and Training Ranges]]<ref name= "Hewitt2009">Hewitt, A.D., Jenkins, T.F., Walsh, M.E., Bigl, S.R. and Brochu, S., 2009. Validation of sampling protocol and the promulgation of method modifications for the characterization of energetic residues on military testing and training ranges (No. ERDC/CRREL-TR-09-6). Engineer Research and Development Center / Cold Regions Research and Engineering Lab (ERDC/CRREL) TR-09-6, Hanover, NH, USA. [[Media:Hewitt-2009 ERDC-CRREL TR-09-6.pdf | Report.pdf]]</ref>  
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*[[media:Epa-2006-method-8330b.pdf| U.S. EPA SW-846 Method 8330B: Nitroaromatics, Nitramines, and Nitrate Esters by High Performance Liquid Chromatography (HPLC)]]<ref name= "USEPA2006M">U.S. Environmental Protection Agency (USEPA), 2006. Method 8330B (SW-846): Nitroaromatics, Nitramines, and Nitrate Esters by High Performance Liquid Chromatography (HPLC), Rev. 2. Washington, D.C. [[media:Epa-2006-method-8330b.pdf | Report.pdf]]</ref>
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*[[media:Epa-2007-method-8095.pdf | U.S. EPA SW-846 Method 8095: Explosives by Gas Chromatography.]]<ref name= "USEPA2007M">U.S. Environmental Protection Agency (US EPA), 2007. Method 8095 (SW-846): Explosives by Gas Chromatography. Washington, D.C. [[media:Epa-2007-method-8095.pdf| Report.pdf]]</ref>
  
 
==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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[[File:Beal1w2 Fig1.png|thumb|200 px|left|Figure 1: Downrange distance of visible propellant plume on snow from the firing of different munitions. Note deposition behind firing line for the 84-mm rocket. Data from: Walsh et al.<ref>Walsh, M.R., Walsh, M.E., Ampleman, G., Thiboutot, S., Brochu, S. and Jenkins, T.F., 2012. Munitions propellants residue deposition rates on military training ranges. Propellants, Explosives, Pyrotechnics, 37(4), pp.393-406. [http://dx.doi.org/10.1002/prep.201100105 doi: 10.1002/prep.201100105]</ref><ref>Walsh, M.R., Walsh, M.E., Hewitt, A.D., Collins, C.M., Bigl, S.R., Gagnon, K., Ampleman, G., Thiboutot, S., Poulin, I. and Brochu, S., 2010. Characterization and Fate of Gun and Rocket Propellant Residues on Testing and Training Ranges: Interim Report 2. (ERDC/CRREL TR-10-13.  Also: ESTCP Project ER-1481)  [[media:Walsh-2010 ERDC-CRREL TR-11-15 ESTCP ER-1481.pdf| Report]]</ref>]]
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[[File:Beal1w2 Fig2.png|thumb|left|200 px|Figure 2: A low-order detonation mortar round (top) with surrounding discrete soil samples produced concentrations spanning six orders of magnitude within a 10m by 10m area (bottom). (Photo and data: A.D. Hewitt)]]
  
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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Munitions constituents are released on military testing and training ranges through several common mechanisms. Some are locally dispersed as solid particles from incomplete combustion during firing and detonation. Also, small residual particles containing propellant compounds (e.g., [[Wikipedia: Nitroglycerin | nitroglycerin [NG]]] and [[Wikipedia: 2,4-Dinitrotoluene | 2,4-dinitrotoluene [2,4-DNT]]]) are distributed in front of and surrounding target practice firing lines (Figure 1). At impact areas and demolition areas, high order detonations typically yield very small amounts (<1 to 10 mg/round) of residual high explosive compounds (e.g., [[Wikipedia: TNT | TNT ]], [[Wikipedia: RDX | RDX ]] and [[Wikipedia: HMX | HMX ]]) that are distributed up to and sometimes greater than) 24 m from the site of detonation<ref name= "Walsh2017">Walsh, M.R., Temple, T., Bigl, M.F., Tshabalala, S.F., Mai, N. and Ladyman, M., 2017. Investigation of Energetic Particle Distribution from High‐Order Detonations of Munitions. Propellants, Explosives, Pyrotechnics, 42(8), pp.932-941. [https://doi.org/10.1002/prep.201700089 doi: 10.1002/prep.201700089] [[media: Walsh-2017-High-Order-Detonation-Residues-Particle-Distribution-PEP.pdf| Report.pdf]]</ref>.
  
==Site Description==
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Low-order detonations and duds are thought to be the primary source of munitions constituents on ranges<ref>Hewitt, A.D., Jenkins, T.F., Walsh, M.E., Walsh, M.R. and Taylor, S., 2005. RDX and TNT residues from live-fire and blow-in-place detonations. Chemosphere, 61(6), pp.888-894. [https://doi.org/10.1016/j.chemosphere.2005.04.058 doi: 10.1016/j.chemosphere.2005.04.058]</ref><ref>Walsh, M.R., Walsh, M.E., Poulin, I., Taylor, S. and Douglas, T.A., 2011. Energetic residues from the detonation of common US ordnance. International Journal of Energetic Materials and Chemical Propulsion, 10(2). [https://doi.org/10.1615/intjenergeticmaterialschemprop.2012004956 doi: 10.1615/IntJEnergeticMaterialsChemProp.2012004956] [[media:Walsh-2011-Energetic-Residues-Common-US-Ordnance.pdf| Report.pdf]]</ref>. Duds are initially intact but may become perforated or fragmented into micrometer to centimeter;o0i0k-sized particles by nearby detonations<ref>Walsh, M.R., Thiboutot, S., Walsh, M.E., Ampleman, G., Martel, R., Poulin, I. and Taylor, S., 2011. Characterization and fate of gun and rocket propellant residues on testing and training ranges (No. ERDC/CRREL-TR-11-13). Engineer Research and Development Center / Cold Regions Research and Engineering Lab (ERDC/CRREL) TR-11-13, Hanover, NH, USA. [[media:Epa-2006-method-8330b.pdf| Report.pdf]]</ref>. Low-order detonations can scatter micrometer to centimeter-sized particles up to 20 m from the site of detonation<ref name= "Taylor2004">Taylor, S., Hewitt, A., Lever, J., Hayes, C., Perovich, L., Thorne, P. and Daghlian, C., 2004. TNT particle size distributions from detonated 155-mm howitzer rounds. Chemosphere, 55(3), pp.357-367.[[media:Taylor-2004 TNT PSDs.pdf| Report.pdf]]</ref>
[[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  
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The particulate nature of munitions constituents in the environment presents a distinct challenge to representative soil sampling. Figure 2 shows an array of discrete soil samples collected around the site of a low-order detonation – resultant soil concentrations vary by orders of magnitude within centimeters of each other. The inadequacy of discrete sampling is apparent in characterization studies from actual ranges which show wide-ranging concentrations and poor precision (Table 1).
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>.  
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In comparison to discrete sampling, incremental sampling tends to yield reproducible concentrations (low relative standard deviation [RSD]) that statistically better represent an area of interest<ref name= "Hewitt2009"/>.
  
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"/>.
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{| class="wikitable" style="float: right; text-align: center; margin-left: auto; margin-right: auto;"
 
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|+ Table 1. Soil Sample Concentrations and Precision from Military Ranges Using Discrete and Incremental Sampling. (Data from Taylor et al. <ref name= "Taylor2011"/> and references therein.)
[[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>.]]
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|-
==Natural Source Zone Depletion (NSZD)==
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! Military Range Type !! Analyte !! Range<br/>(mg/kg) !! Median<br/>(mg/kg) !! RSD<br/>(%)
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.<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.
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|-
 
+
| colspan="5" style="text-align: left;" | '''Discrete Samples'''
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"/>.
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|-
 
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| Artillery FP || 2,4-DNT || <0.04 – 6.4 || 0.65 || 110
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>.  
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|-
 
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| Antitank Rocket || HMX || 5.8 – 1,200 || 200 || 99
{| class="wikitable" style="text-align:center;"
 
|+ Table 1. Oil Loss Measurements*
 
 
|-
 
|-
! rowspan="2" | Reference
+
| Bombing || TNT || 0.15 – 780 || 6.4 || 274
! 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
+
| Mortar || RDX || <0.04 – 2,400 || 1.7 || 441
| 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
+
| Artillery || RDX || <0.04 – 170 || <0.04 || 454
| 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.
+
| colspan="5" style="text-align: left;" | '''Incremental Samples*'''
 
|-
 
|-
| 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.
+
| Artillery FP || 2,4-DNT || 0.60 – 1.4 || 0.92 || 26
 
|-
 
|-
| 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>.
+
| Bombing || TNT || 13 – 17 || 14 || 17
 
|-
 
|-
| 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.  
+
| Artillery/Bombing || RDX || 3.9 – 9.4 || 4.8 || 38
 +
|-  
 +
| Thermal Treatment || HMX || 3.96 – 4.26 || 4.16 || 4
 
|-
 
|-
| 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>.
+
| colspan="5" style="text-align: left; background-color: white;" | * For incremental samples, 30-100 increments and 3-10 replicate samples were collected.
 
|}
 
|}
  
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).
+
==Incremental Sampling Approach==
 +
ISM is a requisite for representative and reproducible sampling of training ranges, but it is an involved process that is detailed thoroughly elsewhere<ref name= "Hewitt2009"/><ref name= "Taylor2011"/><ref name= "USEPA2006M"/>. In short, ISM involves the collection of many (30 to >100) increments in a systematic pattern within a decision unit (DU). The DU may cover an area where releases are thought to have occurred or may represent an area relevant to ecological receptors (e.g., sensitive species). Figure 3 shows the ISM sampling pattern in a simplified (5x5 square) DU. Increments are collected at a random starting point with systematic distances between increments. Replicate samples can be collected by starting at a different random starting point, often at a different corner of the DU. Practically, this grid pattern can often be followed with flagging or lathe marking DU boundaries and/or sampling lanes and with individual pacing keeping systematic distances between increments. As an example, an artillery firing point might include a 100x100 m DU with 81 increments.
 +
[[File:Beal1w2 Fig3.png|thumb|200 px|left|Figure 3. Example ISM sampling pattern on a square decision unit. Replicates are collected in a systematic pattern from a random starting point at a corner of the DU. Typically more than the 25 increments shown are collected]]
 +
 
 +
DUs can vary in shape (Figure 4), size, number of increments, and number of replicates according to a project’s data quality objectives.
  
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.
+
[[File:Beal1w2 Fig4.png|thumb|right|250 px|Figure 4: Incremental sampling of a circular DU on snow shows sampling lanes with a two-person team in process of collecting the second replicate in a perpendicular path to the first replicate. (Photo: Matthew Bigl)]]
  
Two comprehensive modeling studies have focused on NSZD at the site<ref name= "Molins2010"/>.  Molins et al.<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.<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>.
+
==Sampling Tools==
 +
In many cases, energetic compounds are expected to reside within the soil surface. Figure 5 shows soil depth profiles on some studied impact areas and firing points. Overall, the energetic compound concentrations below 5-cm soil depth are negligible relative to overlying soil concentrations. For conventional munitions, this is to be expected as the energetic particles are relatively insoluble, and any dissolved compounds readily adsorb to most soils<ref>Pennington, J.C., Jenkins, T.F., Ampleman, G., Thiboutot, S., Brannon, J.M., Hewitt, A.D., Lewis, J., Brochu, S., 2006. Distribution and fate of energetics on DoD test and training ranges: Final Report. ERDC TR-06-13, Vicksburg, MS, USA. Also: SERDP/ESTCP Project ER-1155. [[media:Pennington-2006_ERDC-TR-06-13_ESTCP-ER-1155-FR.pdf| Report.pdf]]</ref>. Physical disturbance, as on hand grenade ranges, may require deeper sampling either with a soil profile or a corer/auger.
  
[[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"/>.]]
+
[[File:Beal1w2 Fig5.png|thumb|left|200 px|Figure 5. Depth profiles of high explosive compounds at impact areas (bottom) and of propellant compounds at firing points (top). Data from: Hewitt et al. <ref>Hewitt, A.D., Jenkins, T.F., Ramsey, C.A., Bjella, K.L., Ranney, T.A. and Perron, N.M., 2005. Estimating energetic residue loading on military artillery ranges: Large decision units (No. ERDC/CRREL-TR-05-7). [[media:Hewitt-2005 ERDC-CRREL TR-05-7.pdf| Report.pdf]]</ref> and Jenkins et al. <ref>Jenkins, T.F., Ampleman, G., Thiboutot, S., Bigl, S.R., Taylor, S., Walsh, M.R., Faucher, D., Mantel, R., Poulin, I., Dontsova, K.M. and Walsh, M.E., 2008. Characterization and fate of gun and rocket propellant residues on testing and training ranges (No. ERDC-TR-08-1). [[media:Jenkins-2008 ERDC TR-08-1.pdf| Report.pdf]]</ref>]]
  
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.<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.
+
Soil sampling with the Cold Regions Research and Engineering Laboratory (CRREL) Multi-Increment Sampling Tool (CMIST) or similar device is an easy way to collect ISM samples rapidly and reproducibly. This tool has an adjustable diameter size corer and adjustable depth to collect surface soil plugs (Figure 6). The CMIST can be used at almost a walking pace (Figure 7) using a two-person sampling team, with one person operating the CMIST and the other carrying the sample container and recording the number of increments collected. The CMIST with a small diameter tip works best in soils with low cohesion, otherwise conventional scoops may be used. Maintaining consistent soil increment dimensions is critical.
  
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>.
+
The sampling tool should be cleaned between replicates and between DUs to minimize potential for cross-contamination<ref>Walsh, M.R., 2009. User’s manual for the CRREL Multi-Increment Sampling Tool. Engineer Research and Development Center / Cold Regions Research and Engineering Lab (ERDC/CRREL) SR-09-1, Hanover, NH, USA. [[media:Walsh-2009 ERDC-CRREL SR-09-1.pdf | Report.pdf]]</ref>.
  
==Natural Attenuation of the Groundwater Plume==
+
==Sample Processing==
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.<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.
+
While only 10 g of soil is typically used for chemical analysis, incremental sampling generates a sample weighing on the order of 1 kg. Splitting of a sample, either in the field or laboratory, seems like an easy way to reduce sample mass; however this approach has been found to produce high uncertainty for explosives and propellants, with a median RSD of 43.1%<ref name= "Hewitt2009"/>. Even greater error is associated with removing a discrete sub-sample from an unground sample. Appendix A in [https://www.epa.gov/sites/production/files/2015-07/documents/epa-8330b.pdf U.S. EPA Method 8330B]<ref name= "USEPA2006M"/> provides details on recommended ISM sample processing procedures.
[[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"/>.
+
Incremental soil samples are typically air dried over the course of a few days. Oven drying thermally degrades some energetic compounds and should be avoided<ref>Cragin, J.H., Leggett, D.C., Foley, B.T., and Schumacher, P.W., 1985. TNT, RDX and HMX explosives in soils and sediments: Analysis techniques and drying losses. (CRREL Report 85-15) Hanover, NH, USA. [[media:Cragin-1985 CRREL 85-15.pdf| Report.pdf]]</ref>. Once dry, the samples are sieved with a 2-mm screen, with only the less than 2-mm fraction processed further. This size fraction represents the USDA definition of soil. Aggregate soil particles should be broken up and vegetation shredded to pass through the sieve. Samples from impact or demolition areas may contain explosive particles from low order detonations that are greater than 2 mm and should be identified, given appropriate caution, and potentially weighed.
  
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.
+
The <2-mm soil fraction is typically still ≥1 kg and impractical to extract in full for analysis. However, subsampling at this stage is not possible due to compositional heterogeneity, with the energetic compounds generally present as <0.5 mm particles<ref name= "Walsh2017"/><ref name= "Taylor2004"/>. Particle size reduction is required to achieve a representative and precise measure of the sample concentration. Grinding in a puck mill to a soil particle size <75 µm has been found to be required for representative/reproducible sub-sampling (Figure 8). For samples thought to contain propellant particles, a prolonged milling time is required to break down these polymerized particles and achieve acceptable precision (Figure 9). Due to the multi-use nature of some ranges, a 5-minute puck milling period can be used for all soils. Cooling periods between 1-minute milling intervals are recommended to avoid thermal degradation. Similar to field sampling, sub-sampling is done incrementally by spreading the sample out to a thin layer and collecting systematic random increments of consistent volume to a total mass for extraction of 10 g (Figure 10).
  
 +
<li style="display: inline-block;">[[File:Beal1w2 Fig6.png|thumb|200 px|Figure 6: CMIST soil sampling tool (top) and with ejected increment core using a large diameter tip (bottom).]]</li>
 +
<li style="display: inline-block;">[[File:Beal1w2 Fig7.png|thumb|200 px|Figure 7: Two person sampling team using CMIST, bag-lined bucket, and increment counter. (Photos: Matthew Bigl)]]</li>
 +
<li style="display: inline-block;">[[File:Beal1w2 Fig8.png|thumb|200 px|Figure 8: Effect of machine grinding on RDX and TNT concentration and precision in soil from a hand grenade range. Data from Walsh et al.<ref>Walsh, M.E., Ramsey, C.A. and Jenkins, T.F., 2002. The effect of particle size reduction by grinding on subsampling variance for explosives residues in soil. Chemosphere, 49(10), pp.1267-1273. [https://doi.org/10.1016/S0045-6535(02)00528-3 doi: 10.1016/S0045-6535(02)00528-3]</ref> ]]</li>
 +
<li style="display: inline-block;">[[File:Beal1w2 Fig9.png|thumb|200 px|Figure 9: Effect of puck milling time on 2,4-DNT concentration and precision in soil from a firing point. Data from Walsh et al.<ref>Walsh, M.E., Ramsey, C.A., Collins, C.M., Hewitt, A.D., Walsh, M.R., Bjella, K.L., Lambert, D.J. and Perron, N.M., 2005. Collection methods and laboratory processing of samples from Donnelly Training Area Firing Points, Alaska, 2003 (No. ERDC/CRREL-TR-05-6). [[media:Walsh-2005 ERDC-CRREL TR-05-6.pdf| Report.pdf]]</ref>.]]</li>
 +
<li style="display: inline-block;">[[File:Beal1w2 Fig10.png|thumb|200 px|center|Figure 10: Incremental sub-sampling of a milled soil sample spread out on aluminum foil.]]</li>
 +
 +
==Analysis==
 +
Soil sub-samples are extracted and analyzed following [[Media: epa-2006-method-8330b.pdf | EPA Method 8330B]]<ref name= "USEPA2006M"/> and [[Media:epa-2007-method-8095.pdf | Method 8095]]<ref name= "USEPA2007M"/> using [[Wikipedia: High-performance liquid chromatography | High Performance Liquid Chromatography (HPLC)]] and [[Wikipedia: Gas chromatography | Gas Chromatography (GC)]], respectively. Common estimated reporting limits for these analysis methods are listed in Table 2.
 +
 +
{| class="wikitable" style="float: center; text-align: center; margin-left: auto; margin-right: auto;"
 +
|+ Table 2. Typical Method Reporting Limits for Energetic Compounds in Soil. (Data from Hewitt et al.<ref>Hewitt, A., Bigl, S., Walsh, M., Brochu, S., Bjella, K. and Lambert, D., 2007. Processing of training range soils for the analysis of energetic compounds (No. ERDC/CRREL-TR-07-15). Hanover, NH, USA. [[media:Hewitt-2007 ERDC-CRREL TR-07-15.pdf| Report.pdf]]</ref>)
 +
|-
 +
! rowspan="2" | Compound
 +
! colspan="2" | Soil Reporting Limit (mg/kg)
 +
|-
 +
! HPLC (8330)
 +
! GC (8095)
 +
|-
 +
| HMX || 0.04 || 0.01
 +
|-
 +
| RDX || 0.04 || 0.006
 +
|-
 +
| [[Wikipedia: 1,3,5-Trinitrobenzene | TNB]] || 0.04 || 0.003
 +
|-
 +
| TNT || 0.04 || 0.002
 +
|-
 +
| [[Wikipedia: 2,6-Dinitrotoluene | 2,6-DNT]] || 0.08 || 0.002
 +
|-
 +
| 2,4-DNT || 0.04 || 0.002
 +
|-
 +
| 2-ADNT || 0.08 || 0.002
 +
|-
 +
| 4-ADNT || 0.08 || 0.002
 +
|-
 +
| NG || 0.1 || 0.01
 +
|-
 +
| [[Wikipedia: Dinitrobenzene | DNB ]] || 0.04 || 0.002
 +
|-
 +
| [[Wikipedia: Tetryl | Tetryl ]]  || 0.04 || 0.01
 +
|-
 +
| [[Wikipedia: Pentaerythritol tetranitrate | PETN ]] || 0.2 || 0.016
 +
|}
  
 
==References==
 
==References==
 
+
<references/>
<references />
 
  
 
==See Also==
 
==See Also==
 +
*[https://itrcweb.org/ Interstate Technology and Regulatory Council]
 +
*[http://www.hawaiidoh.org/tgm.aspx Hawaii Department of Health]
 +
*[http://envirostat.org/ Envirostat]

Latest revision as of 18:58, 29 April 2020

The heterogeneous distribution of munitions constituents, released as particles from munitions firing and detonations on military training ranges, presents challenges for representative soil sample collection and for defensible decision making. Military range characterization studies and the development of the incremental sampling methodology (ISM) have enabled the development of recommended methods for soil sampling that produce representative and reproducible concentration data for munitions constituents. This article provides a broad overview of recommended soil sampling and processing practices for analysis of munitions constituents on military ranges.

Related Article(s):


CONTRIBUTOR(S): Dr. Samuel Beal


Key Resource(s):

Introduction

Figure 1: Downrange distance of visible propellant plume on snow from the firing of different munitions. Note deposition behind firing line for the 84-mm rocket. Data from: Walsh et al.[5][6]
Figure 2: A low-order detonation mortar round (top) with surrounding discrete soil samples produced concentrations spanning six orders of magnitude within a 10m by 10m area (bottom). (Photo and data: A.D. Hewitt)

Munitions constituents are released on military testing and training ranges through several common mechanisms. Some are locally dispersed as solid particles from incomplete combustion during firing and detonation. Also, small residual particles containing propellant compounds (e.g., nitroglycerin [NG] and 2,4-dinitrotoluene [2,4-DNT]) are distributed in front of and surrounding target practice firing lines (Figure 1). At impact areas and demolition areas, high order detonations typically yield very small amounts (<1 to 10 mg/round) of residual high explosive compounds (e.g., TNT , RDX and HMX ) that are distributed up to and sometimes greater than) 24 m from the site of detonation[7].

Low-order detonations and duds are thought to be the primary source of munitions constituents on ranges[8][9]. Duds are initially intact but may become perforated or fragmented into micrometer to centimeter;o0i0k-sized particles by nearby detonations[10]. Low-order detonations can scatter micrometer to centimeter-sized particles up to 20 m from the site of detonation[11]

The particulate nature of munitions constituents in the environment presents a distinct challenge to representative soil sampling. Figure 2 shows an array of discrete soil samples collected around the site of a low-order detonation – resultant soil concentrations vary by orders of magnitude within centimeters of each other. The inadequacy of discrete sampling is apparent in characterization studies from actual ranges which show wide-ranging concentrations and poor precision (Table 1).

In comparison to discrete sampling, incremental sampling tends to yield reproducible concentrations (low relative standard deviation [RSD]) that statistically better represent an area of interest[2].

Table 1. Soil Sample Concentrations and Precision from Military Ranges Using Discrete and Incremental Sampling. (Data from Taylor et al. [1] and references therein.)
Military Range Type Analyte Range
(mg/kg)
Median
(mg/kg)
RSD
(%)
Discrete Samples
Artillery FP 2,4-DNT <0.04 – 6.4 0.65 110
Antitank Rocket HMX 5.8 – 1,200 200 99
Bombing TNT 0.15 – 780 6.4 274
Mortar RDX <0.04 – 2,400 1.7 441
Artillery RDX <0.04 – 170 <0.04 454
Incremental Samples*
Artillery FP 2,4-DNT 0.60 – 1.4 0.92 26
Bombing TNT 13 – 17 14 17
Artillery/Bombing RDX 3.9 – 9.4 4.8 38
Thermal Treatment HMX 3.96 – 4.26 4.16 4
* For incremental samples, 30-100 increments and 3-10 replicate samples were collected.

Incremental Sampling Approach

ISM is a requisite for representative and reproducible sampling of training ranges, but it is an involved process that is detailed thoroughly elsewhere[2][1][3]. In short, ISM involves the collection of many (30 to >100) increments in a systematic pattern within a decision unit (DU). The DU may cover an area where releases are thought to have occurred or may represent an area relevant to ecological receptors (e.g., sensitive species). Figure 3 shows the ISM sampling pattern in a simplified (5x5 square) DU. Increments are collected at a random starting point with systematic distances between increments. Replicate samples can be collected by starting at a different random starting point, often at a different corner of the DU. Practically, this grid pattern can often be followed with flagging or lathe marking DU boundaries and/or sampling lanes and with individual pacing keeping systematic distances between increments. As an example, an artillery firing point might include a 100x100 m DU with 81 increments.

Figure 3. Example ISM sampling pattern on a square decision unit. Replicates are collected in a systematic pattern from a random starting point at a corner of the DU. Typically more than the 25 increments shown are collected

DUs can vary in shape (Figure 4), size, number of increments, and number of replicates according to a project’s data quality objectives.

Figure 4: Incremental sampling of a circular DU on snow shows sampling lanes with a two-person team in process of collecting the second replicate in a perpendicular path to the first replicate. (Photo: Matthew Bigl)

Sampling Tools

In many cases, energetic compounds are expected to reside within the soil surface. Figure 5 shows soil depth profiles on some studied impact areas and firing points. Overall, the energetic compound concentrations below 5-cm soil depth are negligible relative to overlying soil concentrations. For conventional munitions, this is to be expected as the energetic particles are relatively insoluble, and any dissolved compounds readily adsorb to most soils[12]. Physical disturbance, as on hand grenade ranges, may require deeper sampling either with a soil profile or a corer/auger.

Figure 5. Depth profiles of high explosive compounds at impact areas (bottom) and of propellant compounds at firing points (top). Data from: Hewitt et al. [13] and Jenkins et al. [14]

Soil sampling with the Cold Regions Research and Engineering Laboratory (CRREL) Multi-Increment Sampling Tool (CMIST) or similar device is an easy way to collect ISM samples rapidly and reproducibly. This tool has an adjustable diameter size corer and adjustable depth to collect surface soil plugs (Figure 6). The CMIST can be used at almost a walking pace (Figure 7) using a two-person sampling team, with one person operating the CMIST and the other carrying the sample container and recording the number of increments collected. The CMIST with a small diameter tip works best in soils with low cohesion, otherwise conventional scoops may be used. Maintaining consistent soil increment dimensions is critical.

The sampling tool should be cleaned between replicates and between DUs to minimize potential for cross-contamination[15].

Sample Processing

While only 10 g of soil is typically used for chemical analysis, incremental sampling generates a sample weighing on the order of 1 kg. Splitting of a sample, either in the field or laboratory, seems like an easy way to reduce sample mass; however this approach has been found to produce high uncertainty for explosives and propellants, with a median RSD of 43.1%[2]. Even greater error is associated with removing a discrete sub-sample from an unground sample. Appendix A in U.S. EPA Method 8330B[3] provides details on recommended ISM sample processing procedures.

Incremental soil samples are typically air dried over the course of a few days. Oven drying thermally degrades some energetic compounds and should be avoided[16]. Once dry, the samples are sieved with a 2-mm screen, with only the less than 2-mm fraction processed further. This size fraction represents the USDA definition of soil. Aggregate soil particles should be broken up and vegetation shredded to pass through the sieve. Samples from impact or demolition areas may contain explosive particles from low order detonations that are greater than 2 mm and should be identified, given appropriate caution, and potentially weighed.

The <2-mm soil fraction is typically still ≥1 kg and impractical to extract in full for analysis. However, subsampling at this stage is not possible due to compositional heterogeneity, with the energetic compounds generally present as <0.5 mm particles[7][11]. Particle size reduction is required to achieve a representative and precise measure of the sample concentration. Grinding in a puck mill to a soil particle size <75 µm has been found to be required for representative/reproducible sub-sampling (Figure 8). For samples thought to contain propellant particles, a prolonged milling time is required to break down these polymerized particles and achieve acceptable precision (Figure 9). Due to the multi-use nature of some ranges, a 5-minute puck milling period can be used for all soils. Cooling periods between 1-minute milling intervals are recommended to avoid thermal degradation. Similar to field sampling, sub-sampling is done incrementally by spreading the sample out to a thin layer and collecting systematic random increments of consistent volume to a total mass for extraction of 10 g (Figure 10).

  • Figure 6: CMIST soil sampling tool (top) and with ejected increment core using a large diameter tip (bottom).
  • Figure 7: Two person sampling team using CMIST, bag-lined bucket, and increment counter. (Photos: Matthew Bigl)
  • Figure 8: Effect of machine grinding on RDX and TNT concentration and precision in soil from a hand grenade range. Data from Walsh et al.[17]
  • Figure 9: Effect of puck milling time on 2,4-DNT concentration and precision in soil from a firing point. Data from Walsh et al.[18].
  • Figure 10: Incremental sub-sampling of a milled soil sample spread out on aluminum foil.
  • Analysis

    Soil sub-samples are extracted and analyzed following EPA Method 8330B[3] and Method 8095[4] using High Performance Liquid Chromatography (HPLC) and Gas Chromatography (GC), respectively. Common estimated reporting limits for these analysis methods are listed in Table 2.

    Table 2. Typical Method Reporting Limits for Energetic Compounds in Soil. (Data from Hewitt et al.[19])
    Compound Soil Reporting Limit (mg/kg)
    HPLC (8330) GC (8095)
    HMX 0.04 0.01
    RDX 0.04 0.006
    TNB 0.04 0.003
    TNT 0.04 0.002
    2,6-DNT 0.08 0.002
    2,4-DNT 0.04 0.002
    2-ADNT 0.08 0.002
    4-ADNT 0.08 0.002
    NG 0.1 0.01
    DNB 0.04 0.002
    Tetryl 0.04 0.01
    PETN 0.2 0.016

    References

    1. ^ 1.0 1.1 1.2 Taylor, S., Jenkins, T.F., Bigl, S., Hewitt, A.D., Walsh, M.E. and Walsh, M.R., 2011. Guidance for Soil Sampling for Energetics and Metals (No. ERDC/CRREL-TR-11-15). Report.pdf
    2. ^ 2.0 2.1 2.2 2.3 Hewitt, A.D., Jenkins, T.F., Walsh, M.E., Bigl, S.R. and Brochu, S., 2009. Validation of sampling protocol and the promulgation of method modifications for the characterization of energetic residues on military testing and training ranges (No. ERDC/CRREL-TR-09-6). Engineer Research and Development Center / Cold Regions Research and Engineering Lab (ERDC/CRREL) TR-09-6, Hanover, NH, USA. Report.pdf
    3. ^ 3.0 3.1 3.2 3.3 U.S. Environmental Protection Agency (USEPA), 2006. Method 8330B (SW-846): Nitroaromatics, Nitramines, and Nitrate Esters by High Performance Liquid Chromatography (HPLC), Rev. 2. Washington, D.C. Report.pdf
    4. ^ 4.0 4.1 U.S. Environmental Protection Agency (US EPA), 2007. Method 8095 (SW-846): Explosives by Gas Chromatography. Washington, D.C. Report.pdf
    5. ^ Walsh, M.R., Walsh, M.E., Ampleman, G., Thiboutot, S., Brochu, S. and Jenkins, T.F., 2012. Munitions propellants residue deposition rates on military training ranges. Propellants, Explosives, Pyrotechnics, 37(4), pp.393-406. doi: 10.1002/prep.201100105
    6. ^ Walsh, M.R., Walsh, M.E., Hewitt, A.D., Collins, C.M., Bigl, S.R., Gagnon, K., Ampleman, G., Thiboutot, S., Poulin, I. and Brochu, S., 2010. Characterization and Fate of Gun and Rocket Propellant Residues on Testing and Training Ranges: Interim Report 2. (ERDC/CRREL TR-10-13. Also: ESTCP Project ER-1481) Report
    7. ^ 7.0 7.1 Walsh, M.R., Temple, T., Bigl, M.F., Tshabalala, S.F., Mai, N. and Ladyman, M., 2017. Investigation of Energetic Particle Distribution from High‐Order Detonations of Munitions. Propellants, Explosives, Pyrotechnics, 42(8), pp.932-941. doi: 10.1002/prep.201700089 Report.pdf
    8. ^ Hewitt, A.D., Jenkins, T.F., Walsh, M.E., Walsh, M.R. and Taylor, S., 2005. RDX and TNT residues from live-fire and blow-in-place detonations. Chemosphere, 61(6), pp.888-894. doi: 10.1016/j.chemosphere.2005.04.058
    9. ^ Walsh, M.R., Walsh, M.E., Poulin, I., Taylor, S. and Douglas, T.A., 2011. Energetic residues from the detonation of common US ordnance. International Journal of Energetic Materials and Chemical Propulsion, 10(2). doi: 10.1615/IntJEnergeticMaterialsChemProp.2012004956 Report.pdf
    10. ^ Walsh, M.R., Thiboutot, S., Walsh, M.E., Ampleman, G., Martel, R., Poulin, I. and Taylor, S., 2011. Characterization and fate of gun and rocket propellant residues on testing and training ranges (No. ERDC/CRREL-TR-11-13). Engineer Research and Development Center / Cold Regions Research and Engineering Lab (ERDC/CRREL) TR-11-13, Hanover, NH, USA. Report.pdf
    11. ^ 11.0 11.1 Taylor, S., Hewitt, A., Lever, J., Hayes, C., Perovich, L., Thorne, P. and Daghlian, C., 2004. TNT particle size distributions from detonated 155-mm howitzer rounds. Chemosphere, 55(3), pp.357-367. Report.pdf
    12. ^ Pennington, J.C., Jenkins, T.F., Ampleman, G., Thiboutot, S., Brannon, J.M., Hewitt, A.D., Lewis, J., Brochu, S., 2006. Distribution and fate of energetics on DoD test and training ranges: Final Report. ERDC TR-06-13, Vicksburg, MS, USA. Also: SERDP/ESTCP Project ER-1155. Report.pdf
    13. ^ Hewitt, A.D., Jenkins, T.F., Ramsey, C.A., Bjella, K.L., Ranney, T.A. and Perron, N.M., 2005. Estimating energetic residue loading on military artillery ranges: Large decision units (No. ERDC/CRREL-TR-05-7). Report.pdf
    14. ^ Jenkins, T.F., Ampleman, G., Thiboutot, S., Bigl, S.R., Taylor, S., Walsh, M.R., Faucher, D., Mantel, R., Poulin, I., Dontsova, K.M. and Walsh, M.E., 2008. Characterization and fate of gun and rocket propellant residues on testing and training ranges (No. ERDC-TR-08-1). Report.pdf
    15. ^ Walsh, M.R., 2009. User’s manual for the CRREL Multi-Increment Sampling Tool. Engineer Research and Development Center / Cold Regions Research and Engineering Lab (ERDC/CRREL) SR-09-1, Hanover, NH, USA. Report.pdf
    16. ^ Cragin, J.H., Leggett, D.C., Foley, B.T., and Schumacher, P.W., 1985. TNT, RDX and HMX explosives in soils and sediments: Analysis techniques and drying losses. (CRREL Report 85-15) Hanover, NH, USA. Report.pdf
    17. ^ Walsh, M.E., Ramsey, C.A. and Jenkins, T.F., 2002. The effect of particle size reduction by grinding on subsampling variance for explosives residues in soil. Chemosphere, 49(10), pp.1267-1273. doi: 10.1016/S0045-6535(02)00528-3
    18. ^ Walsh, M.E., Ramsey, C.A., Collins, C.M., Hewitt, A.D., Walsh, M.R., Bjella, K.L., Lambert, D.J. and Perron, N.M., 2005. Collection methods and laboratory processing of samples from Donnelly Training Area Firing Points, Alaska, 2003 (No. ERDC/CRREL-TR-05-6). Report.pdf
    19. ^ Hewitt, A., Bigl, S., Walsh, M., Brochu, S., Bjella, K. and Lambert, D., 2007. Processing of training range soils for the analysis of energetic compounds (No. ERDC/CRREL-TR-07-15). Hanover, NH, USA. Report.pdf

    See Also