Configuring Detection Wells Near Landfills

A graphical method was devised for designing contaminant detection monitoring networks in aquifers. The approach eliminates bias in detection efficiency among well pairs, thereby improving the overall efficiency of a ground water monitoring network. In the equidistant configurations derived by the graphical approach, all wells are located the same distance from a landfill, but the distance is measured parallel to ground water flow, Measured perpendicular to ground water flow, there is also an equal spacing between wells in an equidistant network. A simulation model was used to compare an equidistant network to a peripheral monitoring configuration, in which wells were spaced evenly along the downgradient boundaries of a landfill. The equidistant network yielded a 12.4% higher detection efficiency and also facilitated earlier release detection. In practice, the graphical approach that yields equidistant configurations can be used to identify candidate monitoring networks to detect potential releases from landfills.     

Designing a monitoring network for detecting groundwater pollution with stochastic simulation and a cost model

A method is presented to design monitoring networks for detecting groundwater pollution at industrial sites. The goal is to detect the pollution at some distance from the site’s boundary so that it can be cleaned up or hydrologically contained before contaminating groundwater outside the site. It is assumed that pollution may occur anywhere on the site, that transport is by advection only and that no retardation and chemical reactions take place. However, the approach can be easily extended to include designated (and uncertain) source areas, dispersion and reactive transport. The method starts from the premise that it is impossible to detect 100% of all the contaminant plumes with reasonable costs and therefore seeks a balance between the risk of pollution and network density. The design approach takes account of uncertainty in the flow field by simulating realisations of conductivity, groundwater head and associated flow fields, using geostatistical simulation and a groundwater flow model. The realisations are conditioned to conductivity and head observations that may already be present on the site. The result is an ensemble of flow fields that is further analysed using a particle track program. From this the probability of missing a contaminant plume originating anywhere on the terrain can be estimated for a given network. From this probability follows the risk, i.e. the expected costs of an undetected pollution. The total costs of the monitoring strategy are calculated by adding the risk of pollution to the costs of installing and maintaining the monitoring wells and the routinely performed chemical analyses. By repeating this procedure for networks of varying well numbers, the best network is chosen as the one that minimises total cost. The method is illustrated with a simulated example showing the added worth of exploratory wells for characterising hydraulic conductivity of a site.     

The Geochemistry of Boron in a Landfill Monitoring Program

Ground water monitoring data collected during the past eight years at a permitted municipal solid waste (MSW) disposal facility located in the midwestern United States indicated fluctuations in typical leachate indicator parameter concentrations. Apparent trends in the data inferred leachate outbreak, generating suspicion as to the integrity of the landfill liner. Eight ground water monitoring wells were installed in three distinct geologic units at the landfill facility, including glacial drift, silurian dolomite, and a post-glacial peat fen, which is downgradient from the landfill. Piezometer nests were used to define ground water gradients at the site. Using boron as an indicator, the occurrence of analytes of concern in the downgradient monitoring wells were shown to be indicative of the natural geochemistry of site ground water. This work emphasizes the importance of understanding site hydrogeology during the interpretation of ground water quality data.     

Relationship Between Subsurface Landfill Gas and Arsenic Mobilization into Groundwater

The maximum contaminant level for arsenic was reduced by the U.S. Environmental Protection Agency (U.S. E.P.A.) for Drinking Water Standards from 50 micrograms per liter (µg/L) to 10 µg/L, effective January 23, 2006. The subject site is a double-lined sanitary landfill facility located in the mid-Atlantic region of the United States. Arsenic was reported above the maximum contaminant level in a downgradient monitoring well (MW-18) in July 2005. Since July 2005, arsenic levels in MW-18 fluctuated above and below 10 µg/L. This research focuses on determining whether reducing conditions in groundwater, enhanced by subsurface landfill gas emissions, were causing naturally-occurring arsenic to mobilize from the native variably-saturated vadose zone soils into groundwater. The groundwater data collected from the impacted well (MW-18) were compared to an upgradient well (MW-8) to determine whether significant differences existed during the time period of April 2004 to April 2007. Linear regression analysis was also used to determine whether other parameters had a significant relationship with the arsenic concentrations detected in MW-18. The groundwater located in MW-18 was consistently more reduced than groundwater located in the upgradient/background well MW-8, and this was most likely attributed to the presence of subsurface landfill gas in the area. According to the U.S. E.P.A., oxidation-reduction potential (ORP) values in groundwater less than 50 millivolts (mv) suggest that a reducing environment may be present. The data presented in this study indicate that arsenic can mobilize into groundwater under moderately reducing conditions, with ORP measurements averaging 53 mv.     

Factors Influencing Variability in Groundwater Monitoring Data Sets

“Random” variability in groundwater monitoring data sets reduces the ability to identify long‐term concentration trends. This, in turn, increases the time and cost required to evaluate the effectiveness of natural attenuation and other groundwater remedies. To better understand the factors influencing variability in groundwater monitoring results, we have analyzed three large groundwater monitoring data sets. For the three data sets, the long‐term trend in contaminant concentration in each well accounted for an average of 30% to 40% of the overall variation in contaminant concentration. Understanding the causes of the remaining variability would support the development of improved groundwater monitoring methods. All three data sets show large differences in the temporal monitoring records between individual wells (e.g., coefficient of variation for monitoring results from individual wells ranges from 0.08 to 4.6) indicating that well and aquifer factors are more important contributors to variability than sample collection and analysis factors. However, the depth to groundwater (R² = 0.020) and distance between water level and screened interval (R² = 0.049) accounted for only a portion of the differences in variability between wells and other aquifer characteristics evaluated and were not correlated with the observed variability in monitoring results. Unidentified factors were apparently much more important contributors to variability than these factors. The monitoring data sets exhibited two distinct timescales for variability: Time‐independent variability that was apparent even when wells were re‐sampled within a few days and a long‐term variability likely associated with the long‐term concentration trend. The observation of time‐independent variability suggests that frequent monitoring of contaminated monitoring wells serves primarily to characterize sources of variability unrelated to the long‐term trend of primary interest.     

The Horizontal Reactive Media Treatment Well (HRX Well®) for Passive In‐Situ Remediation

A new in‐situ remediation concept termed a Horizontal Reactive Media Treatment Well (HRX Well®) is presented that utilizes horizontal wells filled with reactive media to passively treat contaminated groundwater in‐situ. The approach involves the use of large‐diameter directionally drilled horizontal wells filled with granular reactive media generally installed parallel to the direction of groundwater flow. The design leverages natural “flow‐focusing” behavior induced by the high in‐well hydraulic conductivity of the reactive media relative to the aquifer hydraulic conductivity to passively capture and treat proportionally large volumes of groundwater within the well. Clean groundwater then exits the horizontal well along its downgradient sections. Many different types of solid granular reactive media are already available (e.g., zero valent iron, activated carbon, ion exchange resins, zeolite, apatite, chitin); therefore, this concept could be used to address a wide range of contaminants. Three‐dimensional flow and transport simulations were completed to assess the general hydraulic performance, capture zones, residence times, effects of aquifer heterogeneity, and treatment effectiveness of the concept. The results demonstrate that capture and treatment widths of up to tens of feet can be achieved for many aquifer settings, and that reductions in downgradient concentrations and contaminant mass flux are nearly immediate. For a representative example, the predicted treatment zone width for the HRX Well is approximately 27 to 44 feet, and contaminant concentrations immediately downgradient of the HRX Well decreased an order of magnitude within 10 days. A series of laboratory‐scale physical tests (i.e., tank tests) were completed that further demonstrate the concept and confirm model prediction performance. For example, the breakthrough time, peak concentration and total mass recovery of methylene blue (reactive tracer) was about 2, 35, and 20 times (respectively) less than chloride (conservative tracer) at the outlet of the tank‐scale HRX Well.     

Improving the reliability of groundwater monitoring networks using combined numerical,geostatistical and neural network-based simulation models

ABSTRACT

A new methodology is proposed for improving the accuracy of groundwater-level estimations and increasing the efficiency of groundwater-level monitoring networks. Three spatio-temporal (S-T) simulation models, numerical groundwater flow, artificial neural network and S-T kriging, are implemented to simulate water-table level variations. Individual models are combined using model fusion techniques and the more accurate of the individual and combined simulation models is selected for the estimation. Leave-one-out cross-validation shows that the estimation error of the best fusion model is significantly less than that of the three individual models. The selected fusion model is then considered for optimal S-T redesign of the groundwater monitoring network of the Dehgolan Plain (Iran). Using a Bayesian maximum entropy interpolation technique, soft data are included in the geostatistical analyses. Different scenarios are defined to incorporate economic considerations and different levels of precision in selecting the best monitoring network; a network of 37 wells is proposed as the best configuration. The mean variance estimation errors of all scenarios decrease significantly compared to that of the existing monitoring network. A reduction in equivalent uniform annual costs of different scenarios is achieved.     

A Systematic Approach to Designing a Multiphase Unsaturated Zone Monitoring Network

A systematic approach is presented for the design of a multiphase vadose zone monitoring system recognizing that, as in ground water monitoring system design, complete subsurface coverage is not practical. The approach includes identification and prioritization of vulnerable areas: select ion of cost-effective indirect monitoring methods that will provide early warning of contaminant migration: selection of direct monitoring methods for diagnostic confirmation; identification of background monitoring locations; and identification of an appropriate temporal monitoring plan. An example of a monitoring system designed for a solid waste landfill is presented and utilized to illustrate the approach and provide details of system implementation. The example design described incorporates the use of neutron moisture probes deployed in both vertical and horizontal access tubes beneath the lcachate recovery collection system of the landfill. Early warning of gaseous phase contaminant migration is monitored utilizing whole-air active soil gas sampling points deployed in gravel- filled trenches beneath the subgrade. Diagnostic confirmation of contaminant migration is provided utilizing pore- liquid samplers. Conservative tracers can be used to distinguish between chemical species released by a landfill from those attributable to other (e.g. off-site) sources or present naturally in the subsurface. A discussion of background monitoring point location is also presented.     

Groundwater Quality Impacts from a Full‐Scale Integrated Constructed Wetland

The concept of integrated constructed wetlands (ICW) promotes in‐situ soils to construct and line wetland cells. The integrity of soil material, however, may provide a potential pathway for contaminants to flow into the underlying groundwater. This study assessed the extent of groundwater quality deterioration due to the establishment of a full‐scale ICW system treating domestic wastewater in Ireland. The ICW is located at Glaslough in Co. Monaghan, Ireland. It consists of two sedimentation ponds and a sequence of five shallow vegetated wetland cells. The ICW cells were lined with 500‐mm thick local subsoil material, which comprised a mixture of alluvium, organic soils, tills, and gravel. Groundwater samples and head data were collected from eight piezometers, which were installed around the ICW cells. The groundwater and wetland water samples were analysed for water quality parameters such as bulk organic matter, nutrients, and pathogens. Overall, the quality of groundwater underlying the ICW system recorded some contamination with bulk organic matter and some inorganic nutrients. Significantly higher contaminant concentrations were recorded in monitoring wells upgradient and near to the distal wetland cells than downgradient ones, which were near to the proximal cells. For the downgradient piezometers, concentrations seldomly exceeded the natural background levels. Detailed analyses through the application of chemometrics models indicated that the source of contamination was largely of geogenic origin. Findings suggest that ICW systems pose a minimal risk to the groundwater quality; the greatest risk was associated with the distal wetland cells.     

Permeable Asphalt: A New Tool to Reduce Road Salt Contamination of Groundwater in Urban Areas

Chloride contamination of groundwater in urban areas due to deicing is a well‐documented phenomenon in northern climates. The objective of this study was to evaluate the effects of permeable pavement on degraded urban groundwater. Although low impact development practices have been shown to improve stormwater quality, no infiltration practice has been found to prevent road salt chlorides from entering groundwater. The few studies that have investigated chlorides in permeable asphalt have involved sampling directly beneath the asphalt; no research has looked more broadly at surrounding groundwater conditions. Monitoring wells were installed upgradient and downgradient of an 860 m² permeable asphalt parking lot at the University of Connecticut (Storrs, Connecticut). Water level and specific conductance were measured continuously, and biweekly samples were analyzed for chloride. Samples were also analyzed for sodium (Na), calcium (Ca), and magnesium (Mg). Analysis of variance analysis indicated a significantly (p < 0.001) lower geometric mean Cl concentration downgradient (303.7 mg/L) as compared to upgradient (1280 mg/L). Concentrations of all alkali metals increased upgradient and downgradient during the winter months as compared to nonwinter months, indicating that cation exchange likely occurred. Despite the frequent high peaks of chloride in the winter months as well as the increases in alkali metals observed, monitoring revealed lower Cl concentrations downgradient than upgradient for the majority of the year. These results suggest that the use of permeable asphalt in impacted urban environments with high ambient chloride concentrations can be beneficial to shallow groundwater quality, although these results may not be generalizable to areas with low ambient chloride concentrations.     

Developing a long-term monitoring network under uncertain flowpaths

A long-term monitoring well network is developed using complementary and simple approaches in conjunction with a stochastic ground water flow and transport model. The development is illustrated for a case study of a U.S. nuclear testing site (Shoal) that is undergoing environmental restoration. The network design builds on three different, yet complementary, tools for locating the monitoring wells with a main objective of detection monitoring. The first tool is applied to select potential siting horizons where monitoring wells could be located. The second tool is used to place monitoring wells in locations with high success probability. The success here is defined as the detection of migrating stochastic plumes before a certain mass percentage reaches a compliance boundary. The third tool is used to analyze detection efficiency of multiple combinations of three wells. Seventy-six different three-well networks are selected from 20 candidate locations and are evaluated for detection efficiency. From the 76 networks analyzed, 28 attain detection efficiency close to or above 70%. The results of the different analyses provide multiple alternatives for the locations of the three wells, which will become part of the long-term monitoring network at Shoal. A number of combinations are equally good, and the final choice will depend on practical considerations and future agreements between model sponsor and regulators.     

A Tracer Test to Characterize Treatment of TCE in a Permeable Reactive Barrier

A tracer test was conducted to characterize the flow of groundwater across a permeable reactive barrier constructed with plant mulch (a biowall) at the OU‐1 site on Altus Air Force Base, Oklahoma. This biowall is intended to intercept and treat groundwater contaminated by trichloroethylene (TCE) in a shallow aquifer. The biowall is 139‐m long, 7.3‐m deep, and 0.5‐m wide. Bromide was injected from an upgradient well into the groundwater as a conservative tracer, and was subsequently observed breaking through in monitoring wells within and downgradient of the biowall. The bromide breakthrough data demonstrate that groundwater entering the biowall migrated across it, following the slope of the local groundwater surface. The average seepage velocity of groundwater was approximately 0.06 m/d. On the basis of the Darcy velocity of groundwater and geometry of the biowall, the average residence time of groundwater in the biowall was estimated at 10 d. Assuming all TCE removal occurred in the biowall, the reduction in TCE concentrations in groundwater across the biowall corresponds to a first‐order attenuation rate constant in the range of 0.38 to 0.15 per d. As an independent estimate of the degradation rate constant, STANMOD software was used to fit curves through data on the breakthrough of bromide and TCE in selected wells downgradient of the injection wells. Best fits to the data required a first‐order degradation rate constant for TCE removal in the range of 0.13 to 0.17 per d. The approach used in this study provides an objective evaluation of the remedial performance of the biowall that can provide a basis for design of other biowalls that are intended to remediate TCE‐contaminated groundwater.     

Innovative Contaminant Mass Flux Monitoring in an Aquifer Subject to Tidal Effects

Exposure from groundwater contamination to aquatic receptors residing in receiving surface water is dependent upon the rate of contaminated groundwater discharge. Characterization of groundwater fluxes is challenging, especially in coastal environments where tidal fluctuations result in transient groundwater flows towards these receptors. This can also be further complicated by the high spatial heterogeneity of subsurface deposits enhanced by anthropogenic influences such as the mixing of natural sediments and backfill materials, the presence of subsurface built structures such as sheet pile walls or even occurrence of other sources of contaminant discharge. In this study, the finite volume point dilution method (FVPDM) was successfully used to characterize highly transient groundwater flows and contaminant mass fluxes within a coastal groundwater flow system influenced by marked tides. FVPDM tests were undertaken continuously for more than 48 h at six groundwater monitoring wells, in order to evaluate groundwater flow dynamics during several tide cycles. Contaminant concentrations were measured simultaneously which allowed calculating contaminant mass fluxes. The study highlighted the importance of the aquifer heterogeneity, with groundwater fluxes ranging from 10⁻⁷ to 10⁻³ m/s. Groundwater flux monitoring enabled a significant refinement of the conceptual site model, including the fact that inversion of groundwater fluxes was not observed at high tide. Results indicated that contaminant mass fluxes were particularly higher at a specific monitoring well, by more than three orders of magnitude, than at other wells of the investigated aquifer. This study provided crucial information for optimizing further field investigations and risk mitigation measures.     

Electrical Resistivity Imaging of a Permanganate Injection During In Situ Treatment of RDX‐Contaminated Groundwater

Groundwater beneath the former Nebraska Ordnance Plant (NOP) is contaminated with the explosive hexahydro‐1,3,5‐trinitro‐1,3,5‐triazine (RDX) and trichloroethene (TCE). Previous treatability experiments confirmed that permanganate could mineralize RDX in NOP aquifer material. The objective of this study was to determine the efficacy of permanganate to transform RDX in the field by monitoring a pilot‐scale in situ chemical oxidation (ISCO) demonstration. In this demonstration, electrical resistivity imaging (ERI) was used to create two‐dimensional (2‐D) images of the test site prior to, during, and after injecting sodium permanganate. The ISCO was performed by using an extraction‐injection well configuration to create a curtain of permanganate. Monitoring wells were positioned downgradient of the injection zone with the intent of capturing the permanganate‐RDX plume. Differencing between ERI taken preinjection and postinjection determined the initial distribution of the injected permanganate. ERI also quantitatively corroborated the hydraulic conductivity distribution across the site. Groundwater samples from 12 downgradient wells and 8 direct‐push profiles did not provide enough data to quantify the distribution and flow of the injected permanganate. ERI, however, showed that the permanganate injection flowed against the regional groundwater gradient and migrated below monitoring well screens. ERI combined with monitoring well samples helped explain the permanganate dynamics in downgradient wells and support the use of ERI as a means of monitoring ISCO injections.     

Statistical Analysis of Secondary Water Quality Impacts from Enhanced Reductive Bioremediation

Enhanced reductive bioremediation (ERB) is effective for treating a broad range of groundwater contaminants, but does result in secondary water quality impacts (SWQIs). Monitoring data from 47 ERB projects were analyzed to gain a better understanding of the formation and extent of SWQIs. The database analysis revealed that SWQIs occur at virtually every site, including reduced levels of background aqueous electron acceptors (O₂, NO₃^?, and SO₄^2?), increases in dissolved‐phase metals (Fe and Mn), and the production of CH₄. However, the SWQI “plume” that is produced is usually confined within the original contaminant plume. As a result, SWQIs from ERB are unlikely to adversely impact potable water supplies. SWQIs do attenuate with distance downgradient, with concentrations often returning to near background levels. The results of the database analysis were combined with previous research to develop a general conceptual model (CM) of SWQI production, mobilization, and attenuation. This CM can assist in identifying conditions where SWQIs may pose a concern. These can include sites with low iron/high sulfate (H₂S mobilization), high groundwater velocity (SWQIs at distances far downgradient), and sites with low CH₄ anaerobic oxidation rates (CH₄ migration).     

Field Demonstration of the Horizontal Treatment Well (HRX Well®) for Passive In Situ Remediation

The horizontal reactive media treatment well (HRX Well®) uses directionally drilled horizontal wells filled with a treatment media to induce flow-focusing behavior created by the well-to-aquifer permeability contrast to passively capture proportionally large volumes of groundwater. Groundwater is treated in situ as it flows through the HRX Well and downgradient portions of the aquifer are cleaned via elution as these zones are flushed with clean water discharging from the HRX Well. The HRX Well concept is particularly well suited for sites where long-term mass discharge control is a primary performance objective. This concept is appropriate for recalcitrant and difficult-to-treat constituents, including chlorinated solvents, per- and polyfluoroalkyl substances (PFAS), 1,4-dioxane, and metals. A full-scale HRX Well was installed and operated to treat trichloroethene (TCE) with zero valent iron (ZVI). The model-predicted enhanced flow through the HRX Well (compared to the flow in and equivalent cross-sectional area orthogonal to flow in the natural formation before HRX Well installation) and treatment zone width was consistent with flows and widths estimated independently by point velocity probe (PVP) testing, HRX Well tracer testing, and observed treatment in downgradient monitoring wells. The actual average capture zone width was estimated to be between 45 and 69 feet. Total TCE mass discharge reduction was maintained through the duration of the performance monitoring period and exceeded 99.99% (%). Decreases in TCE concentrations were observed at all four downgradient monitoring wells within the treatment zone (ranging from 50 to 74% at day 436), and the first arrival of treated water was consistent with model predictions. The field demonstration confirmed the HRX Well technology is best suited for long-term mass discharge control, can be installed under active infrastructure, requires limited ongoing operation and maintenance, and has low life cycle energy and water requirements.     

Identification of Multiple Sources of Groundwater Contamination by Dual Isotopes

Chlorinated solvents are one of the most commonly detected groundwater contaminants in industrial areas. Identification of polluters and allocation of contaminant sources are important concerns in the evaluation of complex subsurface contamination with multiple sources. In recent years, compound‐specific isotope analyses (CSIA) have been employed to discriminate among different contaminant sources and to better understand the fate of contaminants in field‐site studies. In this study, the usefulness of dual isotopes (carbon and chlorine) was shown in assessments of groundwater contamination at an industrial complex in Wonju, Korea, where groundwater contamination with chlorinated solvents such as trichloroethene (TCE) and carbon tetrachloride (CT) was observed. In November 2009, the detected TCE concentrations at the study site ranged between nondetected and 10,066 µg/L, and the CT concentrations ranged between nondetected and 985 µg/L. In the upgradient area, TCE and CT metabolites were detected, whereas only TCE metabolites were detected in the downgradient area. The study revealed the presence of separate small but concentrated TCE pockets in the downgradient area, suggesting the possibility of multiple contaminant sources that created multiple comingling plumes. Furthermore, the variation of the isotopic (δ¹³C and δ³⁷Cl) TCE values between the upgradient and downgradient areas lends support to the idea of multiple contamination sources even in the presence of detectable biodegradation. This case study found it useful to apply a spatial distribution of contaminants coupled with their dual isotopic values for evaluation of the contaminated sites and identification of the presence of multiple sources in the study area.     

Assessing the Impact of a Heated Basement on Groundwater Temperatures in Bratislava,Slovakia

Groundwater temperature is a useful hydrogeological parameter that is easy to measure and can provide much insight into groundwater flow systems, but can be difficult to interpret. For measuring temperature directly in the ground, dedicated specifically designed monitoring wells are recommended since conventional groundwater wells are not optimal for temperature monitoring. Multilevel monitoring of groundwater temperature is required to identify contributions of different possible heat inputs (sources) on measured temperature signals. Interpreting temperature data as a cosine function, including period, average temperature, amplitude, and phase offset, is helpful. Amplitude dampening and increasing phase shift with distance from a boundary can be used for estimation of transport parameters. Temperature measurements at different depths can be used for evaluation of unknown parameters of analytical functions by optimization of regression fits in Python. These estimated parameters can be used to calculate temperatures at known water table depths which can be applied as a fixed transient boundary condition in MT3DMS to overcome the limitations of MT3DMS heat transport modeling in the unsaturated zone. In this study, temperature monitoring and modeling was used to evaluate the influence of a department store's heated basement foundation on groundwater temperature within a green space (city park), with the main outcome that 17 years after construction, the department store foundation has increased the mean groundwater temperature by 3.2 °C. Heat input evaluated by the MT3DMS model varied from 0.1 W/m² at a distance of 100 m up to 12 W/m² next to the building.     

Single-Well Injection-Drift Test to Estimate Groundwater Velocity

A simple algebraic equation is presented here to estimate the magnitude of groundwater velocity based on data from a single-well injection-drift test thereby eliminating the time-consuming and costly extraction phase. A volume of tracer-amended water was injected by forced-gradient into a single well followed by monitoring of the conservative solute tracers under natural-gradient conditions as their upgradient portions drifted back through the well. The breakthrough curve data from the single well during the drift phase was analyzed to determine the mean travel times of the tracers. The estimated mean upgradient travel distance back through the single well and the mean travel times of the tracers were used in a simple algebraic equation to estimate groundwater velocity. The groundwater velocity based on the single-well injection-drift test was estimated to be approximately 0.64 ft per day. Two transects of observation wells were used to monitor the natural-gradient tracer transport downgradient of the injection well. The one-dimensional, or dual-well, transport of the tracer from the injection well to the nearest downgradient observation well indicated that the groundwater velocity was 0.55 ft per day. The two-dimensional, or multi-well, transport of the center of mass of the tracers indicated that the groundwater velocity was 0.60 ft per day; the dual- and multi-well results were in excellent agreement with those from the single-well and validated the simple algebraic equation. The new single-well method presented here is relatively simple, rapid, and does not require an extraction phase.     

In Situ Chemical Oxidation of RDX-Contaminated Groundwater with Permanganate at the Nebraska Ordnance Plant

Groundwater beneath the former Nebraska Ordnance Plant (NOP) is contaminated with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The current pump and treat facility is preventing offsite migration but does not offer a short-term solution. Our objective was to quantify the effectiveness of permanganate to degrade RDX in situ. This was accomplished by performing laboratory treatability experiments, aquifer characterization, and a pilot-scale in situ chemical oxidation (ISCO) demonstration. Treatability experiments confirmed that permanganate could mineralize RDX in the presence of NOP aquifer solids. The pilot-scale ISCO demonstration was performed using an extraction-injection well configuration to create a curtain of permanganate between two injection wells. RDX destruction was then quantified as the RDX-permanganate plume migrated downgradient through a monitoring well field. Electrical resistivity imaging (ERI) was used to identify the subsurface distribution of permanganate after injection. Results showed that RDX concentrations temporally decreased in wells closest to the injection wells by 70% to 80%. Observed degradation rates (0.12 and 0.087/d) were lower than those observed under laboratory batch conditions at 11.5 °C (0.20/d) and resulted from lower than projected permanganate concentrations. Both ERI and spatial electrical conductivity measurements verified that permanganate distribution was not uniform throughout the 6.1-m (20 feet) well screens and that groundwater sampling captured both treated and nontreated groundwater during pumping. Although heterogeneous flow paths precluded a uniform permanganate distribution, pilot-scale results provided proof-of-concept that permanganate can degrade RDX in situ and support permanganate as a possible remedial treatment for RDX-contaminated groundwater.