Downgradient Monitoring Wells Research Articles

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

AbstractThe 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.

Boron as a contaminant at construction and demolition (C&D) debris landfills

Elevated boron concentrations above the regulatory standard were inadvertently discovered in downgradient groundwater monitoring wells at 22 construction and demolition (C&D) debris landfills in Florida, US. This created a unique opportunity to evaluate whether C&D debris can be considered a plausible source of boron at unlined landfills. Approximately 1200 historical landfill-leachate and groundwater records were surveyed from semi-annual and annual monitoring reports covering a 9-year period. Laboratory leaching experiments were conducted on soils from each of these sites to determine if the source could have been boron mobilized from naturally occurring soils. Historical leachate quality data from lined landfills near four of the unlined C&D debris landfills were examined to determine if leachate from the unlined landfills could be the boron source. The US Environmental Protection Agency (EPA) Method 1312, or Synthetic Precipitation Leaching Procedure (SPLP), and the EPA Method 1316 were performed on materials commonly found in C&D debris to see if these products have the potential to leach appreciable levels of boron. The results of this work indicate leachate from unlined C&D debris landfills as the most plausible source of elevated boron concentrations in downgradient monitoring wells.

Diseño de una barrera permeable reactiva para el tratamiento de aguas subterráneas contaminadas con tricloroetileno: modelo numérico

A 3-D groundwater numerical model was developed with the software Visual MODFLOW, to design a PRB with brown coal and compost as reactive materials to treat contaminated groundwater with trichloroethene (TCE) in the vicinity of Nowa Deba waterworks (South-East Poland). The results of the analysis allowed the evaluation of the flow changes due to the installation of diverse PRB systems (continuous wall, funnel and gate) and proposed the optimum configuration, length and thickness of the PRB. Furthermore, the effect of decreasing hydraulic conductivity of the reactive materials over time, and its impact on key design parameters, such as: Capture zone, residence time and discharge rate were also evaluated. Simulation results suggest that the best design for the contaminated site is a funnel and gate system with two funnels and one gate (1 310 m long and 3 m thick). In addition, results were used to select the location of downgradient monitoring wells to assess PRB performance.

Downhole pressure and chemical monitoring for CO2 and brine leak detection in aquifers above a CO2 storage reservoir

We assess the effectiveness of downhole monitoring of pressure and chemistry for detecting CO2 and brine leakage into underground sources of drinking water (USDW) overlying a geologic CO2 storage (GCS) reservoir. This assessment uses synthetic data sets generated in an uncertainty quantification (UQ) model framework. This framework combines the results of three model types: (1) a model of a hypothetical, 50-yr, 5-MT/yr GCS operation in the Vedder Fm. at the Kimberlina site in the southern San Joaquin Basin in California, USA, (2) models of CO2/brine leakage in legacy wells into overlying aquifers, and (3) models of CO2/brine plume migration in those aquifers. Leakage into overlying aquifers causes changes in pressure, CO2 saturation, and total dissolved solids (TDS), the latter being related to the solubility and speciation of CO2. This study captures the influence of leakage depth along the legacy well, regional groundwater flow, and aquifer heterogeneity on leak detection using downhole pressure and TDS monitoring.We consider CO2/brine leakage for legacy wells located at various distances from the CO2 injector at the Kimberlina site. Because gaseous CO2 is much less dense than supercritical CO2, the depth where leakage occurs along the wellbore is a key factor affecting the CO2, TDS, and pressure plumes. Supercritical CO2 that migrates upwards to depths less than 800 m undergoes a phase change to gaseous CO2, with the large volumetric expansion causing a large rise in pressure and a large spread in the TDS and pressure plumes. Because the pressure and TDS plumes migrate with the CO2 plume, they are strongly affected by regional groundwater flow and aquifer heterogeneity.We evaluated the likelihood of leak detection over a wide range of CO2/brine leakage-mass magnitude for downhole monitoring at various sensor depths, as well as for locations upgradient, downgradient, and laterally from the leaking well. Leak-detection effectiveness of downhole monitoring improves as the volumes of the CO2 and TDS plumes increase with cumulative CO2/brine leakage mass, groundwater gradient, and aquifer permeability. The effectiveness of pressure monitoring also increases strongly with CO2 leakage rate and can be useful for early detection of large leaks. Because TDS and pressure plumes migrate with the regional groundwater flow, downgradient monitoring wells are more effective than lateral or upgradient wells. Pressure monitoring is found to be more effective than TDS monitoring for shallow depths, while TDS monitoring is more effective at depths greater than about 600 m for this site. The addition of two monitoring depths to a single-depth monitoring well greatly improves leak detection. Monitoring three (shallow, medium, and deep) depths in the same well is found to be equally effective for pressure and TDS for the Kimberlina site. However, these comparisons are affected by the choice of detectable TDS and pressure thresholds, which depend on the level of unfilterable background noise for a specific site. For GCS sites like the Kimberlina site, with a thick sequence of overlying aquifers, unbroken by confining, low-permeability aquitard layers, an effective use of an individual monitoring well would be to monitor pressure at a shallow (<600 m) depth and to monitor TDS at a medium-depth (600−900 m) interval and for at least one deep (>900 m) interval. Because pressure and TDS changes evolve so gradually, continuous monitoring is not necessary.

Assessment of the use of a zero-valent iron permeable reactive barrier for nitrate removal from groundwater in the alluvial plain of the Dagu River, China

Groundwater in a shallow aquifer used for potable supply in Qingdao, China, has been contaminated by nitrate by the excessive use of chemical fertilisers for vegetable production in the area. In this study, a representative site was selected to construct a non-continuous permeable reactive barrier (PRB) back-filled with a mixed reactive medium containing of zero-valent iron, activated carbon and medium- to coarse-grained sand. The PRB consists of alternate well and pillars and was constructed to provide a funnel and gate treatment system for two pumping wells. Groundwater monitoring during a trial of the system indicated that the concentrations of nitrate in monitoring wells within the treatment system rapidly declined (within a day) from about 120 mg/L to 20 mg/L. Reductions in nitrate concentrations were also observed in the pumping wells and downgradient monitoring wells, but changes in nitrate concentrations were moderated by mixing with contaminated groundwater. The PRB construction technique of using alternative wells and pillars combined with jet grouting was found to be effective for nitrate removal from groundwater at a small scale.

Enhanced reductive dechlorination of trichloroethene in an acidic DNAPL impacted aquifer

A field pilot test was conducted using an emulsified vegetable oil (EVO) and colloidal magnesium hydroxide [Mg(OH)2] formulation to enhance reductive dechlorination of dense non-aqueous phase liquid (DNAPL) trichloroethene (TCE) in an acidic (pH < 4), heterogeneous aquifer. The field test consisted of i) a single well injection test to evaluate Mg(OH)2 distribution and ii) installation of two EVO-Mg(OH)2 permeable reactive barriers (PRBs; PRB-1 & PRB-2) at varying distances downgradient of the DNAPL source area. Distribution of Mg(OH)2 was observed up to 2.3 m away from the injection point within a permeable coarse sand layer; however, Mg(OH)2 transport in the overlying clayey-silty sand was minimal. Downgradient of the PRBs, colloidal Mg(OH)2 increased the pH of the coarse sand to levels appropriate for biological reductive dechlorination (pH >∼5); however, some settling of Mg(OH)2 in the injection wells generated persistent high pH (∼9–10) within the PRBs. A redesigned suspension of colloidal Mg(OH)2 was tested and proved to be more effective at raising aquifer pH without an excessive rise in pH within the PRBs. At PRB-1 (located closest to the DNAPL source area), limited TCE biodegradation was observed due to the influx of high TCE concentrations (up to 400 mg/L) and inhibition of dechlorinating bacteria. At PRB-2 (located 25 m downgradient of the DNAPL source area), TCE concentrations were much lower (13–26 mg/L) and production of cis-1,2-dichloroethene (cDCE) and some vinyl chloride (VC) was observed. Subsequent bioaugmentation with a commercial dechlorinating culture at PRB-2 improved conversion of cDCE to VC and ethene at downgradient monitoring wells over the duration of the study. These results emphasize the importance of PRB location (relative to the DNAPL source), base selection for pH adjustment, source strength, and local heterogeneities for the design and long-term performance of ERD in acidic DNAPL-impacted aquifers.

Remember Groundwater Flow

Groundwater flow is governed by groundwater hydraulics and is key to understanding the subsurface migration of groundwater, dissolved contaminants, and emplaced injection fluids. Yet groundwater flow appears to be largely forgotten or ignored in many subsurface injection-based remedial studies. Over the past decade, numerous studies of subsurface injection technologies (in situ oxidation, enhanced biodegradation, etc.) have been presented at conferences and published as peer-reviewed papers. These studies discuss the effectiveness of these remediation technologies and present chemical and biological data to support their results. However, rarely do these studies consider the impact of groundwater flow on data interpretation and their results. Often the studies do not even provide basic information on groundwater seepage velocities. I recently read a professional paper, an in situ oxidation (ISCO) study, that prompted me to write this editorial. I will use this unnamed study as an example, but I could equally use one of numerous other studies as an example. This ISCO study showed reductions in contaminant concentrations with some rebound and, in many respects, was well designed and well documented. The groundwater seepage velocity was reported to be about 46 ft/year. Three ISCO injection events were completed, during a period of about 9 months, at the source and at other injection locations extending about 30 ft downgradient from the source. The monitoring network consisted of three transects of discrete monitoring wells located 15, 45, and 75 ft downgradient from the source injection location. Evaluations of the effectiveness of the injections were completed based on a groundwater sampling event 3 months after the last injection. A final groundwater sampling event, which occurred 11 months after the last injection event, was used to evaluate rebound. Do you see what troubles me here? Consider this ISCO study and assume idealized horizontal groundwater flow while disregarding aquifer heterogeneity that complicates groundwater flow. Based on the distances of the monitoring transects (15, 45, and 75 ft downgradient) and the groundwater seepage velocity (46 ft/year), it would take approximately 119 (almost 4 months), 357, and 595 days for the injected ISCO fluid from the source injection to flow past the three monitoring transects, respectively. Yet this ISCO study evaluated the results and effectiveness of the ISCO injections based on groundwater samples collected 3 months after the last injection, before the injected fluid had migrated past the closest monitoring well transect. During the final sampling event, 11 months postinjection, the injected fluid would be expected to be found around two of the three well transects under these idealized groundwater flow conditions. The real world is not homogeneous and isotropic. Heterogeneity can result in varying migration rates for injected fluid and can affect the composition of well samples. Injected fluids in higher permeable zones will flow past downgradient monitoring wells faster than injected fluids in lower permeable zones. Groundwater samples reflect a combination of groundwater from the different permeability zones intersected by the well. If a groundwater sample is collected from a well that intersects both a higher permeable zone, where the original contaminated groundwater has displaced the injected fluid, and a lower permeable zone, which still has the noncontaminated injected fluid, the resulting groundwater sample would reflect a mixture of groundwater from these two zones. If, for example, 70% of the sample water comes from the higher permeable zone and 30% of the sample water comes from the lower permeable zone, the resulting groundwater sample would indicate a contaminant concentration 30% lower than the original groundwater contaminant concentration. This could be interpreted to suggest that the postinjection groundwater concentrations had declined by 30%. Groundwater flow needs to be understood and specifically addressed during studies of injection-based in situ remediation because groundwater flow is key to understanding the subsurface migration of groundwater and emplaced injection fluids. The characterization and heterogeneity of groundwater flow need to be integral to the design, data collection, and data interpretation of injection-based remedial studies if we wish to truly understand the effectiveness of our in situ injection remedial technologies. Therefore, for the design, implementation, interpretation, and presentation of studies involving injection based in situ remedial technologies, remember groundwater flow. Guy Swenson, BA, MS, Marcellus, NY; [email protected]

Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit.

We use uranium (U) isotope ratios to detect and quantify the extent of natural U reduction in groundwater across a roll front redox gradient. Our study was conducted at the Smith Ranch-Highland in situ recovery (ISR) U mine in eastern Wyoming, USA, where economic U deposits occur in the Paleocene Fort Union formation. To evaluate the fate of aqueous U in and adjacent to the ore body, we investigated the chemical composition and isotope ratios of groundwater samples from the roll-front type ore body and surrounding monitoring wells of a previously mined area. The (238)U/(235)U of groundwater varies by approximately 3‰ and is correlated with U concentrations. Fluid samples down-gradient of the ore zone are the most depleted in (238)U and have the lowest U concentrations. Activity ratios of (234)U/(238)U are ∼5.5 up-gradient of the ore zone, ∼1.0 in the ore zone, and between 2.3 and 3.7 in the down-gradient monitoring wells. High-precision measurements of (234)U/(238)U and (238)U/(235)U allow for development of a conceptual model that evaluates both the migration of U from the ore body and the extent of natural attenuation due to reduction. We find that the premining migration of U down-gradient of the delineated ore body is minimal along eight transects due to reduction in or adjacent to the ore body, whereas two other transects show little or no sign of reduction in the down-gradient region. These results suggest that characterization of U isotopic ratios at the mine planning stage, in conjunction with routine geochemical analyses, can be used to identify where more or less postmining remediation will be necessary.

Fingerprinting and Groundwater Model Application to Evaluate Hydraulic Barrier Efficiency (Italy)

Remediation actions at contaminated sites are based on multiple numerical model scenarios considering different parameter distributions, source positions and contaminant transport paths. In some cases the excess of scenarios is due to uncertainties in the conceptual model as a result of the spread of contamination through heterogeneities in the physical system. Reduction of project hypotheses and conceptual model uncertainty is therefore needed. This result can be achieved by coupling hydrogeological investigations with environmental forensic techniques, better localization of the source and understanding of contamination history. In this respect, in the present study, compositional fingerprinting and groundwater flow modeling were applied to a former oil storage facility where, even though a hydraulic barrier had been built to stop the hydrocarbon plume, the presence of some hydrocarbons was still found in downgradient monitoring wells. The final aim was to evaluate the efficacy of the hydraulic barrier and identify of the source of pollution. Fingerprinting results indicated pollution with a gasoline-diesel mixture much altered by water washing and/or biodegradation. Comparison of seven groundwater samples collected in wells and monitoring wells was performed by analyzing the volatile fraction (BTEX) and the total ion chromatogram (TIC), focusing attention on: n-alkanes (m/z 85), alkylcyclohexanes (m/z 83), isoprenoids (m/z 113), C4-alkylbenzenes (m/z 134), C3-C6 alkylbenzenes and polycyclic aromatic hydrocarbons (PAHs). The most probable scenario was then identified by combining the results of fingerprinting with different contaminant paths obtained using the numerical model.

Vegetative Treatment System Impacts on Groundwater Quality

<abstract><title><italic>Abstract.</italic></title> Increased environmental awareness has prompted the need for improved feedlot runoff control. Vegetative treatment systems (VTSs) provide a cost-effective option that may enhance environmental security by protecting water quality. Vegetative treatment systems are typically designed on the basis of hydraulic performance, which may result in excess application of some nutrients, specifically nitrogen and phosphorus. Groundwater quality monitoring is required to determine the effect, if any, that VTSs have on groundwater. Shallow groundwater (2 to 10 m) quality beneath six VTSs in Iowa was monitored over a four-year period. Monitoring wells were located upgradient, within, and downgradient of the VTSs. Groundwater samples were collected on a monthly basis and analyzed for ammoniacal nitrogen, chloride, nitrate-nitrogen, and fecal coliforms. A trend analysis was conducted to evaluate groundwater response patterns to VTS construction and use. In general, monitoring wells located within and downgradient of the VTS showed increasing trends in chloride and decreasing trends in nitrate concentrations. No trends for fecal coliforms or ammoniacal nitrogen were seen. Statistical analysis was performed to test for concentration differences between upgradient, within, and downgradient monitoring wells. In general, no differences in ammoniacal nitrogen concentration were seen. Fecal coliform concentrations were generally highest at the monitoring well within the VTS, but no difference was found between upgradient and downgradient concentrations. Chloride concentrations were generally significantly higher within and downgradient of the VTS when compared to the upgradient well; nitrate concentrations were generally significantly lower within and downgradient of the VTA than upgradient.

Performance of Deep-Rooted Phreatophytic Trees at a Site Containing Total Petroleum Hydrocarbons

Poplar and willow tree stands were installed in 2003 at a site in Raleigh, North Carolina containing total petroleum hydrocarbon – contaminated groundwater. The objective was groundwater uptake and plume control. The water table was 5 to 6 m below ground surface (bgs) and therefore methods were used to encourage deep root development. Growth rates, rooting depth and sap flow were measured for trees in Plot A located in the center of the plume and in Plot B peripheral to the plume. The trees were initially sub-irrigated with vertically installed drip-lines and by 2005 had roots 4 to 5 m bgs. Water balance calculations suggested groundwater uptake. In 2007, the average sap flow was higher for Plot B (∼59 L per day per tree) than for Plot A (∼23 L per day per tree), probably as a result of TPH-induced stress in Plot A. Nevertheless, the estimated rate of groundwater uptake for Plot A was sufficient, relative to the calculated rate of groundwater flux beneath the stand, that a high level of plume control was achieved based on MODFLOW modeling results. Down-gradient groundwater monitoring wells installed in late 2011 should provide quantitative data for plume control.

Application of enhanced bioremediation for TCE-contaminated groundwater: a pilot-scale study

The industrial solvent trichloroethylene (TCE) is among the most ubiquitous chlorinated compounds found in groundwater contamination. The objective of this pilot-scale study was to apply the combined biosparging and enhanced in situ bioremediation technology to remediate TCE-contaminated groundwater at a TCE-spill site. A biosparging well was installed inside the TCE plume for oxygen supplement. Primary substrate (cane molasses) was injected into the TCE plume through the biosparging well to enhance the rate of TCE co-metabolism. Three monitoring wells were installed in series downgradient of the biosparging well along the groundwater flow path. Results of polymerase chain reaction and nucleotide sequence analysis revealed that no appropriate TCE-degradation enzymes were observed in site groundwater. Thus, aerobic-activated sludge containing TCE-degraders collected from an industrial wastewater treatment plant were injected into the biosparging and three monitoring wells as an inoculum to provide microbial consortia for TCE biodegradation. After sludge injection, TCE-degraders (type I and II methanotrophs) and TCE-degrading enzymes (e.g. toluene monooxygenase, phenol monooxygenase) were detected in the injection and downgradient monitoring wells and remained in the aquifer during the 140-day pilot-scale study. Results indicate that significant TCE removal was observed (with TCE concentration dropped from 210 to 18 μg/L in substrate injection well). This reveals that appropriate substrates and inocula are required to effectively enhance the aerobic co-metabolic rate of TCE. Results from this study indicate that the enhanced in situ bioremediation is a promising technology to remediate TCE-contaminated groundwater.

Field assessment of carboxymethyl cellulose stabilized iron nanoparticles for in situ destruction of chlorinated solvents in source zones

This study pilot-tested carboxymethyl cellulose (CMC) stabilized zero-valent iron (ZVI) nanoparticles (with a trace amount of Pd catalyst) for in situ destruction of chlorinated ethenes such as perchloroethylene (PCE) and trichloroethylene (TCE) and polychlorinated biphenyls (PCBs) that had been in groundwater for decades. The test site was located in a well-characterized secondary source zone of PCBs and chlorinated ethenes. Four test wells were installed along the groundwater flow direction (spaced 5 ft apart), including one injection well (IW), one up-gradient monitoring well (MW-3) and two down-gradient monitoring wells (MW-1 and MW-2). Stabilized nanoparticle suspension was prepared on-site and injected into the 50-ft deep, unconfined aquifer. Approximately 150 gallons of 0.2 g/L Fe–Pd (CMC = 0.1 wt%, Pd/Fe = 0.1 wt%) was gravity-fed through IW-1 over a 4-h period (Injection #1). One month later, another 150 gallons of 1.0 g/L Fe–Pd (CMC = 0.6 wt%, Pd/Fe = 0.1 wt%) was injected into IW-1 at an injection pressure <5 psi (Injection #2). When benchmarked against the tracer, approximately 37.4% and 70.0% of the injected Fe was detected in MW-1 during injection #1 and #2, respectively, confirming the soil mobility of the nanoparticles through the aquifer, and higher mobility of the particles was observed when the injection was performed under higher pressure. Rapid degradation of PCE and TCE was observed in both MW-1 and MW-2 following each injection, with the maximum degradation being observed during the first week of the injections. The chlorinated ethenes concentrations gradually returned to their pre-injection levels after ∼2 weeks, indicating exhaustion of the ZVI's reducing power. However, the injection of CMC-stabilized nanoparticle and the abiotic reductive dechlorination process appeared to have boosted a long-term in situ biological dechlorination thereafter, which was evidenced by the fact that PCE and TCE concentrations showed further reduction after two weeks. After 596 days from the first injection, the total chlorinated ethenes concentration decreased by about 40% and 61% in MW-1 and MW-2, respectively. No significant long-term reduction of PCB 1242 was observed in MW-1, but a reduction of 87% was evident in MW-2. During the 596 days of testing, the total concentrations of cis-DCE (dichloroethylene) and VC (vinyl chloride) decreased by 20% and 38% in MW-1 and MW-2, respectively. However, the combined fraction of cis-DCE and VC in the total chlorinated ethenes (PCE, TCE, cis-DCE and VC) increased from 73% to 98% and from 62% to 98%, respectively, which supports the notion that biological dechlorination of PCE and TCE was active. It is proposed that CMC-stabilized ZVI-Pd nanoparticles facilitated the early stage rapid abiotic degradation. Over the long run, the existing biological degradation process was boosted with CMC as the carbon source and hydrogen from the abiotic/biotic processes as the electron donor, resulting in the sustained enhanced destruction of the chlorinated organic chlorinated ethenes in the subsurface.

Sulfur Isotopes as Indicators of Amended Bacterial Sulfate Reduction Processes Influencing Field Scale Uranium Bioremediation

Aqueous uranium (U(VI)) concentrations in a contaminated aquifer in Rifle Colorado have been successfully lowered through electron donor amended bioreduction. Samples collected during the acetate amendment experiment were analyzed for aqueous concentrations of Fe(ll), sulfate, sulfide, acetate, U(VI), and delta(34)S of sulfate and sulfide to explore the utility of sulfur isotopes as indicators of in situ acetate amended sulfate and uranium bioreduction processes. Enrichment of up to 7% per hundred in delta(34)S of sulfate in down-gradient monitoring wells indicates a transition to elevated bacterial sulfate reduction. A depletion in Fe(II), sulfate, and sulfide concentrations atthe height of sulfate reduction, along with an increase in the delta(34)S of sulfide to levels approaching the delta(34)S values of sulfate, indicates sulfate limited conditions concurrent with a rebound in U(VI) concentrations. Upon cessation of acetate amendment, sulfate and sulfide concentrations increased, while delta(34)S values of sulfide returned to less than -20% per hundred and sulfate delta(34)S decreased to near-background values, indicating lower levels of sulfate reduction accompanied by a corresponding drop in U(VI). Results indicate a transition between electron donor and sulfate-limited conditions at the height of sulfate reduction and suggest stability of biogenic FeS precipitates following the end of acetate amendment.

Seven‐year performance evaluation of a permeable reactive barrier

AbstractIn June and July 2001, the Massachusetts Department of Environmental Protection (MassDEP) installed a permeable reactive barrier (PRB) to treat a groundwater plume of chlorinated solvents migrating from an electronics manufacturer in Needham, Massachusetts, toward the Town of Wellesley's Rosemary Valley wellfield. The primary contaminant of concern at the site is trichloroethene (TCE), which at the time had a maximum average concentration of approximately 300 micrograms per liter directly upgradient of the PRB. The PRB is composed of a mix of granular zero‐valent iron (ZVI) filings and sand with a pure‐iron thickness design along its length between 0.5 and 1.7 feet. The PRB was designed to intercept the entire overburden plume; a previous study had indicated that the contaminant flux in the bedrock was negligible. Groundwater samples have been collected from monitoring wells upgradient and downgradient of the PRB on a quarterly basis since installation of the PRB. Inorganic parameters, such as oxidation/reduction potential, dissolved oxygen, and pH, are also measured to determine stabilization during the sampling process. Review of the analytical data indicates that the PRB is significantly reducing TCE concentrations along its length. However, in two discrete locations, TCE concentrations show little decrease in the downgradient monitoring wells, particularly in the deep overburden. Data available for review include the organic and inorganic analytical data, slug test results from nearby bedrock and overburden wells, and upgradient and downgradient groundwater‐level information. These data aid in refining the conceptual site model for the PRB, evaluating its performance, and provide clues as to the reasons for the PRB's underperformance in certain locations. © 2008 Wiley Periodicals, Inc.

Virus Transport Experiments in a Sandy Aquifer

The occurrence of human enteric viruses in ground water has been well documented in the literature. Bacteriophages such as MS-2 and PRD1 have properties similar to pathogenic human viruses suggesting that bacteriophages can be used as proxies for virus transport. The objective of this study is to investigate a “worst case scenario” for virus transport in a ground water aquifer, i.e., sand and gravel aquifer under a forced-gradient, by using bacteriophages. Field studies have been conducted to trace large-scale (34 m) and small-scale (10 m) virus transport under natural- and forced-gradients through a sand and gravel aquifer at a ground water research site at the Texas A&M University. Virus transport was monitored under natural and forced-gradient conditions using MS-2 and PRD-1 as virus tracers and bromide as a conservative tracer. Results indicate virus and bromide transport to down-gradient monitoring wells in both the large- and small-scale field tests. During the tests conducted, MS-2 transport appears to exhibit little longitudinal dispersion, showing a narrow peak at the well nest 34 m down-gradient in 13 days which is 20 days before the bromide breakthroughs, indicating that bacteriophage transport through the aquifer was mainly by advective flow. Differences in tracer transport can be attributed to the heterogeneity of the sand and gravel aquifer at the site, different injection methods, different sampling methods, and different tracer properties. Heterogeneity of the aquifer would cause virus transport through preferential flow paths.

Microbial Incorporation of 13C-Labeled Acetate at the Field Scale: Detection of Microbes Responsible for Reduction of U(VI)

A field-scale acetate amendment experiment was performed in a contaminated aquifer at Old Rifle, CO to stimulate in situ microbial reduction of U(VI) in groundwater. To evaluate the microorganisms responsible for microbial uranium reduction during the experiment, 13C-labeled acetate was introduced into well bores via bio-traps containing porous activated carbon beads (Bio-Sep). Incorporation of the 13C from labeled acetate into cellular DNA and phospholipid fatty acid (PLFA) biomarkers was analyzed in parallel with geochemical parameters. An enrichment of active sigma-proteobacteria was demonstrated in downgradient monitoring wells: Geobacter dominated in wells closer to the acetate injection gallery, while various sulfate reducers were prominent in different downgradient wells. These results were consistent with the geochemical evidence of Fe(III), U(VI), and SO(4)2- reduction. PLFA profiling of bio-traps suspended in the monitoring wells also showed the incorporation of 13C into bacterial cellular lipids. Community composition of downgradient monitoring wells based on quinone and PLFA profiling was in general agreement with the 13C-DNA result. The direct application of 13C label to biosystems, coupled with DNA and PLFA analysis,

Reliability assessment of groundwater monitoring networks at landfill sites

Landfills represent a significant threat to groundwater contamination due to their nature of operation and their abundance. Monitoring well networks at these sites are of vital importance in detecting leakage plumes. This study presents a reliability assessment to estimate the performance of groundwater monitoring systems at landfill sites. A hypothetical problem is presented where the detection probability of several monitoring systems is compared. A Monte–Carlo approach is used to incorporate uncertainties due to subsurface heterogeneity and the leak location. Hydraulic conductivity and leak location are considered as random variables with prescribed probability density functions. A finite difference groundwater model coupled with a random walk particle-tracking model simulates a contaminant plume released from the landfill for each Monte–Carlo realization. The analysis shows that lateral dispersivity of the medium has a significant influence on the reliability of the monitoring system, since it is the primary parameter controlling the width of the contaminant plume. Furthermore the number and the location of the monitoring wells are dependent on the heterogeneity of the medium and size of the contaminant leak. It is concluded that the reliability of the common practice of three downgradient monitoring wells is inadequate from the point of view of prevention of groundwater contamination due to landfills.

Estimation of Methyl tert -Butyl Ether Plume Length Using the Domenico Analytical Model

Methyl tert -butyl ether (MTBE) plume is controlled by many factors, primarily by groundwater flow velocity, dispersion, natural attenuation. This study employed an analytical model introduced by 3, J. Hydrol. 91, 49–58.) to describe the MTBE concentration distribution horizontal pattern and estimated the MTBE plume length. The model was applied to 90 leaking underground storage tank cases in Los Angeles, CA, U.S.A. The analytical model was calibrated with field data for each case using a Microsoft Excel spreadsheet program. Methyl tert -butyl ether concentrations in one source monitoring well and one to two downgradient centerline monitoring wells were used for each case study. When the centerline well is not available, the closest off-centerline wells were projected to the centerline using an ellipse trigonometry method. The model parameter values for longitudinal dispersivity, groundwater velocity, and degradation rate constant were calibrated using the field data and then used to estimate the maximum distance between source well and the plume edge. This study demonstrates that the Domenico model can be applied to MTBE plume investigation when adequate field data are available. The correlation coefficients calculated based on the results of the 90 case studies indicate that MTBE plume length has a poor correlation with MTBE concentration at the source well, and a moderate negative correlation with the degradation rate constant (−0.65) and λ/ν ratio (0.64). Furthermore, MTBE plume length has a poor correlation with the longitudinal dispersivity (−0.4), hydraulic gradient (−0.1), and groundwater velocity (0.17).

Estimation of Methyl tert -Butyl Ether Plume Length Using the Domenico Analytical Model

Methyl tert -butyl ether (MTBE) plume is controlled by many factors, primarily by groundwater flow velocity, dispersion, natural attenuation. This study employed an analytical model introduced by Domemico (1987, J. Hydrol 91 , 49-58.) to describe the MTBE concentration distribution horizontal pattern and estimated the MTBE plume length. The model was applied to 90 leaking underground storage tank cases in Los Angeles, CA, U.S.A. The analytical model was calibrated with field data for each ease using a Microsoft Excel spreadsheet program. Methyl tert -butyl ether concentrations in one source monitoring well and one to two downgradient centerline monitoring wells were used for each case study. When the centerline well is not available, the closest off-centerline wells were projected to the centerline using an ellipse trigonometry method. The model parameter values for longitudinal dispersivity, groundwater velocity, and degradation rate constant were calibrated using the field data and then used to estimate the maximum distance between source well and the plume edge. This study demonstrates that the Domenico model can be applied to MTBE plume investigation when adequate field data are available. The correlation coefficients calculated based on the results of the 90 case studies indicate that MTBE plume length has a poor correlation with MTBE concentration at the source well, and a moderate negative correlation with the degradation rate constant ( m 0.65) and u / v ratio (0.64). Furthermore, MTBE plume length has a poor correlation with the longitudinal dispersivity ( m 0.4), hydraulic gradient ( m 0.1), and groundwater velocity (0.17).