Solvent release into a sandy aquifer. 2. Estimation of DNAPL mass based on a multiple-component dissolution model

A chlorinated solvent mixture (2.0 L of trichloroethylene, 0.5 L of chloroform, and 2.5 L of tetrachloroethylene) was released into a sandy aquifer to create a heterogeneously distributed DNAPL (dense nonaqueous-phase liquid) source. The dissolution and dissolved-phase plume development from this source were studied in detail along a cross-section downgradient of the source for a period of approximately 1 year. At the conclusion of the experiment, the site was excavated to map the actual distribution of solvent residuals in the subsurface. Multiple-component dissolution theory provides a tool for the estimation of the mass of a multiple-component DNAPL source present in the groundwater. Concentration ratios between the compounds change with time, and those changes can be used to estimate the mass of DNAPL upgradient of the monitoring point(s) or well(s). The method is independent of the dilution occurring in the groundwater and only requires observations of time series of the contaminants in one or more monitoring points. For the field experiment, the method was applied using the measured concentrations of individual sampling points, the depth-integrated concentrations, the area-integrated concentrations, and the effluent concentrations of the cell. The experiment showed that multiple-component dissolution theory may be a valuable tool for the estimation of the mass of multiple-component DNAPL residuals in the saturated zone.     

Comparison between donor substrates for biologically enhanced tetrachloroethene DNAPL dissolution

Tetrachloroethene (PCE) dense nonaqueous-phase liquid (DNAPL) can act as a persistent groundwater contamination source for decades. Biologically enhanced dissolution of pure PCE DNAPL has potential for reducing DNAPL longevity as indicated previously (Environ. Sci. Technol. 2000, 34, 2979). Reported here are expanded studies to evaluate donor substrates that offer different remediation strategies for bioenhanced DNAPL dissolution, including pentanol (soluble substrate, fed continuously), calcium oleate (insoluble substrate, placed in column initially by alternate pumping of sodium oleate and calcium chloride), and olive oil (mixed with PCE and placed in column initially). Compared with a no-substrate column control, the DNAPL dissolution rate was enhanced about three times when directly coupled with biological transformation. The major degradation product formed was cDCE, but significant amounts of VC and ethene were also found with some columns. Extensive methanogenesis, which reduced PCE transformation, occurred in both the pentanol-fed and oleate-amended columns, but not in the olive-oil-amended column, suggesting that methanogens managed to colonize column niches where PCE DNAPL was not present. Detrimental methane production in the pentanol-fed column was nearly eliminated by presaturating the feed solution with PCE. These results suggest potential DNAPL remediation strategies to enhance dehalogenation while controlling competitive methanogenic utilization of donor substrates.     

Remediation of polycyclic aromatic hydrocarbon compounds in groundwater using poplar trees

A seven-year study was conducted to assess the effectiveness of hybrid poplar trees to remediate polycyclic aromatic hydrocarbon (PAH) compounds in soil and groundwater at a creosote-contaminated site. A reduction in the areal extent of the PAH plume was observed in the upper half of the 2-m-thick saturated zone, and PAH concentration levels in the groundwater declined throughout the plume. PAH concentrations began to decline during the period between the third and fourth growing seasons, which coincided with the propagation of the tree roots to the water table region. Remediation was limited to naphthalene and several three-ring PAHs (acenaphthylene and acenaphthene). PAH concentrations in soil and aquifer sediment samples also declined over time; however, levels of four-ring PAHs persisted at the lower depths during the study period. The naphthalene to total PAH concentration ratio in the most contaminated groundwater decreased from >0.90 at the beginning of the second growing season to approximately 0.70 at the end the study. Remediation in the lower region of the saturated zone was limited bythe presence of a 0.3-m-thick layer of creosote present as a dense nonaqueous phase liquid (DNAPL). The nearly steady-state condition of the PAH concentrations observed during the last three years of the study suggests that the effectiveness of the phytoremediation system is limited by the rate of PAH dissolution from the DNAPL source.     

Biological enhancement of tetrachloroethene dissolution and associated microbial community changes

A bench-scale study was performed to evaluate the enhancement of tetrachloroethene (PCE) dissolution from a dense nonaqueous phase liquid (DNAPL) source zone due to reductive dechlorination. The study was conducted in a pair of two-dimensional bench-scale aquifer systems using soil and groundwater from Dover Air Force Base, DE. After establishment of PCE source zones in each aquifer system, one was biostimulated (addition of electron donor) while the other was biostimulated and then bioaugmented with the KB1 dechlorinating culture. Biostimulation resulted in the growth of iron-reducing bacteria (Geobacter) in both systems as a result of the high iron content of the Dover soil. After prolonged electron donor addition methanogenesis dominated, but no dechlorination was observed. Following bioaugmentation of one system, dechlorination to ethene was achieved, coincident with growth of introduced Dehalococcoides and other microbes in the vicinity and downgradient of the PCE DNAPL (detected using DGGE and qPCR). Dechlorination was not detected in the nonbioaugmented system over the course of the study, indicating that the native microbial community, although containing a member of the Dehalococcoides group, was not able to dechlorinate PCE. Over 890 days, 65% of the initial emplaced PCE was removed in the bioaugmented, dechlorinating system, in comparison to 39% removal by dissolution from the nondechlorinating system. The maximum total ethenes concentration (3 mM) in the bioaugmented system occurred approximately 100 days after bioaugmentation, indicating that there was at least a 3-fold enhancement of PCE dissolution atthis time. Removal rates decreased substantially beyond this time, particularly during the last 200 days of the study, when the maximum concentrations of total ethenes were only about 0.5 mM. However, PCE removal rates in the dechlorinating system remained more than twice the removal rates of the nondechlorinating system. The reductions in removal rates over time are attributed to both a shrinking DNAPL source area, and reduced flow through the DNAPL source area due to bioclogging and pore blockage from methane gas generation.     

Reactive tracer tests to predict dense nonaqueous phase liquid dissolution dynamics in laboratory flow chambers

Reactive tracer tests were conducted to evaluate the relationship between contaminant mass reduction, Rm, and flux reduction, Rj, in laboratory experiments with porous media contaminated with a dense nonaqueous phase liquid (DNAPL). The reduction in groundwater contaminant flux resulting from partial mass removal was obtained from continuous and pulsed cosolvent and surfactant flushing dissolution tests in laboratory flow chambers packed with heterogeneous porous media. Using the streamtubes concept a Lagrangian analytical solution was applied to study the contaminant dissolution. The analytical solution was independently parametrized using nonreactive and reactive tracertests and the predicted dissolution was compared to the observed data. Analytical solution parameters related to aquifer hydrodynamic heterogeneities were determined from a nonreactive tracer, while those related to DNAPL spatial distribution heterogeneity were obtained from a reactive tracer. Reactive travel time variance, derived from this combination of tracers, was used to predict the relationship between Rm and Rj. Predictions based on the tracer tests closely matched measured dissolution data, suggesting that tracers can be used to characterize the DNAPL spatial distribution heterogeneity controlling the dissolution behavior. Experimental results demonstrated that increased reactive travel time variance led to greater flux reduction for a given partial mass removal.     

Field evaluation of the solvent extraction residual biotreatment technology

The Solvent Extraction Residual Biotreatment (SERB) technology was evaluated at a former dry cleaner site in Jacksonville, FL, where an area of tetrachloroethylene (PCE) contamination was identified. The SERB technology is a treatmenttrain approach for complete site restoration, which combines an active in situ dense nonaqueous-phase liquid (DNAPL) removal technology, cosolvent extraction, with a passive enhanced in situ bioremediation technology, reductive dechlorination. During the in situ cosolvent extraction test, approximately 34 kL of 95% ethanol/5% water (v:v) was flushed through the contaminated zone, which removed approximately 60% of the estimated PCE mass. Approximately 2.72 kL of ethanol was left in the subsurface, which provided electron donorfor enhancement of biological processes in the source zone and downgradient areas. Quarterly groundwater monitoring for over 3 yr showed decreasing concentrations of PCE in the source zone from initial values of 4-350 microM to less than 150 microM during the last sampling event. Initially there was little to no daughter product formation in the source zone, but after 3 yr, measured concentrations were 242 microM for cis-dichloroethylene (cis-DCE), 13 microM for vinyl chloride, and 0.43 microM for ethene. In conjunction with the production of dissolved methane and hydrogen and the removal of sulfate, these measurements indicate that in situ biotransformations were enhanced in areas exposed to the residual ethanol. First-order rate constants calculated from concentration data for individual wells ranged from -0.63 to -2.14 yr(-1) for PCE removal and from 0.88 to 2.39 yr(-1) for cis-DCE formation. First-order rate constants based on the change in total mass estimated from contour plots of the groundwater concentration data were 0.75 yr(-1) for cis-DCE, -0.50 yr(-1) for PCE, and -0.33 yr(-1) for ethanol. Although these attenuation rate constants include additional processes, such as sorption, dispersion, and advection, they provide an indication of the overall system dynamics. Evaluation of the groundwater data from the former dry cleaner site showed that cosolvent flushing systems can be designed and utilized to aid in the enhancement of biodegradation processes at DNAPL sites.     

Monitoring oxidation of chlorinated ethenes by permanganate in groundwater using stable isotopes: laboratory and field studies

Permanganate injection is increasingly applied for in situ destruction of chlorinated ethenes in groundwater. This laboratory and field study demonstrates the roles that carbon isotope analysis can play in the assessment of oxidation of trichloroethene (TCE) by permanganate. In laboratory experiments a strong carbon isotope fractionation was observed during oxidation of TCE with similar isotopic enrichment factors (-25.1 to -26.8 per thousand) for initial KMnO4 concentrations between 67 and 1,250 mg/L. At the field site, a single permanganate injection episode was conducted in a sandy aquifer contaminated with TCE as dense nonaqueous liquid (DNAPL). After injection, enriched delta13C values of up to +204% and elevated Cl- concentrations were observed at distances of up to 4 m from the injection point. Farther away, the Cl- increased without any change in delta13C of TCE suggesting that Cl- was not produced locally but migrated to the sampling point Except for the closest sampling location to the injection point, the delta13C rebounded to the initial 613C again, likely due to dissolution of DNAPL Isotope mass balance calculations made it possible to identify zones where TCE oxidation continued to occur during the rebound phase. The study indicates that delta13C values can be used to assess the dynamics between TCE oxidation and dissolution and to locate zones of oxidation of chlorinated ethenes that cannot be identified from the Cl- distribution alone.     

Biologically enhanced mass transfer of tetrachloroethene from DNAPL in source zones: experimental evaluation and influence of pool morphology

High-saturation pools of dense nonaqueous phase liquid (DNAPL) are long-term sources of groundwater contamination at many hazardous-waste sites. DNAPL pools consist of a high saturation zone with slow dissolution overlaid by a transition zone with lower saturations and more rapid dissolution. Effects of biological activity on pool dissolution must be understood to evaluate and implement bioremediation strategies. Bioenhanced dissolution of tetrachloroethene (PCE) in transition zones of high-saturation pools was investigated in a custom-designed 5-cm flow cell. Experiments were conducted to characterize mass transfer following DNAPL emplacement, with and without an active microbial culture capable of reductive dehalogenation. For average pool saturations < or = 0.55, mass transfer during biodegradation was enhanced by factors of 4-13, due primarily to high mass flux of PCE degradation products. However, at an average pool saturation of 0.74, mass transfer was enhanced by factors less than 1.5. Mass transfer was significantly greater from pools with an observable transition zone than without. Advective flow through multiphase transition zones enhanced dissolution and biological activity. These laboratory-scale experimental results suggest that biotechnologies may be effective remediation strategies for depletion of source zones within pool transition zones.     

Geochemical reactions resulting from in situ oxidation of PCE-DNAPL by KMnO4 in a sandy aquifer

Although the potential for KMnO4 to destroy chlorinated ethenes in situ was first recognized more than a decade ago, the geochemical processes that accompany the oxidation have not previously been examined. In this study, aqueous KMnO4 solutions (10-30 g/L) were injected into an unconfined sand aquifer contaminated by the dense non-aqueous-phase liquid (DNAPL) tetrachloroethylene (PCE). The effects of the injections were monitored using depth-specific, multilevel groundwater samplers, and continuous cores. Two distinct geochemical zones evolved within several days after injection. In one zone where DNAPL is present, reactions between KMnO4 and dissolved PCE resulted in the release of abundant chloride and hydrogen ions to the water. Calcite and dolomite dissolved, buffering the pH in the range of 5.8-6.5, releasing Ca, Mg, and CO2 to the pore water. In this zone, the aqueous Ca/Cl concentration ratio is close to 5:12, consistent with the following reaction for the oxidation of PCE in a carbonate-rich aquifer: 3C2Cl4 + 5CaCO3(s) + 4KMnO4 + 2H+ --> 11CO2 + 4MnO2(s) + H2O + 12Cl- + 5Ca2+ + 4K+. In addition to Mg from dolomite dissolution, increases in the concentration of Mg as well as Na may result from exchange with K at cation-exchange sites. In the second zone, where lesser amounts of PCE were present, KMnO4 persisted in the aquifer for more than 14 months, and the porewater pH increased graduallyto between 9 and 10 as a resultof reaction between KMnO4 and H2O. A small increase in SO4 concentrations in the zones invaded by KMnO4 suggests that KMnO4 injections caused oxidation of sulfide minerals. There are important benefits of carbonate mineral buffering during DNAPL remediation by in situ oxidation. In a carbonate-buffered system, Mn(VII) is reduced to Mn(IV) and is immobilized in the groundwater by precipitating as insoluble manganese oxide. Energy-dispersive X-ray spectroscopy analyses of the manganese oxide coatings on aquifer mineral grains have detected the impurities Al, Ca, Cl, Cu, Pb, P, K, Si, S, Ti, U, and Zn indicating that, similar to natural systems, precipitation of manganese oxide is accompanied by coprecipitation of other elements. In addition, the consumption of excess KMnO4 by reaction with reduced minerals such as magnetite will be minimized because the rates of these reactions increase with decreasing pH. Aquifer cores collected after the KMnO4 injections exhibit dark brown to black bands of manganese oxide reaction products in sand layers where DNAPL was originally present. Mineralogical investigations indicate that the manganese oxide coatings are uniformly distributed over the mineral grains. Observations of the coatings using transmission electron microscopy indicate that they are on the order of 1 microm thick, and consequently, the decrease in porosity through the formation of the coatings is negligible.     

Dissolution-induced contact angle modification in dense nonaqueous phase liquid/water systems

The contact angle between DNAPL, water, and aquifer material interfaces influences the spatial distribution of DNAPLs as they infiltrate into the aquifer, and may ultimately influence their remediation. The objective of this work was to evaluate the effects of dissolution on contact angle. Just as physically retracting a sessile drop reduces its contact angle with a surface, it was speculated that dissolution could cause contact angles to be reduced. Long-term dissolution experiments were conducted over the course of days to weeks, examining the dissolution of sessile drops of two DNAPLs, trichloroethylene (TCE) and tetrachloroethylene (PCE), in water and low concentration surfactant solutions, on glass surfaces. Experiments found that dissolution led to a continuous decrease of contact angle measured through the DNAPL drop, in most cases to near 0 degrees, far lower than angles achievable through measurements of receding contact angles for the same systems. Pinning of drop contact diameter was observed in most experiments. A model developed on the basis of the Bashforth-Adams equation to predict the effect of dissolution on contact angle for drops with a pinned contact diameter showed very good agreement with experimental observations.     

Pump-and-treat remediation of chlorinated solvent contamination at a controlled field-experiment site

Pump-and-treat (P&T) remediation and associated concentration tailing are investigated at the field scale in a mildly heterogeneous sandy aquifer through the extraction of dissolved chlorinated solvent plumes that had developed over 475 d from a multicomponent dense nonaqueous-phase liquid (DNAPL) source intentionally emplaced in the aquifer at the Borden (ON) research site. Extraction was accomplished via a source-containment well located 25 m from the source and two further downgradient plume-centerline wells to remove the advancing high-concentration dissolved plumes. The 550 days of detailed P&T field data demonstrated the following: remediation, albeit slowly, of the leading 25-60 m plume section to around typical drinking water standard concentrations; concentration tailing (reduction) over 4 orders of magnitude in the plume; a steady-state concentration "plateau" in the source-containment well capturing the steadily dissolving DNAPL source; influences of extraction rate changes (concentration rebounds); and, lengthy tailing from inter-well stagnation-zone areas. Much of the contaminant behavior during the P&T appeared to be "ideal" in the sense that with appropriate specification of the source term and pumping regime, it was reasonably predicted by 3-dimensional numerical model (HydroGeoSphere) simulations that assumed ideal (macrodispersion, linear sorption, etc.) transport. Supporting lab studies confirmed nonideal sorption was, however, important at the point sample scale with enhanced PCE (tetrachloroethene) sorption to low- and high-permeability strata and moderate nonlinear and competitive sorption influences. Although there was limited evidence of nonideal tailing contributions to the field data (underprediction of some tailing curve gradients), such contributions to P&T tailing were not easily discerned and appeared to play a relatively minor role within the mildly heterogeneous aquifer studied.     

Dehalorespiration model that incorporates the self-inhibition and biomass inactivation effects of high tetrachloroethene concentrations

In the vicinity of dense nonaqueous phase liquid (DNAPL) contaminant source zones, aqueous concentrations of tetrachloroethene (PCE) in groundwater may approach saturation levels. In this study, the ability of two PCE-respiring strains (Desulfuromonas michiganensis and Desulfitobacterium strain PCE1) to dechlorinate high concentrations of PCE was experimentally evaluated and depended on the initial biomass concentration. This suggests high PCE concentrations permanently inactivated a fraction of biomass, which, if sufficiently large, prevented dechlorination from proceeding. The toxic effects of PCE were incorporated into a model of dehalorespirer growth by adapting the transformation capacity concept previously applied to describe biomass inactivation by products of cometabolic TCE oxidation. The inactivation growth model was coupled to the Andrews substrate utilization model, which accounts for the self-inhibitory effects of PCE on dechlorination rates, and fit to the experimental data. The importance of incorporating biomass inactivation and self-inhibition effects when modeling reductive dechlorination of high PCE concentrations was demonstrated by comparing the goodness-of-fit of the Andrews biomass inactivation and three alternate models that do capture these factors. The new dehalorespiration model should improve our ability to predict contaminant removal in DNAPL source zones and determine the inoculum size needed to successfully implement bioaugmentation of DNAPL source zones.     

Barrier-controlled monitored natural attenuation

Three existing technologies (source containment, source reduction, and monitored natural attenuation) are integrated in barrier-controlled monitored natural attenuation (BCMNA)--a new approach for managing plumes of contaminated groundwater and remediating contaminated sites. The basic BCMNA concept uses a low-permeability, nonreactive barrier to release contaminants into an aquifer at a rate that optimizes natural attenuation. A simplified, one-dimensional model of the process is developed, and a hypothetical example of BCMNA is presented for a site contaminated with benzene. The analytical solution is used to demonstrate how contaminant concentrations can be controlled at a downgradient point of environmental compliance by manipulating design variables. BCMNA provides a greater degree of process control and risk reduction than monitored natural attenuation alone. BCMNA also holds promise for reducing remediation costs because (1) barriers can be constructed relatively inexpensively and (2) a cost-effective amount of source reduction can be applied inside the contained area with the BCMNA system remaining in place to safely complete the remediation process after source reduction is terminated. Further numerical modeling and a demonstration project are recommended to address important details and prove the concept.     

Field evaluation of in situ source reduction of trichloroethylene in groundwater using bioenhanced in-well vapor stripping

Two technologies in combination, cometabolic bioremediation and in-well vapor stripping, were applied to reduce trichloroethylene (TCE) concentrations in groundwater at a contaminant source area without the need to pump contaminated groundwater to the surface for treatment. The vapor-stripping well reduced source TCE concentrations (as high as 6-9 mg/L) by over 95%. Effluent from the well then flowed to two bioremediation wells, where additional reductions of approximately 60% were achieved. TCE removal was extensively monitored (for research and not regulatory purposes) using an automated system that collected samples about every 45 min at 55 locations over an area of approximately 50 x 60 m2. During 4.5 months of system operation, total TCE mass removal was 8.1 kg, 7.1 kg of which resulted from in-well vapor stripping and 1.0 kg from biotreatment. The system reduced the average TCE concentration of about 3000 microg/L in the source-zone groundwater to about 250 microg/L in water leaving the treatment zone, effecting greater than 92% TCE removal. A 6 month rebound study after system operation ceased found TCE concentrations then increased significantly in the treatment zone due to diffusion from the fractured rock below and perhaps other processes, with mass increases of about 1.5 kg in the lower aquifer and 0.3 kg in the upper aquifer.     

Simulation of the mobility of metal-EDTA complexes in groundwater: the influence of contaminant metals

Reactive transport simulations were conducted to model chemical reactions between metal-EDTA (ethylenediaminetetraacetic acid) complexes during transport in a mildly acidic quartz-sand aquifer. Simulations were compared with the results of small-scale tracer tests wherein nickel-, zinc-, and calcium-EDTA complexes and free EDTA were injected into three distinct chemical zones of a plume of sewage-contaminated groundwater. One zone had a large mass of adsorbed, sewage-derived zinc; one zone had a large mass of adsorbed manganese resulting from mildly reducing conditions created by the sewage plume; and one zone had significantly less adsorbed manganese and negligible zinc background. The chemical model assumed that the dissolution of iron(III) from metal-hydroxypolymer coatings on the aquifer sediments by the metal-EDTA complexes was kinetically restricted. All other reactions, including metal-EDTA complexation, zinc and manganese adsorption, and aluminum hydroxide dissolution were assumed to reach equilibrium on the time scale of transport; equilibrium constants were either taken from the literature or determined independently in the laboratory. A single iron(III) dissolution rate constant was used to fit the breakthrough curves observed in the zone with negligible zinc background. Simulation results agreed well with the experimental data in all three zones, which included temporal moments derived from breakthrough curves at different distances downgradient from the injections and spatial moments calculated from synoptic samplings conducted at different times. Results show that the tracer cloud was near equilibrium with respect to Fe in the sediment after 11 m of transport in the Zn-contaminated region but remained far from equilibrium in the other two zones. Sensitivity studies showed that the relative rate of iron(III) dissolution by the different metal-EDTA complexes was less important than the fact that these reactions are rate controlled. Results suggest that the published solubility for ferrihydrite reasonably approximates the Fe solubility of the hydroxypolymer coatings on the sediments. Aluminum may be somewhat more soluble than represented by the equilibrium constant for gibbsite, and its dissolution may be rate controlled when reacting with Ca-EDTA complexes.     

Destruction efficiencies and dynamics of reaction fronts associated with the permanganate oxidation of trichloroethylene

Although potassium permanganate (KMnO4) flushing is commonly used to destroy chlorinated solvents in groundwater, many of the problems associated with this treatment scheme have not been examined in detail. We conducted a KMnO4 flushing experiment in a large sand-filled flow tank (L x W x D = 180 cm x 60 cm x 90 cm) to remove TCE emplaced as a DNAPL in a source zone. The study was specifically designed to investigate cleanup progress and problems of pore plugging associated with the dynamics of the solid-phase reaction front (i.e., MnO2) using chemical and optical monitoring techniques. Ambient flow through the source zone formed a plume of dissolved TCE across the flow tank. The volume and concentration of TCE plume diminished with time because of the in situ oxidation of the DNAPL source. The migration velocity of the MnO2 reaction front decreased with time, suggesting that the kinetics of the DNAPL oxidation process became diffusion-controlled because of the pore plugging. A mass balance calculation indicated that only approximately 18% of the total applied KMnO4 (MnO4- = 1250 mg/ L) participated in the oxidation reaction to destroy approximately 41% of emplaced TCE. Evidently, the efficiency of KMnO4 flushing scheme diminished with time due to pore plugging by MnO2 and likely CO2, particularly in the TCE source zone. In addition, the excess KMnO4 used for flushing may cause secondary aquifer contamination. One needs to be concerned about the efficacy of KMnO4 flushing in the field applications. Development of a new approach that can provide both contaminant destruction and plugging/ MnO4- control is required.     

Influence of biodegradation processes on the duration of CaCO3 as a pH buffer in municipal solid waste incinerator bottom ash

The long-term leachability of heavy metals from municipal solid waste incinerator (MSWI) bottom ash is of concern because of its potential use as a secondary construction material. Calcite is the most important long-term buffer in MSWI bottom ash as it buffers solutions during percolation and is an important factor in the control of heavy-metal mobility. It has been argued that biodegradation of residual organic material in the MSWI is a significant source of acidity. Model calculations have therefore been carried out to determine the influence of biodegradation on the longevity of the calcite buffer. Using the program STEADYQL, which couples thermodynamic equilibrium with kinetically controlled reactions, solution composition was estimated at steady state. The concentration of Ca dissolved from calcite was estimated in the presence and absence of gypsum as a function of the reaction rate of a number of slow reactions: aerobic, ferrogenic, sulfogenic, and methanogenic biodegradation; diffusion of O2 into the system; degassing of CO2 out of the system; and dissolution of Ca silicate. It was found that, independent of the rate, the biodegradation of organic matter had little influence on the longevity of the calcite buffer (between 2,000 and 3,000 yr for a deposit of 1 m in depth), that anaerobic biodegradation may have a slight retarding effect, and that calcite dissolution due to acid input via precipitation was negligible (around 3% of the total at reference conditions for rainwater with a pH value of 4.3).     

Reductions in contaminant mass discharge following partial mass removal from DNAPL source zones

Although in situ remediation technologies have been used to aggressively treat dense nonaqueous phase liquid (DNAPL) source zones, complete contaminant removal or destruction is rarely achieved. To evaluate the effects of partial source zone mass removal on dissolved-phase contaminant flux, four experiments were conducted in a two-dimensional aquifer cell that contained a tetrachloroethene (PCE) source zone and down-gradient plume region. Initial source zone PCE saturation distributions, quantified using a light transmission system, were expressed in terms of a ganglia-to-pool ratio (GTP), which ranged from 0.16 (13.8% ganglia) to 1.6 (61.5% ganglia). The cells were flushed sequentially with a 4% (wt.) Tween 80 surfactant solution to achieve incremental PCE mass removal, followed by water flooding until steady-state mass discharge and plume concentrations were established. In all cases, the GTP ratio decreased with increasing mass removal, consistent with the observed preferential dissolution of PCE ganglia and persistence of high-saturation pools. In the ganglia-dominated system (GTP = 1.6), greater than 70% mass removal was required before measurable reductions in plume concentrations and mass discharge were observed. For pool-dominated source zones (GTP < 0.3), substantial reductions (>50%) in mass discharge were realized after only 50% mass removal.     

Decadal geochemical and isotopic trends for nitrate in a transboundary aquifer and implications for agricultural beneficial management practices

Nitrate contamination of aquifers is a global agricultural problem. Agricultural beneficial management practices (BMPs) are often promoted as a means to reduce nitrate contamination in aquifers through producer optimized management of inorganic fertilizer and animal manure inputs. In this study, decadal trends (1991-2004) in nitrate concentrations in conjunction with 3H/3He groundwater ages and nitrate stable isotopes (delta15N, delta18O) were examined to determine whether BMPs aimed at reducing aquifer-scale nitrate contamination in the transboundary Abbotsford-Sumas aquifer were effective. A general trend of increasing nitrate concentrations in young groundwater (< approximately 5 yr) suggested that voluntary BMPs were not having a positive impact in achieving groundwater quality targets. While the stable isotope data showed that animal manure was and still is the prevalent source of nitrate in the aquifer, a recent decrease in delta15N in nitrate suggests a BMP driven shift away from animal wastes toward inorganic fertilizers. The coupling of long-term monitoring of nitrate concentrations, nitrate isotopes, and 3H/3He age dating proved to be invaluable, and they should be considered in future assessments of the impact of BMPs on nutrients in groundwaters. The findings reveal that BMPs should be better linked to groundwater nutrient monitoring programs in order to more quickly identify BMP deficiencies, and to dynamically adjust nutrient loadings to help achieve water quality objectives.     

Fate and origin of 1,2-dichloropropane in an unconfined shallow aquifer

A shallow aquifer with different redox zones overlain by intensive agricultural activity was monitored for the occurrence of 1,2-dichloropropane (DCP) to assess the fate and origin of this pollutant. DCP was detected more frequently in groundwater samples collected in aerobic and nitrate-reducing zones than those collected from iron-reducing zones. Simulated DCP concentrations for groundwater entering an iron-reducing zone were calculated from a fate and transport model that included dispersion, sorption, and hydrolysis but not degradation. Simulated concentrations were well in excess of measured values, suggesting that microbial degradation occurred in the iron-reducing zone. Microcosm experiments were conducted using aquifer samples collected from iron-reducing and aerobic zones to evaluate the potential for microbial degradation of DCP and to explain field observations. Hydrogenolysis of DCP and production of monochlorinated propanes in microcosm experiments occurred only with aquifer materials collected from the iron-reducing zone, and no dechlorination was observed in microcosms established with aquifer materials collected from the aerobic zones. Careful analyses of the DCP/1,2,2-trichloropropane ratios in groundwater indicated that older fumigant formulations were responsible for the high levels of DCP present in this aquifer.