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Many contaminated sites use active remedies such as pump-and-treat or ''in situ'' remediation to clean up impacted groundwater. Natural attenuation processes such as natural degradation or hydrodynamic dispersion also contribute to the cleanup.  As remediation progresses, a point is often reached when the time required to reach the remedial objectives using the active remedy is roughly the same as the time required if the active remedy is shut down, and the continuing remediation of the site is provided by natural attenuation processes alone.  From that point forward, the extra effort and expense of the active remedy provides no benefit over natural attenuation, and it may be appropriate to transition the site to Monitored Natural Attenuation (MNA). This article deals with currently available tools and approaches that can be used to support a decision to transition from active remediation to MNA.
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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)''':  
 
'''Related Article(s)''':  
*[Monitored Natural Attenuation (MNA)]
 
*[Alternative Endpoints]
 
*[Source Zone Modeling]
 
*[Plume Response Modeling]
 
*[REMChlor-MD]
 
  
  
'''CONTRIBUTOR(S):''' [[Dr. John Wilson]]
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'''CONTRIBUTOR(S):''' [[Dr. Samuel Beal]]
  
  
 
'''Key Resource(s)''':  
 
'''Key Resource(s)''':  
*[[media:2002-Newell-Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies.pdf| Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies]]<ref name= "Newell2002">Newell, C.J., Rifai, H.S., Wilson, J.T., Connor, J.A., Aziz, J.A., Suarez, M.P., 2002. Calculation and use of first-order rate constants for monitored natural attenuation studies. 28p. EPA/540/S-02/500. [[media:2002-Newell-Calculation and Use of First-Order Rate Constants for Monitored Natural Attenuation Studies.pdf| Report.pdf]]</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://www.nas.cee.vt.edu/index.php Natural Attenuation Software (NAS) Version 2.3.3]<ref name= "Widdowson2008">Widdowson, M.A., Mendez, E., Chapelle, F.H., Casey, C.C., 2008. Natural Attenuation Software (NAS) Version 2.3.3. Virginia Polytechnic Institute and State University, the United States Geological Survey, and the United States Naval Facilities Engineering Command. </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>  
*[[media:2002-Aziz-Biochlor Natural Attenuation Decision Support System Vs 2.2.pdf| BIOCHLOR Natural Attenuation Support System, Version 2.2]]<ref name= "Aziz2002">Aziz, C.E., Newell, C.J. and Gonzales, J.R., 2002. BIOCHLOR Natural Attenuation Decision Support System Version 2.2 User’s Manual Addendum. Groundwater Services, Inc., Houston, Texas for the Air Force Center for Environmental Excellence.[[media:2002-Aziz-Biochlor Natural Attenuation Decision Support System Vs 2.2.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==
Many active remedies are effective at treating higher concentrations of contaminants, but as the contaminant concentrations decrease, the rate of cleanup may slow until it is not significantly different from the rate of cleanup provided by the natural attenuation processes that occur at the site. The United States Environmental Protection Agency (USEPA 1999)<ref> U.S. Environmental Protection Agency (USEPA), 1999. Use of monitored natural attenuation at superfund, RCRA corrective action, and underground storage tank sites. [[media:1999 USEPA- Use of monitored natural attenuation at superfund.pdf| Report.pdf]]</ref> allows the use of [[Monitored Natural Attenuation (MNA) | monitored natural attenuation (MNA)]] to attain the cleanup goals when the site-specific remediation objectives can be attained within a time frame that is reasonable compared to that offered by other more active methods.  Many CERCLA<ref>U.S. Environmental Protection Agency (USEAP), 2019a. Summary of the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund)</ref> and RCRA<ref>U.S. Environmental Protection Agency (USEPA), 2019b. Resource Conservation and Recovery Act (RCRA) Laws and Regulations</ref> sites take advantage of this policy. An active remedy is typically used initially to treat high concentrations of contaminants followed by MNA to treat the lower concentrations that remain.
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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)]]
  
Unfortunately, there is no well-established approach to determine when it is appropriate to discontinue the active remedy. This article reviews available tools and approaches to evaluate a transition to MNA.  
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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>.
The tools and approaches depend on calculations of rate constants for natural attenuation with distance in flowing groundwater or rate constants for attenuation over time in individual monitoring wells. The next section provides some background on first-order constants and how they are extracted from monitoring data.
 
  
==Background on Rate Constants ==
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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>
  
A general formula to describe the rate of a chemical reaction is
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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).
                   
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:{|
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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"/>.
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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.)
 
|-
 
|-
| '''Equation 1:''' || || <big>''r = k [C]</big><sup> m</sup>''
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! Military Range Type !! Analyte !! Range<br/>(mg/kg) !! Median<br/>(mg/kg) !! RSD<br/>(%)
 
|-
 
|-
|where:
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| colspan="5" style="text-align: left;" | '''Discrete Samples'''
 
|-
 
|-
| || ''r''&nbsp;&nbsp;&nbsp;&nbsp;|| is the rate of the reaction,
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| Artillery FP || 2,4-DNT || <0.04 – 6.4 || 0.65 || 110
 
|-
 
|-
| || ''k''|| is the rate constant,
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| Antitank Rocket || HMX || 5.8 – 1,200 || 200 || 99
 
|-
 
|-
| || ''C''|| is the concentration of the chemical undergoing the reaction, and
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| Bombing || TNT || 0.15 – 780 || 6.4 || 274
 
|-
 
|-
|the exponent&nbsp;&nbsp;&nbsp;&nbsp;|| ''m''|| is the order of the reaction.
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| Mortar || RDX || <0.04 – 2,400 || 1.7 || 441
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|-
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| Artillery || RDX || <0.04 – 170 || <0.04 || 454
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|-
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| colspan="5" style="text-align: left;" | '''Incremental Samples*'''
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|-
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| Artillery FP || 2,4-DNT || 0.60 – 1.4 || 0.92 || 26
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|-
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| Bombing || TNT || 13 – 17 || 14 || 17
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|-
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| Artillery/Bombing || RDX || 3.9 – 9.4 || 4.8 || 38
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|-
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| Thermal Treatment || HMX || 3.96 – 4.26 || 4.16 || 4
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|-
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| colspan="5" style="text-align: left; background-color: white;" | * For incremental samples, 30-100 increments and 3-10 replicate samples were collected.
 
|}
 
|}
  
When the rate of the reaction is proportional to the concentration of the contaminant, the value of ''m'' is 1. Therefore, the reaction is described as a first-order reaction, and the rate constant is described as a first-order rate constant. In Equation 1, concentration could go up or down, but ''k'' is a constant of proportionality for the rate of increase in concentration. The rate constant for attenuation is the negative of ''k''. If the rate of degradation is a fixed value regardless of concentration, the value of ''m'' is 0, and degradation is a zero-order process.    
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==Incremental Sampling Approach==
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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.
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[[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]]
  
Natural attenuation of concentrations over time in monitoring wells is frequently described by a first-order rate constant, and natural biological or abiotic degradation of contaminants in flowing groundwater is typically also described by a first-order rate constant. Figure 1 provides an example of monitoring data that is described by a first-order rate constant.
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DUs can vary in shape (Figure 4), size, number of increments, and number of replicates according to a project’s data quality objectives.
  
[[File:Wilson1w2Fig1.png|thumb|Figure 1.  Attenuation of Trichloroethene (TCE) over time in a monitoring well at a site in Michigan.  The concentration vs. time rate constant is 0.326 per year and largely represents the rate of the attenuation of the source of contaminants in the aquifer.]]
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[[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)]]
  
The rate constant for attenuation over time in a well and the rate constant for attenuation with distance along a flow path in an aquifer describe different situations that are controlled by different processes. ''Attenuation over time'' in a well is largely controlled by the rate of attenuation of the source of contamination in the aquifer. ''Attenuation with distance'' along a flow path includes attenuation of concentrations in the source along with contributions from biological degradation processes, abiotic degradation processes and hydrodynamic dispersion of the contaminated groundwater into clean groundwater<ref name= "Newell2002"/>.
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==Sampling Tools==
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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.
  
The first-order rate constant for attenuation over time in a well is commonly referred to as ''k''<sub>point</sub><ref name= "Newell2002"/>. A chart in Microsoft EXCEL of concentration on date of sampling can be used to extract a value for ''k''<sub>point</sub>. Select the data, and then insert an exponential trend line and display the equation on the chart. The value of ''k''<sub>point</sub> can also be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak. Note that the rate constants extracted in EXCEL are constants for the rate of change, not the rate of attenuation. Take the negative of the rate of change to get ''k''<sub>point</sub>. In the example in Figure 1, the units of time on the X axis is years, and the value of ''k''<sub>point</sub> is 0.326 per year. 
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[[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>]]
  
Attenuation versus distance rate coefficients describe a bulk attenuation rate of both degradation and dispersion processes.  To extract values for rate constants for degradation alone, it is necessary to calibrate a groundwater flow and transport model to the data at the site. The model is calibrated with values for the hydrogeological properties of the aquifer (effective porosity, hydraulic gradient, hydraulic conductivity, hydrodynamic dispersion and the organic carbon content of the aquifer matrix). After the hydrogeological properties of the aquifer are fixed in the model, the most appropriate values for the degradation rate constants are the values that produce the best fit between the contaminant concentrations that are predicted by the model and the contaminant monitoring data at the site.
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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.
  
There are a number of reasons why natural attenuation processes are better described as first-order relationship instead of zero-order or some other order.  The attenuation over time in a monitoring well tracks the attenuation over time of the source of contamination that sustains the plume<ref name= "Newell2002"/>.  Sites go through a lifecycle, and attenuation of sources at mature sites is often a first-order process<ref>Sale, T., Newell, C., Stroo, H., Hinchee, R. and Johnson, P., 2008. Frequently asked questions regarding management of chlorinated solvents in soils and groundwater. Environmental Security Technology Certification Program Office (DoD), Arlington, VA (ER-200530). [[media: 2008-Sale-Frequently Asked Questions Regarding Management of Chlorinated Solvent in Soils and Groundwater.pdf| Report.pdf]]</ref>.  If a chlorinated solvent site is mature, the contamination in the source area that was originally present as nonaqueous phase liquids (NAPL) has been redistributed and is now sequestered in a sorbed phase to aquifer solids or has diffused into non-transmissive portions of the aquifer matrix .  Transfer of contaminants back into the more transmissive portions of the aquifer occurs by diffusion along a fixed path length, and the rate of transfer is controlled by the concentration of the contaminant remaining in the source material.  Because the rate of transfer is proportional to the concentration of contaminant in the source material, attenuation of the source is a first-order process.  These processes are discussed in more detail in [[Source Zone Modeling]].
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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>.
  
Degradation process are also usually first-order. Abiotic reactions are almost always first-order.   Biodegradation reactions are zero-order at high concentrations because the enzymes are saturated with substrate but are first-order at lower concentrations that are typical of natural attenuation conditions in groundwater.  
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==Sample Processing==
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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.
  
==Goals for MNA at Sites==
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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 information necessary to evaluate whether a site can be transitioned to MNA depends on the goal for MNA at the site. For many cleanup actions, the goal is to confine contamination within a waste management area where the contamination is left in place, in which case the cleanup goal applies to point-of-compliance wells that are outside the waste management area. For other cleanup actions, the entire site must be cleaned up, in which case the cleanup goal applies to any monitoring well on the site. The time by which the goal is to be attained is specified at CERCLA sites in the Record of Decision (the ROD). At RCRA sites, the time allowed for the cleanup to be attained may be specified in the permit.
 
  
==When the Goal Applies to Point-of-Compliance Wells==
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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).
  
Consider the following framework for evaluating a transition to MNA:
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<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>
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<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>
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<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>
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<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>
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<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>
  
#Use a computer model to extract rate constants for the natural degradation of the contaminant that occurred in groundwater at the site before the active remedy was installed.
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==Analysis==
#Assume that the same rate constants will apply after the active remedy is no longer in operation.  Note that this assumption may not be valid if the active remedy changes the geochemistry of the aquifer in the flow path to the point-of-compliance well.  
+
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.
#Calibrate a computer groundwater flow and transport model with the hydrogeological properties of the aquifer that pertain after the active remedy is no longer in operation, the concentration of contaminant after the active remedy, and the rate constants for natural degradation that are assumed to apply after the active remedy.
 
#Use the computer model to project the concentrations of the contaminant at the point-of-compliance well over time.
 
#If the concentrations at the point-of-compliance wells are predicted to be less than the goal before the specified date, that is a quantitative line of evidence in support of a transition to MNA.
 
  
There are two computer applications that are particularly useful to extract rate constants at a site from monitoring data that were collected before the active remedy was installed.  Natural Attenuation Software (NAS)<ref name= "Widdowson2008"/> and BIOCHLOR<ref name= "Aziz2002"/> can both be downloaded from the internet at no cost. 
+
{| 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>)
In NAS, the user inputs the hydrogeological data, the distance of wells along the flow path, and the concentrations of contaminants in the wells.  The NAS application extracts rate constants and makes projections at the point-of-compliance.  With NAS, it is possible to extract different rate constants for specific geochemical environments along the flow path.
+
|-
 
+
! rowspan="2" | Compound
[[File:Wilson1w2Fig2.png|thumb|left|Figure 2. Example calibration of NAS to natural attenuation of total BTEX at a site (Figure 17 of NAS User’s Manual).]]
+
! colspan="2" | Soil Reporting Limit (mg/kg)
[[File:Wilson1w2Fig3.png|thumb|left|Figure 3.  The data input screen for BIOCHLOR before remediation with cis-1,2-Dichloroethene (DCE) and vinyl chloride (VC) source concentrations of 500 and 87 mg/L respectively at the source when the release first occurred.]]
+
|-
[[File:Wilson1w2Fig4.png|thumb|left|Figure 4. Output of the RUN CENTERLINE simulation in BIOCHLOR comparing the fit between the simulation and the field data for vinyl chloride before an active remedy was implemented]]
+
! HPLC (8330)
[[File:Wilson1w2Fig5.png|thumb|Figure 5. Output of the RUN CENTERLINE simulation of conditions after an active remedy was implemented with a source concentration of 1.1 mg/L, projecting the concentration of vinyl chloride at a distance corresponding to a point-of-compliance well.]]
+
! GC (8095)
Figure 2 provides an example calibration of NAS. The concentrations in the monitoring wells used to calibrate the model are compared to the simulation provided by the model. The values of the rate constants that are extracted from the field data are available in the “Output” tab under “Data and Results Table.
+
|-
 
+
| HMX || 0.04 || 0.01
Figure 3 depicts the input screen for BIOCHLOR. The user inputs the hydrogeological parameters, the first-order rate constants (1st Order Decay Coefficient), the distribution of the wells along the flow path, and the concentrations of contaminants in the wells. The model is set up for conditions that apply before the installation of the active remedy.
+
|-
 
+
| RDX || 0.04 || 0.006
BIOCHLOR does not automatically fit the rate constants to the field data. Instead, the user examines the output of the model, and adjusts the rate constants until they provide the best fit between the model prediction and the monitoring data for wells at the site. This comparison is illustrated in Figure 4.  
+
|-
 
+
| [[Wikipedia: 1,3,5-Trinitrobenzene | TNB]] || 0.04 || 0.003
If the distance from the source well to the point-of-compliance well is set as the “Modeled Area Length” in Section 5 of the input screen, the “Run Centerline” output will provide the projected concentrations at that length. Assume the distance from the source well to the point-of-compliance well is 250 feet.  The projected concentration in Figure 3 of vinyl chloride at a point-of-compliance well is 0.042 mg/L.  If the regulatory goal were the federal drinking water MCL<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> of 0.002 mg/L, the projected concentration would exceed the goal, and MNA would not be adequate as a remedy.
+
|-
 
+
| TNT || 0.04 || 0.002
For the sake of illustration, assume that an active remedy has been implemented, and the concentrations in the source well are 5.4 mg/L for DCE and 1.1 mg/L for vinyl chloride.  To evaluate whether it is now appropriate to transition to MNA, BIOCHLOR could be calibrated with these concentrations to predict concentrations in the point-of-compliance well.  (See Figure 5.) In this example, the projected concentration at the point-of-compliance well does meet the goal.
+
|-
 
+
| [[Wikipedia: 2,6-Dinitrotoluene | 2,6-DNT]] || 0.08 || 0.002
Some active remedies are subject to rebound. If this is the case, the evaluation should begin at the point in time when it is clear that the trend in concentrations is downward.
+
|-
 
+
| 2,4-DNT || 0.04 || 0.002
==When the Goal Applies to All the Wells==
 
 
 
Each well at the site is evaluated independently, and the rate constant that is applicable is the rate constant for attenuation over time in the well (''k''<sub>point</sub>).  To evaluate whether the region in an aquifer that is sampled by a particular monitoring well is ready to transition to MNA, it is necessary to have monitoring data from a period of time before the remedy was implemented. This data is used to extract a value for ''k''<sub>point</sub> in the aquifer under natural attenuation conditions. The evaluation of a transition to MNA will assume that the same value for ''k''<sub>point</sub> will apply after the active remedy is complete. This assumption may not be appropriate if the active remedy caused a permanent change in the geochemistry of the aquifer. The assumption is usually appropriate for pump-and-treat remedies. 
 
 
 
If ''k''<sub>point</sub> before implementation of the active remedy describes the time course of natural attenuation after the active remedy is completed, the time required to attain the cleanup goal is predicted from the following:
 
:{|
 
| || ||rowspan="2"| <big>''ln (''</big>
 
| style="border-style:solid; border-width: 0px 0px 1px 0px" | ''<small>C<sub>goal</sub></small>''
 
|rowspan="2"| <big>'')''</big> || &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
 
 
|-
 
|-
|'''Equation 2:'''&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;||''<big>t =''</big>&nbsp;&nbsp;&nbsp;&nbsp;
+
| 2-ADNT || 0.08 || 0.002
| style="vertical-align:top;"|''<small>C<sub>current</sub></small>'' ||
 
 
|-
 
|-
| || || colspan="3" style="text-align:center; border-style:solid; border-width: 1px 0 0 0"| ''<big>-k</big><sub>point</sub>''||
+
| 4-ADNT || 0.08 || 0.002
 
|-
 
|-
|where:
+
| NG || 0.1 || 0.01
 
|-
 
|-
| ||''C<sub>current</sub>''&nbsp;&nbsp;&nbsp;&nbsp;||colspan="5"| is the current concentration after active remediation,
+
| [[Wikipedia: Dinitrobenzene | DNB ]] || 0.04 || 0.002
 
|-
 
|-
| ||''C<sub>goal</sub>''|| colspan="5"| is the cleanup goal, and
+
| [[Wikipedia: Tetryl | Tetryl ]]  || 0.04 || 0.01
 
|-
 
|-
| ||''t''|| colspan="5"| is the time required for concentrations to attenuate from ''C<sub>current</sub>'' to ''C<sub>goal</sub>.'' 
+
| [[Wikipedia: Pentaerythritol tetranitrate | PETN ]] || 0.2 || 0.016
|}                     
+
|}
 
 
If the value of ''t'' estimated using Equation 2 is less than the difference between the current date and the date specified by the site stakeholders to attain the goal, that is evidence in support of a transition to MNA. 
 
 
 
Some active remedies are subject to contaminant concentration rebound.  If this is the case, the evaluation should use a value of ''C''<sub>current</sub> that is attained after the rebound has stabilized. 
 
 
 
This approach depends on a robust value for ''k''<sub>point</sub>.  It is worthwhile to do a sensitivity analysis on ''k''<sub>point</sub> where the lower 95% or 90% confidence interval on ''k''<sub>point</sub> is used in Equation 2 to see if that changes the outcome of the evaluation.  The confidence intervals can be calculated in EXCEL using the Regression Analysis Tool in the Data Analysis Toolpak.  Wilson (2011)<ref name= "Wilson2011">Wilson, J.T. 2011.  An Approach for Evaluating the Progress of Natural Attenuation in Groundwater.  EPA 600-R-11-204. [[media:Wilson-2011-An Approach for Evaluating Progress.pdf| Report.pdf]]</ref> provides detailed discussion of the use of linear regression to extract ''k''<sub>point</sub> and confidence intervals on ''k''<sub>point</sub>.  Wilson (2011)<ref name= "Wilson2011"/>  also discusses the use of goodness-of-fit tests to determine if there is evidence that a first-order rate equation is not the best fit to the monitoring data, and as a result the use of Equation 2 would not be appropriate.   
 
 
 
At many sites, there is no specified date when the cleanup goal must be attained.  In this situation, the monitoring data can be evaluated to determine if the current rate of attenuation under the active remedy is faster than the rate of natural attenuation before the active remedy was installed.  The monitoring data can be examined to identify a time interval when the benefit of the active remedy has approached an asymptote.  A second value of for ''k''<sub>point</sub> can be extracted for that time interval.  The two values for ''k''<sub>point</sub> can be evaluated statistically to see if the current rate is faster at some appropriate level of confidence.  If there is no statistical evidence that the rate of attenuation is faster, that determination can support a decision to transition to MNA.
 
 
 
==Extent of Treatment Necessary to Transition to MNA==
 
[[File:Wilson1w2Fig6.png|thumb|Figure 6. Example calibration of NAS to predict the reduced concentration at the source that is necessary to meet the remediation goal at a point-of-compliance well (Figure 19 of NAS User’s Manual).]]
 
There are several computer applications that can predict the extent of treatment that must be achieved by the active remedy before it is worthwhile to evaluate the site for transition to MNA. Based on the distribution of contamination along the flow path, the NAS application will automatically predict a reduced concentration at the source well that will bring concentrations to the goal in the point-of-compliance well (Figure 6).  A table that opens under the “DOS/TOS” tab provides the “Time of Equilibration” required to meet the goal at the reduced concentration.
 
 
 
Modules in NAS allow the user to evaluate the effect of various pump-and-treat and source removal scenarios on the time required to attain the goal at the point-of-compliance well. 
 
 
 
The REMChlor-MD<ref name= "Falta2018">Falta, R.W., Farhat, S.K., Newell, C.J. and Lynch, K., 2018. A Practical Approach for Modeling Matrix Diffusion Effects inREMChlor. ER-201426 [[media:2018-Falta-REMChlor Modeling Matrix Diffusion Effects.pdf| Report.pdf]]</ref> and REMFuel<ref name= "Falta2012">Falta, R.W., Ahsanuzzaman, A.N., Stacy, M.B., Earle, R.C. and Wilson, J.T., 2012. Remediation Evaluation Model for Fuel hydrocarbons (REMFuel). Users manual version 1.0. U.S. Environmental Protection Agency. EPA/600/R-12/028. [[media:2012-Falta-REMFuel Remediation Evaluation-Model for Fuel hydrocarbons users manual.PDF| Report.pdf]]</ref> models are flexible screening tools that allow a simultaneous evaluation of the extent of treatment provided by (1) source removal, (2) ''in situ'' remediation of the contaminated groundwater, or (3) natural attenuation processes in three discrete intervals along the flow path and three discrete time periods.  Both REMChlor-MD<ref name= "Falta2018"/> and REMFuel<ref name= "Falta2012"/> can be downloaded from the internet at no cost.  Liang et al. (2012)<ref>Liang, H., Falta, R.W., Newell, C.J., Farhat, S.K., Rao, P.S. and Basu, N., 2010. Decision & Management Tools for DNAPL Sites: Optimization of Chlorinated Solvent Source and Plume Remediation Considering Uncertainty. ER-200704 [[media:2010-Liang-Decision and Management Tools for DNAPL sites-ER-200704-FR.pdf| Report.pdf]]</ref> provide a modeling program that uses Monte Carlo simulations to evaluate the effects of the uncertainties in the modeling parameters on the predictions of REMChlor-MD.
 
 
 
==Summary==
 
 
 
Tools and approaches are available that can be adapted to determine when a site is ready to transition from active remedy to MNA.  However, these tools and approaches have not been applied for this purpose at a significant number of sites, and at the present time, they are not generally accepted by the regulatory authorities.  There is an opportunity to expand the state-of-practice for MNA and create a logical and consistent framework that can be used to evaluate sites for transition from active remedy to MNA.
 
  
 
==References==
 
==References==
Line 140: Line 127:
  
 
==See Also==
 
==See Also==
*[http://dx.doi.org/10.1007/978-1-4614-6922-3  Newell, C.J., Kueper, B.H., Wilson, J.T., Johnson, P.C., 2014. Natural Attenuation of Chlorinated Solvent Source Zones. In: Chlorinated Solvent Source Zone Remediation, Editors: Kueper, B.H., Stroo, H.F., Vogel, C.M., Ward. SERDP ESTCP Environmental Remediation Technology, vol 7. Springer, New York, NY. pgs. 459-508. doi: 10.1007/978-1-4614-6922-3]
+
*[https://itrcweb.org/ Interstate Technology and Regulatory Council]
*[https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Monitoring/ER-200436/ER-200436/(language)/eng-US Kram, Mark, and Widdowson, Mark, 2008. Estimating Cleanup Times Associated with Combining Source-Area Remediation with Monitored Natural Attenuation. ESTCP ER-200436]
+
*[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