MNA PERFORMANCE EVALUATION FOR CONTAMINATED SITE REMEDIATION: FATE &TRANSPORT MODELING BASED APPROACH

Transcription

1 Congrès annuel de la Société canadienne de génie civil Annual Conference of the Canadian Society for Civil Engineering Montréal, Québec, Canada 5- juin / June 5-, MNA PERFORMANCE EVALUATION FOR CONTAMINATED SITE REMEDIATION: FATE &TRANSPORT MODELING BASED APPROACH Faisal I Khan, Tahir Husain Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St John s, NF, A1B 3X5, Canada ABSTRACT: Natural attenuation is a cost effective, feasible, and acceptable option for contaminated site remediation as long as the reduction of contaminants to an acceptable level takes place within a reasonable timeframe without causing any detrimental effects to human health and to the local ecological systems. The natural attenuating processes include biodegradation, dispersion, dilution, sorption, volatilization, and chemical or biological stabilization of contaminants. A natural attenuation site may include any or all the above-mentioned processes. To comply with the environmental regulations and enforcement, it is required to study, systematically and scientifically, the extent of contamination, associated risk, and effectiveness of natural attenuation. It is a tricky problem, as on release contaminants distribute themselves among various media (soil, water, and soil vapor) and can exist for long in different phases (e.g. aqueous, non-aqueous and vapor phase). Analytical and numerical models are used to study the fate and transport of contaminants in different medium and also evaluation of natural attenuation. These authors have recently developed a systematic methodology entitled, risk based monitored natural attenuation for site evaluation for natural attenuation. The methodology comprises of seven steps that involve multimedia fate and transport modeling of contaminant, and risk based evaluation. This paper aims to present outline of the methodology and to discuss in detail the application of multimedia fate and transport models in natural attenuation evaluation. 1. INTRODUCTION Monitored Natural Attenuation (MNA) is the reliance on natural attenuation processes, within the context of carefully controlled and monitored site cleanup, to achieve site-specific remedial objectives within time frame that is reasonable compared to that offered by more active methods (DOE 1999). Site characterization, reactive flow-and-transport modeling, and long-term monitoring are required to evaluate natural attenuation (NA) effectiveness. Site characterization determines the extent of contamination, which is subsequently used in a reactive transport model to predict the fate of the contaminants and their transport to receptors. NA (biotic and abiotic) processes occur at most sites, but to varying degrees of effectiveness depending on the type and concentration of the contaminants and physical, chemical, and biological characteristics of the soil and the groundwater. NA processes reduce the potential risk posed by site contaminants in three ways: i) the contaminant is converted to a less toxic form through the destructive processes of biodegradation or abiotic transformations, ii) potential exposure levels are reduced by lowering the contaminant concentration, and iii) contaminant mobility and bioavailability are reduced by sorption to the soil or rock matrix (DOE 1999; Husain et al. ). 1

2 The increased reliance on NA is not only because of high costs of cleanup technologies, but also due to their limited effectiveness. Studies by US Environmental Protection Agency (EPA) and US National Research Council (NRC) indicated that conventional technologies did not reach the prescribed regulatory cleanup standards at many sites. In 199, the NRC concluded that conventional technologies had restored contaminated groundwater to regulatory standards at eight out of the 77 sites evaluated in previous studies (ASTM 1995). The use of NA in the Superfund program during the 199s climbed from % to more than 5% of contaminated sites and further increased in the underground storage tank program. Since 1997, NA is the leading remedy for conventional sites contaminated by leaking underground storage tanks (ASTM 1995). The recent risk-based remediation approach seeks to calibrate cleanup efforts to reflect human and environmental health risks. Office of Environmental Management published a decision-making framework guide for the evaluation and selection of MNA remedies at Department of Energy sites (DOE 1999). This approach centers on risk estimation and its management. Recently these authors have developed Risk based Monitored Natural Attenuation (RBMNA) methodology for MNA (Khan and Husain 1a; 1b). The latest version of methodology and its application are detailed at Khan and Husain (). This paper briefly recapitulates the methodology and demonstrates its application.. METHODOLOGY FOR RISK BASED MONITORED NATURAL ATTENUATION (RBMNA) EVALUATION 1.1 Module 1: Risk-based Screening Step1.1: Review of available data and risk characterization. Available site data is used to compute the risk to on-site receptors (Tier 1). If the computed risk is higher than the threshold, either engineering remediation or further analysis is required (Tiers and 3). If not, the available data is evaluated to support NA as the remedial option. Step 1.: Development of a conceptual model. The development of the conceptual model required for the integration of contaminant history, assessment of biogeochemical attenuation, site hydrology data, exposure duration and pathways. It includes, source characterization, plume perimeter assessment, and identification of attenuation mechanism, (e.g., sorption, precipitation, volatilization, biodegradation). Step 1.3: Refining the conceptual model. The conceptual model developed in step 1. is verified and modified using information collected from various monitoring locations, historical data of the site, and past studies of similar sites. Wiedemeier et al. (1999) suggest the use of contaminant contour maps, electron acceptor maps, metabolic by-product maps, and alkalinity maps for such purposes. After this verification, the conceptual site model is ready, and the study enters the prediction mode. 1. Module : Quantitative Evaluation of MNA Step.1: Fate-and-transport modeling of contaminant. This step, the most important of RBMNA methodology, model contaminant transport in soil and groundwater, and its attenuation by dilution, transformation, degradation, and adsorption. The contaminated zone is divided into an unsaturated (vadose) and a saturated zone. Modeling and simulation are conducted for each zone independently and the results subsequently used to model NA in the region. Vadose Zone Modeling This step models migration of the contaminant in the vadose zone through leaching, evapotranspiration, and ion-exchange. Many computer-based models are available to conduct this step, among them SESOIL is the one suggested by these authors considering its i) simplicity, ii) precision, iii) flexibility to model various release and dispersion scenarios, and iv) ability to model fate and transport of non-aqueous phase liquid (NAPLs). SESOIL, an acronym for SEasonal Soil compartment model, is a one-dimensional vertical fate-andtransport model for unsaturated soil zones. It was developed for EPA s Office of Water and the Office of

3 Toxic Substances in 191 by Arthur D Little, Inc. (Bonazountas and Wagner 19). SESOIL was updated in 19 to include a fourth soil compartment (original model included up to three layers) and soil erosion algorithms. SESOIL is based on mass balance and partition of the contaminant into dissolved, sorbed, vapor and pure phases. SESOIL describes: i) hydrologic cycle of the unsaturated soil zone, ii) pollutant concentrations and masses in water, soil, and air phases, iii) pollutant migration to the groundwater, iv) pollutant volatilization at the ground surface, and v) pollutant transport in the washload due to surface runoff and erosion at the ground surface (ESC ). SESOIL does the computation on monthly basis for up to 999 years of simulation time. In SESOIL, soil column may be composed of up to four layers, each with different properties which affect the pollutant fate. Each soil layer may also be subdivided into a maximum of sublayers to provide an enhanced resolution of pollutant fate and migration in the column. Saturated Zone Modeling Among the many computer-based models available to perform this saturated zone modeling AT13D (AT13D 1997) is the one preferred by these authors considering its analytical capability, simplicity, and preciseness. AT13D, an acronym for Analytical Transient 1-, and 3 Dimensional simulation of waste transport in aquifer systems is a generalized three-dimensional groundwater model developed by G.T. Yeh at Oak Ridge National Laboratory. John Seymor in 19, Darry Holman in 19, Howard Trussell in 19, and Robert A. Schneiker in 1997 made subsequent modifications. This model estimates the concentration of contaminants transported, dispersed, degraded and sorbed in groundwater flow. The transport mechanisms simulated by AT13D include advection, dispersion, sorption, decay/degradation, and losses to the atmosphere. The model can estimate how far a contaminant plume will migrate, and can be compared to groundwater standards for the evaluation of risk at specific locations and times. Contaminant transport in AT13D can be modeled using either (i) contaminant transport without decay, or (ii) contaminant transport with biodegradation as a first-order decay process. AT13D can run a total of about 5 scenarios (run options) with varying combinations of three-contaminant types, eight-source configurations, three-source release types, and four types of aquifer dimensions defining these run options (AT13D 1997). Step.: Modeling of Natural Attenuation. NA modeling predicts the migration and degradation of the dissolved contaminant plume. Models predict contaminant concentrations in a receptor well, and estimate the time required for contaminants to reach a potential receptor. NA modeling involves: i) a conceptual model as an idealized representation of the natural system, ii) a mathematical model representing controlling mechanisms in mathematical terms, iii) solution of the mathematical model, iv) calibration of the solution by adjusting the computed to the observed response of the natural system, v) validation of the accuracy of the model s prediction, and vi) simulation based on the calibrated solution of the conceptual model. Among the available computer-based tools, BIOPLUME II or III (Rifai et al. 1997; ) is good in modeling NA of fuel hydrocarbons contaminated sites. MNA tool Box (SNL 1997) and BIOSCREEN (Newell et al. 1997) also study NA. Selection of appropriate model is dependent upon the availability of site-specific data and scope of the study. Step.3: Exposure and Quantitative Risk Assessment. This step analyses the most likely pathways for contaminant exposure and computes the associated risk for current and reasonable future scenarios. To compute risk, it uses contaminant concentration computed by detailed fate-and-transport model. Subsequently, remediation options are selected according to the severity of the risk; if the risk is not severe, the feasibility of MNA as a remediation option is explored. If NA is demonstrably effective, a longterm monitoring plan is developed. 1.3 Module 3: MNA Planning and Implementation Step 3.1: Preparation of Long Term Monitoring Plan. Ones the effectiveness of NA in a contaminated area has been ascertained a long-term monitoring plan, which includes numbers and specific location of monitoring wells, frequency of monitoring, and analysis, is developed. Two type of monitoring wells are advised: long-term monitoring (LTM) wells to determine plume behavior, and point of compliance (POC) wells to detect contaminant movement outside the negotiated perimeter of the contaminant. POC wells 3

4 are placed at receptor positions; LTM well distribution depends on the size and type of contaminant at the site, and the number should be sufficient to represent the plume behavior. By their function, LTM wells should include analyses for contaminant levels, and if the latter are organic, likely electron acceptors, and likely by-products of degradation, and redox potential. In the same situation, POC wells should sample for contaminant levels as well as for dissolved oxygen (Wiedemeier et al. 1999). Step 3.: Obtaining Approval for MNA. Negotiations with regulators are critically important. For fuel hydrocarbons, the argument for NA relies primarily on three observations: Trend of contaminant disappearance. The best supportive evidence for NA is the demonstration of a rapid decrease in contaminant concentration. Loss of electron donors. A decrease in electron donors (e.g., O, NO - 3, SO -, etc.) levels compared to background levels is strong evidence that organic matter is being degraded and that NA is effective. Degradation products. Elevated levels of by-products Fe +, H S, CO, and methane, produced during the degradation of fuel hydrocarbons, relative to their background level points to contaminant degradation and effectiveness of NA. 3. APPLICATION OF RBMNA IN EVALUATING FUEL HYDROCARBON CONTAMINATED SITE At a fuel oil storage unit, it is found that one of the underground storage vessels was leaking for almost previous years resulting in the leakage of about 99 gallons of gasoline. Preliminary study revealed that concentration of benzene, toluene, ethylbenzene, or xylene (BTEX) levels at the contaminated zone are far than 5 mg/kg. Groundwater study found that an area of about sq ft has concentration more than 5 mg/l. This site has been evaluated for MNA using RBMNA methodology, detailed finding are illustrated at Khan and Husain (); in subsequent section a summarized version of this study is presented. 3.1 Risk based Screening As per first step of this module step onsite and offsite risk were computed using RBCA Chemical Release System 1.3 system (GSI 199). It is found that on-site receptors (residential) poses risk more than 5 times higher than regulatory standard. Similar is the case with offsite receptors. The compiled information on hydrology and electron receptors is summarized in Table 1. The tabulated data reveals that the site has good potential for NA, as the concentrations of electron acceptors in the background are high, and hydrological conditions favor NA. A conceptual model of the site and various attenuation processes effective on the site is developed on the basis of available information. Table 1. Initial setting of parameters used in present study Parameters Values Thickness of soil layers, ft 15, 15, 15, 5 Soil type Sandy with porous shell Application area, cm 5.E+7 Loading rate of Benzene, µg/cm 3.E+ Loading rate of Toluene, µg/cm 3.E+3 Loading rate of Ethylbenzene, µg/cm 3.E+3 Loading rate Xylene, µg/cm 1.E+ Background concentration of O, mg/l 1.5 Background concentration of NO 3 -, mg/l.7 Background concentration of SO -, mg/l. Background concentration of Fe 3+, mg/l 1. Background concentration of CH, mg/l 1.5 Size of aquifer, cm Infinite Depth of aquifer, cm

5 3. Quantitative Evaluation of MNA Fate and transport of contaminant with natural attenuation is studied using SESOIL for contaminant modeling in the unsaturated zone, AT13D for fate and migration of contaminants in the saturated zone (ESC ), and BIOSCREEN for NA (ESC ). Modeling of Vadose Zone The soil column is modeled in four different layers with the contaminant loaded on the second layer from the down. The modeled results for four contaminants (for a -year duration) are compared with the observed data. It is observed that that the contour of modeled BTEX concentration of mg/kg runs over an area of about 3 ft by 5 ft; while the observed concentration run over an area of 5 ft by 15 ft. The detailed results for benzene predict that most of the benzene is volatilized (~9%) and the remaining biodegraded (7%), leaving only a small amount to leach to the groundwater to cause a maximum leachate concentration of.17 mg/l. Toluene shows a comparatively slower movement; a major portion is volatilized (99%), and only an infinitesimal amount leaches causing a maximum leachate concentration of. mg/l. SESOIL results for ethylbenzene predict that about % is volatilized, and the remaining portion is distributed in soil air, soil moisture, biodegradation, and groundwater runoff giving a maximum leachate concentration of.33 mg/l. Similarly, a substantial portion of xylene is volatilized (7%), and the remaining portion diluted in the groundwater (~1%), causing a maximum leachate concentration of. mg/l. Modeling of Saturated Zone The saturated zone is about 55 ft below the ground surface. The transportation and transformation of chemicals in the aquifer is modeled using AT13D. The trend for benzene concentration (Figure 1) indicates that initially for four years there is a negligible concentration in the groundwater. Benzene reaches its maximum concentration in the fifth year, and thereafter decreases nonlinearly. During the last years the maximum concentration zone for benzene is shifts about 5 ft from its original position (source zone). The maximum concentration of benzene at the centerline of the plume is.13 mg/l and plume of 1 µg/l extends to 5 ft downstream. Compared to benzene, the migration of ethylbenzene is relatively slow. A significant concentration of ethylbenzene starts appearing in the aquifer only after eight years of release (Figure 1). The concentration increases for one year, reaches a maximum in the ninth year, and then starts to decrease. Like benzene, the ethylbenzene plume of 1 µg/l concentration persists over an area of about ft radius. The results for toluene are similar to benzene; the maximum concentration is observed in the seventh year after the release, and thereafter decreases nonlinearly (Figure 1). The maximum concentration of toluene in the aquifer is comparatively low (. mg/l). Compared to other BTEX compounds, xylene shows a more persistent trend. In this case, the maximum concentration is observed in the fifth year (comparatively higher than other three constituents) and thereafter decreases (Figure 1). However, this decreasing trend is slow compared to previous ones. The maximum plume centerline concentration is. mg/l and a plume of 1 µg/l concentration extends more than 9 ft downstream. Analysis of these results reveals that BTEX concentrations initially show an increasing trend, followed by a depleting trend. Contaminant depletion results from biodegradation, dilution, and dispersion. An increasing trend is observed because during the initial period the rate of leachate addition to the aquifer was higher than the depleting rate (biodegradation, dilution, and dispersion). Natural Attenuation Modeling The remediation of the aquifer contamination is modeled using the NA processes of biodegradation, absorption, dilution, and dispersion (predominately biodegradation is the dominating one). The biodegradation of organic contaminants can be modeled by: i) first-order decay, and ii) instantaneous reaction models. The first approach models the exponential decay of contaminant, where degradation is dependent on the contaminant concentration. The second approach assumes that the microbial degradation kinetic is fast compared to the transport of oxygen (electron acceptor) and utilization of contaminant; therefore reaction is simulated as an instantaneous reaction between the contaminant and the oxygen. In recent years, it has been observed that the limited availability of electron acceptors (oxygen for aerobic reactions and NO 3 -, SO -, Fe 3+, ions in case of anaerobic reactions) controls the reaction rate 5

6 rather than the contaminant concentration. It is further verified by many experimental studies (Rifai et al. ; Wiedemeier et al. 1999) first-order decay models overestimate biodegradation..3 1 Leachate concentration (toulene and ethylbenzene) mg/l Ethylbenzene Toulene Benzene Xylene 1 Leachate concentration (benzene and xylene) mg/l 1 Time, year Figure 1. Time series analysis of leachate concentration reaching groundwater Figure 1. Time series analysis of leachate concentration reaching groundwater In the present study we used the instantaneous approach to study NA for BTEX using BIOSCREEN (ESC ). A study of benzene plume geometry reveals that the plume is becoming exhausted. The plume for 1 µg/l (the outermost contour), which initially extended about 1 ft by 5 ft, is squeezed to 9 ft by ft after years of release; in the next 3 years the concentration significantly decreases and the plume is confined to 35 ft by ft (Figure ). Initially the benzene plume spread to a large area (Figure ) close to the residential complex, but, after 5 years of release, it is depleted and is now close only to the premises of the storage facility. The ethylbenzene plume shows a similar trend; however, its rate of exhaustion is comparatively slow and after 5 years, the plume dimensions are predicted as 5 ft by 3 ft, which is relatively larger than for benzene. Toluene plume geometries show a similar trend to ethylbenzene; however, the dimensions of its plume are comparatively small (even smaller than benzene). Xylene shows a maximum spread (Figure 3). After years, the xylene plume extends more than 1 ft by 5 ft. However, in the next years the plume shrinks; after 3 years of release, it is ft by 5 ft, and in next years 75 ft by ft. Unlike benzene, after years of release, the xylene plume reaches the residential area, and after 5 years it is exhausted considerably; but its perimeter extends beyond the boundary of the storage area (Figure 3). Above analysis conclude that BTEX plumes are shrinking, though in the initial periods they extend to some sensitive areas. Now, it may be said confidently that NA is active in the area; however, the suitability of natural NA depends on the risk these plumes pose to potential receptors. As contaminant concentrations at receptors are known, the risk to potential on and/or offsite receptors can be estimated. In the present study, a hypothetical residential location at 5 ft from the release source has been considered (futuristic approach) as onsite receptor. Offsite potential receptors are located about 5 ft from the point of release in the downstream direction, and ft in the cross-stream direction. RBCA Chemical Release version 1. (GSI 199) is again used for base risk computations. It is clear from the results presented in Table that for an initial period of years of release, the site poses a considerable risk to on-site (futuristic risk) as well as off-site receptors. However, the risk becomes acceptable after 3 years of release. If the site is maintained for NA, the present onsite and offsite risk must be minimized by taking appropriate measures.

7 Groundwater flow direction 15 1 Monitoring wells River Groundwater flow direction 1 Monitoring wells River Figure. Benzene plume geometry after and 5 years release 7

8 Table. Summary of risk factors and hazard quotient at different duration After years After 3 years After 5 years On site* Off site** On site* Off site** On site* Off site** Exposure pathways Outdoor air exposure Indoor air exposure Soil exposure Groundwat er exposure Surface water exposure CRF HQ CRF HQ CRF HQ CRF HQ CRF HQ CRF HQ.9e-.1 1.e e-. 7.5e-. 3.e-. 5.e-1..e- 3 NA NA.5e-. NA NA 7.e-.3 NA NA 5.3e- 1 NA NA 3.e-.55 NA NA NA NA NA NA 1.e e e-5.1.5e-7.15.e e-9. NA NA NA NA NA NA NA NA NA NA NA NA CRF-Cumulative Risk Factor estimated for carcinogens constituents HQ-Hazard quotient estimated for non-carcinogen constituents NA- Not applicable 3.3 MNA Planning and Implementation It is evident from the above study that MNA is an effective remediation alternative for this site if the appropriate control measures are taken to contain the present and futuristic risks. An extensive monitoring and contingency program is developed to ensure that the constituents are behaving as predicted. The important points of the long-term monitoring plan are highlighted below. Wells,,,, 13, 15 and 17 are used for the long-term monitoring of contaminant and other parameters. For the initial years, the sampling is carried out every four months and the results are evaluated. If there are significant differences between predicted and observed behaviors, the model is calibrated with a new set of data, and a prediction made for the next years. If this discrepancy exists for more than four years, despite calibration and prediction attempts, a contingency plan implemented. Five new wells are installed for POC monitoring. Once they are operative these wells along with wells 1, 1, and 1 are used as POC wells. The contaminant concentration in these wells must be analyzed on a monthly basis. If the concentration of the contaminant exceeds the setup limit, a contingency plan is called. As indicated earlier, the on-site risk is computed considering hypothetical residential receptors; in the present situation there are no potential on-site receptors. As evident from Table 1, the groundwater exposure route is the main concern. In order to control this futuristic risk, groundwater for any portable use is prohibited. Municipal water is available to the receptor for this purpose. At Off-site municipal water is the main source of portable water. Though there are many wells in the area, they are seldom operational. However, to be on the safe side, strict guidelines are issued to offsite residential receptors to avoid the use of groundwater for portable use. The development of any new community facility in the contaminated area (~ 5 m ) for the next years is prohibited. A detailed contingency plan to deals with any adverse situation. It involves the immediate implementation of engineering remediation options to control the situation. A discussion of the details of such a contingency plan is beyond the scope of this paper.

9 Groundwater flow direction 15 1 Monitoring wells River Figure Xylene plume after years of release Groundwater flow direction 15 1 Monitoring wells River Figure 3. Xylene plume geometry after and 5 years release 9

10 . SUMMARY AND CONCLUSION Hydrocarbon spills which cause contamination of soil and groundwater is a worldwide problem. The cleanup of these contaminated sites using conventional remediation processes is cost prohibitive and does not ensure complete site recovery. NA is a remediation approach where physical, chemical, and/or biological processes under favorable conditions act without human intervention to reduce the mass, toxicity, mobility, volume or concentration of contaminants in soil and groundwater. In this paper authored summarized a revised version of the risk based site evaluation and remediation through NA methodology. The application of the discussed methodology has been demonstrated using a case study. In case study it is observed that though there is substantial risk to the site at the initial period of release particularly due to benzene and xylene, it becomes acceptable as the plume is diluted, degraded, and drifts from the point of release. The total onsite risk becomes acceptable only after 3 years of release as the xylene plume persists longer. In brief, the study concludes that NA can be used as an effective measure of remediation at the present site, but efforts are required to implement extensive monitoring programs to track the behavior of the contaminant and to control risks. A contingency plan involving engineering remediation option is needed to deal with any untoward situation. 5. REFERENCES 1. ASTM (1995) Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites, publication # E , Bar Harbor, West Conshohocken, PA.. AT13D (1997) Analytical Transient one, two and Three Dimensional Model for Waster Transport in Groundwater, International Ground Water Modeling Center, Colorado School of Mines, Golden, CO. 3. Bonazountas, M., and Wagner, J., (19) SESOIL: A Seasonal Soil Compartment Model, Arthur D Little, Inc, Cambridge, MA. Prepared for US EPA Office of Toxic Substances, Publication # PB- 11, Washington, DC.. DOE (1999) Decision-making framework guide for the evaluation and selection of MNA remedies at department of energy sites, Office of Environment Management, Department of Energy, available via 5. ESC () SESOIL and AT13D, Integrated Contaminant Transport and Fate modeling users guide, Environmental Software Consultants, Inc., Madison, WI.. GSI (199) RBCA Tool kit for Atlantic RBCA, Groundwater Services, Inc, 11 Norfolk, Houston, TX. 7. Husain, T., Hejazi, R., and Khan, F.I. () Remediation of Petroleum Contaminated Sites using Monitored Natural Attenuation Approach, published in the proceeding of Chemistry and Industry-, October 3-November 1, Bahrain.. Khan, F.I. and Husain, T. () Cleanup contaminated sites, Chemical Engineering Progress, 9(1): Khan, F.I. and Husain, T. (1a) Risk Based Natural Attenuation- A Case Study, J. of Hazardous Materials, B5:3-7.. Khan, F.I., and Husain, T. (1b) Risk based monitored natural attenuation and its application, published in the proceedings of at 9 th Annual Conference-CSCE 1, May 3-June, Victoria, BC. 11. Newell, C.J., McLeord, R.K., and Gonzales, J. (1997) BIOSCREEN Natural attenuation Decision Support System, version 1. revision, Groundwater Services Inc, Houston, TX. 1. NRC (1997) Natural Attenuation for Groundwater Remediation, National Research Council, National Academy Press, Washington, DC. 13. Rifai, H.S., Newell, C.J., Gonzales, J.R., Dendrou, S., Kennedy, L., and Wilson, J. (1997) BIOPLUME III natural attenuation Decision Support System, Version 1., US Air Force Center for Environmental Excellence, Brooks Air Force Base, San Antonio, TX. 1. Rifai, S.H., Newell, C.J., Gonzales, J.R., and Wilson, J.T. () Modeling Natural Attenuation of Fuels with BIOPLUME III, J. of Environmental Engineering, 1(5): SNL, (1999) MNA Tool Box: A Tool for Monitored Natural Attenuation Evaluation, Sandia National Laboratories, Albuquerque, New MO Wiedemeier, T.H., Newell, C.J. Rifai, H.S., and Wilson, J.T. (1999) Natural attenuation of fuels and chlorinated solvents in the subsurface, John Wiley & Sons, New York, pp1-.