Determining effective diffusion coefficients of chlorohydrocarbons in natural clays: Unique results from highly resolved controlled release field experiments.

This study aims to precisely determine the effective diffusion coefficients of chlorohydrocarbons in low permeable units under in-situ field conditions. To this end, two controlled release field experiments using TCE and PCE as dense non-aqueous phase liquids (DNAPLs) were conducted in two natural clayey deposits. Several months to years after the controlled DNAPL release, highly resolved concentration profiles were determined for the chlorohydrocarbons that had diffused into the clayey deposits. Effective diffusion coefficients for TCE and PCE were then determined by calibrating a 3D numerical and 1D analytical model, respectively, to the measured high-resolution concentration profiles. The simulations revealed that the effective diffusion coefficients vary by as much as a factor of four within the same low permeability unit being consistent with observed small-scale heterogeneities. The determined chlorohydrocarbon effective diffusion coefficients were further used to determine the equivalent thickness of DNAPL that would completely dissolve in an idealized, parallel-plate fracture by diffusion transport into clayey deposits for the time periods of the controlled release field experiments. The equivalent TCE and PCE DNAPL film thicknesses ranged between 36 and 581 µm, respectively, comparable and exceeding fracture apertures measured in naturally fractured clay rich deposits. Hence, films of DNAPL initially contained within fractures in clay-rich deposits can completely dissolve away within a few months to a few years due to diffusion. This stored contaminant mass poses a risk to adjacent aquifers if it is re-released due to diffusion out of the matrix after source depletion or remediation.

Global Groundwater Solute Composition and Concentrations.

Informed analysis of policies related to food security, global climate change, wetland ecology, environmental nutrient flux, element cycling, groundwater weathering, continental denudation, human health, and others depends to a large extent on quantitative estimates of solute mass fluxes into and out of all global element pools including the enigmatic global aquifer systems. Herein for the first time, we proffer the mean global solute concentration of all major and selected minor and trace solutes in the active groundwater that represents 99% of liquid fresh water on Earth. Concentrations in this significant element pool have yielded to a geospatial machine learning kNN-nearest neighbors' algorithm with numerous geospatial predictors utilizing a large new lithology/climate/aquifer age/elevation based solute database. The predicted concentrations are consistent with traditional solute ratios, concentrations, and thermodynamic saturation indices.

Chlorinated Ethene Degradation Rate Coefficients Simulated with Intact Sandstone Core Microcosms.

Abiotic transformation of trichloroethene (TCE) in fractured porous rock such as sandstone is challenging to characterize and quantify. The objective of this study was to estimate the pseudo first-order abiotic reaction rate coefficients in diffusion-dominated intact core microcosms. The microcosms imitated clean flow through a fracture next to a contaminated rock matrix by exchanging uncontaminated groundwater, unamended or lactate-amended, in a chamber above a TCE-infused sandstone core. Rate coefficients were assessed using a numerical model of the microcosms that were calibrated to monitoring data. Average initial rate coefficients for complete dechlorination of TCE to acetylene, ethene, and ethane were estimated as 0.019 y-1 in unamended microcosms and 0.024 y-1 in lactate-amended microcosms. Moderately higher values (0.026 y-1 for unamended and 0.035 y-1 for lactate-amended) were obtained based on 13C enrichment data. Abiotic transformation rate coefficients based on gas formation were decreased in unamended microcosms after ∼25 days, to an average of 0.0008 y-1. This was presumably due to depletion of reductive capacity (average values of 0.12 ± 0.10 µeeq/g iron and 18 ± 15 µeeq/g extractable iron). Model-derived rate coefficients and reductive capacities for the intact core microcosms aligned well with results from a previous microcosm study using crushed sandstone from the same site.

Hydro-biogeochemical impacts of fugitive methane on a shallow unconfined aquifer.

Oil and gas development can result in natural gas migration into shallow groundwater. Methane (CH4), the primary component of natural gas, can subsequently react with solutes and minerals in the aquifer to create byproducts that affect groundwater chemistry. Hydro-biogeochemical processes induced by fugitive gas from leaky oil and gas wells are currently not well understood. We monitored the hydro-biogeochemical responses of a controlled natural gas release into a well-studied Pleistocene beach sand aquifer (Canadian Forces Base Borden, Ontario, Canada). Groundwater samples were collected before, during, and up to 700 days after gas injection and analyzed for pH, major and minor ions, alkalinity, dissolved gases, stable carbon isotope ratios of CO2 and CH4, and microbial community composition. Gas injection resulted in a dispersed plume of free and dissolved phase natural gas, affecting groundwater chemistry in two distinct temporal phases. Initially (i.e. during and immediately after gas injection), pH declined and major ions and trace elements fluctuated; at times increasing above baseline concentrations. Changes in the short-term were due to invasion of deep groundwater with elevated total dissolved solids entrained with the upward migration of free phase gas and, reactions that were instigated through the introduction of constituents other than CH4 present in the injected gas (e.g. CO2). At later times, more pronounced aerobic and anaerobic CH4 oxidation led to subtle increases in major ions (e.g. Ca2+, H4SiO4) and trace elements (e.g. As, Cr). Microbial community profiling indicated a persistent perturbation to community composition with a conspicuous ingrowth of taxa implicated in aerobic CH4 oxidation as well anaerobic S, N and Fe species metabolism.

Diffusion-Coupled Degradation of Chlorinated Ethenes in Sandstone: An Intact Core Microcosm Study.

Matrix diffusion must be considered when assessing natural attenuation and remediation of chlorinated ethenes in fractured porous bedrock aquifers. In this study, intact sandstone rock and groundwater from a trichloroethene (TCE)-contaminated site were used in microcosms (maintained for approximately 600 days) to simulate a single fracture-matrix system with a chamber at the top of the core allowing advection to represent fracture flow. Diffusion-coupled degradation with and without biostimulation were evaluated and compared to crushed-rock, batch microcosms. In the diffusion-transport microcosms, lactate stimulated reductive dechlorination of TCE to cis-1,2-dichloroethene (cDCE) and sulfate reduction. Reduction of TCE to cDCE led to a higher rate of chlorinated ethene removal from the cores, likely due to higher concentration gradients, along with lower sorption and a higher diffusion coefficient for cDCE relative to TCE. Reduction of cDCE to vinyl chloride or ethene did not occur as in crushed rock microcosms, inferring an absence of Dehalococcoides in the intact cores. Abiotic transformation was evident in the core microcosms based on the appearance of acetylene and enrichment in δ13C-TCE and δ13C-cDCE. Core microcosms permit a more realistic representation of the behavior of chlorinated ethenes in water-saturated fractured porous rock by incorporating the combined influence of fracture flow and matrix diffusion on transport and transformation.

Significant chemical fluxes from natural terrestrial groundwater rival anthropogenic and fluvial input in a large-river deltaic estuary.

The shores of the Pearl River estuary are home to 35 million people. Their wastes are discharged into the large river delta-front estuary (LDE), one of the most highly polluted systems in the world. Here we construct a radium reactive transport model to estimate the terrestrial groundwater discharge (TGD) into the highly urbanized Pearl River LDE. We find the TGD comprises only approximately 0.9% in term of water discharge compared to the river discharge. The TGD in the Pearl River LDE delivers significant chemical fluxes to the coast, which are comparable to the fluvial loadings from Pearl River and other world major rivers. Of particular importance is the flux of ammonium because of its considerable role in Pearl River estuary eutrophication and hypoxia. Unlike the ammonium in many other aquifers, the ammonium in the Pearl River aquifer system is natural and originated from organic matter remineralization by sulfate reduction in the extremely reducing environment. The TGD derived NH4+ is as much as 5% of the upstream Pearl River fluvial loading and 42% of the anthropogenic inputs. This high groundwater NH4+ flux may greatly intensify the eutrophication, shift the trophic states, and lead to alarming hypoxia within the affected ecosystems in the Pearl River LDE. The large TGD derived chemical fluxes will lead to deterioration of water and will potentially affect human health.

High resolution spatial and temporal evolution of dissolved gases in groundwater during a controlled natural gas release experiment.

Fugitive gas comprised primarily of methane (CH4) with traces of ethane and propane (collectively termed C1-3) may negatively impact shallow groundwater when unintentionally released from oil and natural gas wells. Currently, knowledge of fugitive gas migration, subsurface source identification and oxidation potential in groundwater is limited. To advance understanding, a controlled release experiment was performed at the Borden Research Aquifer, Canada, whereby 51m3 of natural gas was injected into an unconfined sand aquifer over 72days with dissolved gases monitored over 323days. During active gas injection, a dispersed plume of dissolved C1-3 evolved in a depth discrete and spatially complex manner. Evolution of the dissolved gas plume was driven by free-phase gas migration controlled by small-scale sediment layering and anisotropy. Upon cessation of gas injection, C1-3 concentrations increased to the greatest levels observed, particularly at 2 and 6m depths, reaching up to 31.5, 1.5 and 0.1mg/L respectively before stabilizing and persisting. At no time did groundwater become fully saturated with natural gas at the scale of sampling undertaken. Throughout the experiment the isotopic composition of injected methane (δ13C of -42.2‰) and the wetness parameter (i.e. the ratio of C1 to C2+) constituted excellent tracers for the presence of fugitive gas at concentrations >2mg/L. At discrete times C1-3 concentrations varied by up to 4 orders of magnitude over 8m of aquifer thickness (e.g. from <0.01 to 30mg/L for CH4), while some groundwater samples lacked evidence of fugitive gas, despite being within 10m of the injection zone. Meanwhile, carbon isotope ratios of dissolved CH4 showed no evidence of oxidation. Our results show that while impacts to aquifers from a fugitive gas event are readily detectable at discrete depths, they are spatially and temporally variable and dissolved methane has propensity to persist.

Improving estimates of groundwater velocity in a fractured rock borehole using hydraulic and tracer dilution methods.

A straddle-packer system for use in boreholes in fractured rock was modified to investigate the average linear groundwater velocity (v¯f) in fractures under ambient flow conditions. This packer system allows two different tests to be conducted in the same interval between packers without redeploying the system: (1) forced gradient hydraulic tests to determine the interval transmissivity (T), and (2) borehole dilution experiments to determine the groundwater flow rate (Qt) across the test interval. The constant head step test method provides assurance that flow is Darcian when determining T for each interval and identifies the flow rate at the onset of non-Darcian flow. The critical Reynolds number method uses this flow rate to provide the number of hydraulically active fractures (N) in each interval, the average hydraulic aperture for the test interval and the effective bulk fracture porosity. The borehole dilution method provides Qt values for the interval at the time of the test, and v¯f can be estimated from Qt using the flow area derived from the hydraulic tests. The method was assessed by application to seven test intervals in a borehole 73 m deep in a densely fractured dolostone aquifer used for municipal water supply. The critical Reynolds number method identified one or two fractures in each test interval (1.1 m long), which provided v¯f values in the range of 10 to 8000 m/day. This velocity range is consistent with values reported in the literature for ambient flow in this aquifer. However, when hydraulically active fractures in each interval is identified and measured from acoustic and optical televiewer logs, the calculated v¯f values are unreasonably low as are the calculated values of the hydraulic gradient needed to provide the Qt value for each tested interval. The combination of hydraulic and dilution tests in the same interval is an improved method to obtain values of groundwater velocity in fractured rock aquifers.

DFN-M field characterization of sandstone for a process-based site conceptual model and numerical simulations of TCE transport with degradation.

Plumes of trichloroethene (TCE) with degradation products occur at a large industrial site in California where TCE as a dense non-aqueous phase liquid (DNAPL) entered the fractured sandstone bedrock at many locations beginning in the late 1940s. Groundwater flows rapidly in closely spaced fractures but plume fronts are strongly retarded relative to groundwater flow velocities owing largely to matrix diffusion in early decades and degradation processes in later decades and going forward. Multiple data types show field evidence for both biotic and abiotic dechlorination of TCE and its degradation products, resulting in non-chlorinated compounds. Analyses were conducted on groundwater samples from hundreds of monitoring wells and on thousands of rock samples from continuous core over depths ranging from 6 to 426 metres below ground surface. Nearly all of the present-day mass of TCE and degradation products resides in the water-saturated, low-permeability rock matrix blocks. Although groundwater and DNAPL flow primarily occur in the fractures, DNAPL dissolution followed by diffusion and sorption readily transfers contaminant mass into the rock matrix. The presence of non-chlorinated degradation products (ethene, ethane, acetylene) and compound specific isotope analysis (CSIA) of TCE and cis-1,2-dichloroethene (cDCE) indicate at least some complete dechlorination by both biotic and abiotic pathways, consistent with the observed mineralogy and hydrogeochemistry and with published results from crushed rock microcosms. The rock matrix contains abundant iron-bearing minerals and solid-phase organic carbon with large surface areas and long contact times, suggesting degradation processes are occurring in the rock matrix. Multiple, high-resolution datasets provide strong evidence for spatially heterogeneous distributions of TCE and degradation products with varying degrees of degradation observed only when using new methods that achieve better detection of dissolved gases (i.e., Snap Sampler™) and contaminant mass stored in the low permeability rock matrix (i.e., CORE-DFN™). Simulations using a discrete fracture-matrix (DFN-M) numerical model capable of rigorously simulating flow and transport in both the fractures and matrix, including interactions, show that even slow, first-order degradation rates (i.e., 5- to 20-year half-lives) informed by site-derived parameters can contribute strongly to natural attenuation, resulting in TCE plumes that become stationary in space and might even retreat after 50 to 100 years, if the DNAPL sources become depleted due to the combination of diffusion and degradation processes.

New method for continuous transmissivity profiling in fractured rock.

A new method is presented to search for hydraulically transmissive features in open boreholes in bedrock. A flexible borehole liner made of a watertight, nylon fabric is filled with water to create a constant driving head to evert (reverse of invert) the liner down the hole so that the liner pushes the borehole water out into transmissive fractures or other permeable features. The descent rate is governed by the bulk transmissivity of the remaining permeable features below the liner. Initially, the liner descent rate or velocity is a measure of transmissivity (T) of the entire hole. As the everting liner passes and seals each permeable feature, changes in the liner velocity indicate the position of each feature and an estimate of T using the Thiem equation for steady radial flow. This method has been performed in boreholes with diameters ranging from 96 to 330 mm. Profiling commonly takes a few hours in holes 200- to 300-m long. After arrival of the liner at the bottom of the hole, the liner acts as a seal preventing borehole cross connection between transmissive features at different depths. Liner removal allows the hole to be used for other purposes. The T values determined using this method in a dolostone aquifer were found to be similar to the values from injection tests using conventional straddle packers. This method is not a replacement for straddle-packer hydraulic testing of specific zones where greater accuracy is desired; however, it is effective and efficient for scanning entire holes for transmissive features.

Contribution of the aquitard to the regional groundwater hydrochemistry of the underlying confined aquifer in the Pearl River Delta, China.

Aquitards are capable of generating and preserving large amounts of chemicals. The release of the chemicals from the aquitards poses a potential contamination risk to groundwater that may be used as a drinking water source. This work aimed to identify the contribution of hydrogeochemical processes in the aquitards to groundwater hydrochemistry in the underlying confined basal aquifer by studying the coastal Quaternary aquifer-aquitard system of the Pearl River Delta, China. The system was submerged by paleo-seawater in the early Holocene and mainly receives infiltration of precipitation at present, as indicated by investigations on stable isotopes (δ(2)H, δ(18)O), water chemistry (SO4(2-) and Cl(-)) and salinity. Significant correlations between total dissolved solids in the basal aquifer and the thickness of the overlying aquitard further suggested the contribution of the aquitard to the groundwater hydrochemistry in the aquifer. Significant correlations between the chloride concentrations in aquitard porewater and that in groundwater in the aquifer, and between the thickness of the aquitard and the chloride concentrations in groundwater indicated the strong influence of the aquitard on the chloride in the aquifer. This is probably because the low-permeability aquitard is capable of preserving the paleo-seawater in the aquifer and releasing the salinity from the aquitard down to the aquifer via downward flow or diffusion. Isotopic and geochemical studies revealed that the aquitard is also responsible for generating and preserving large amounts of naturally occurring ammonium. Analysis between the concentrations of ammonium in groundwater in the basal aquifer and the total available ammonium in aquitard sediments suggested that the former is significantly controlled by the latter.

Occurrence and geochemical behavior of arsenic in a coastal aquifer-aquitard system of the Pearl River Delta, China.

Elevated concentrations of arsenic, up to 161 µg/L, have been identified in groundwater samples from the confined basal aquifer underlying the aquitard of the Pearl River Delta (PRD). Both aquatic arsenic in pore water and solid arsenic in the sediments in the basal aquifer and aquitard were identified. Arsenic speciation of groundwater in the basal aquifer was elucidated on a pH-Eh diagram. In the PRD, arsenic is enriched in groundwater having both low and high salinity, and arsenic enriched groundwater is devoid of dissolved oxygen, has negative Eh values, is slightly alkaline, and has abnormally high concentrations of ammonium and dissolved organic carbon, but low concentrations of nitrate and nitrite. Results of geochemical and hydrochemical analyses and sequential extraction analysis suggest that reductive dissolution of iron oxyhydroxide could be one of the important processes that mobilized solid arsenic. We speculate that mineralization of sedimentary organic matter could also contribute to aquatic arsenic. Scanning electron microscope analysis confirms that abundant authigenic pyrite is present in the sediments. Sulphate derived from paleo-seawater served as the important sulfur source for authigenic pyrite formation. Co-precipitation of arsenic with authigenic pyrite significantly controlled concentrations of aquatic arsenic in the coastal aquifer-aquitard system.

Fingerprinting TCE in a bedrock aquifer using compound-specific isotope analysis.

A dual isotope approach based on compound-specific isotope analysis (CSIA) of carbon (C) and chlorine (Cl) was used to identify sources of persistent trichloroethylene (TCE) that caused the shut-down in 1994 of a municipal well in an extensive fractured dolostone aquifer beneath Guelph, Ontario. Several nearby industrial properties have known subsurface TCE contamination; however, only one has created a comprehensive monitoring network in the bedrock. The impacted municipal well and many monitoring wells were sampled for volatile organic compounds (VOCs), inorganic parameters, and CSIA. A wide range in isotope values was observed at the study site. The TCE varies between -35.6‰ and -21.8‰ and from 1.6‰ to 3.2‰ for δ(13) C and δ(37) Cl, respectively. In case of cis-1,2-dichloroethene, the isotope values range between -36.3‰ and -18.9‰ and from 2.4‰ to 4.7‰ for δ(13) C and δ(37) Cl, respectively. The dual isotope approach represented by a plot of δ(13) C vs. δ(37) Cl shows the municipal well samples grouped in a domain clearly separate from all other samples from the property with the comprehensive well network. The CSIA results collected under non-pumping and short-term pumping conditions thus indicate that this particular property, which has been studied intensively for several years, is not a substantial contributor of the TCE presently in the municipal well under non-pumping conditions. This case study demonstrates that CSIA signatures would have been useful much earlier in the quest to examine sources of the TCE in the municipal well if bedrock monitoring wells had been located at several depths beneath each of the potential TCE-contributing properties. Moreover, the CSIA results show that microbial reductive dechlorination of TCE occurs in some parts of the bedrock aquifer. At this site, the use of CSIA for C and Cl in combination with analyses of VOC and redox parameters proved to be important due to the complexity introduced by biodegradation in the complex fractured rock aquifer. It is highly recommended to revisit the study when the municipal well is back into full operation.

Using constant head step tests to determine hydraulic apertures in fractured rock.

The initial step in the analysis of contaminant transport in fractured rock requires the consideration of groundwater velocity. Practical methods for estimating the average linear groundwater velocity (v¯) in fractured rock require determination of hydraulic apertures which are commonly calculated by applying the cubic law using transmissivity (T) values and the number of hydraulically active fractures in the test interval. High-resolution, constant-head step injection testing of cored boreholes in a 100 m thick fractured dolostone aquifer was conducted using inflatable packers to isolate specific test intervals from the rest of the borehole. The steps in each test interval were gradually increased from very low to much higher injection rates. At smaller injection rates, the flow rate vs. applied pressure graph projects through the origin and indicates Darcian flow; non Darcian flow is evident at higher injection rates. Non-Darcian flow results in significantly lower calculated T values, which translates to smaller hydraulic aperture values. Further error in the calculated hydraulic aperture stems from uncertainty in the number of hydraulically active fractures in each test interval. This estimate can be inferred from borehole image and core logs, however, all of the fractures identified are not necessarily hydraulically active. This study proposes a method based on Reynolds number calculations aimed at improving confidence in the selection of the number of active fractures in each test interval.

Abnormally high ammonium of natural origin in a coastal aquifer-aquitard system in the Pearl River Delta, China.

High-nitrogen loadings of rivers and aquifers systems are a major concern because of potential effects on human health and water quality impacts such as eutrophication of lakes and coastal zones. This nitrogen enrichment is commonly attributed to anthropogenic sources such as sewage and agricultural and industrial wastes. The aims of this study were to delineate spatial distribution of groundwater ammonium in the coastal aquifer system in Pearl River Delta (PRD), China and to identify the origin of the abnormally high ammonium. A total of 40 boreholes were drilled to collect core samples of the aquitard and groundwater samples in the basal aquifer. The core samples were used for extraction of pore water for centrifugation and bulk chemical analyses in laboratory. Unlike previous studies which focused mainly on the aquifer, this study treated the aquifer-aquitard system as a hydrogeochemical continuum. The results show that the aquifer-aquitard system contains an exceptionally large total ammonium mass. Ammonium occurred at concentrations up to 390 mg/L in the basal sand Pleistocene aquifer 20-50 m deep, the largest concentration reported for groundwater globally. This ammonium was natural, areally extensive (1600 km(2)) and originated in the overlying Holocene-Pleistocene aquitard and entered the aquifer by groundwater transport and diffusion. Total ammonium in the aquifer (190 × 10(6) kg) was exceeded by total ammonium in the aquitard (8600 × 10(6) kg) by a factor of 45. Much organic nitrogen remained in the aquitard available for conversion to ammonium. This natural ammonium in the aquifer was slowly transported into the PRD river channels and the estuary of the South China Sea. The rate of this contribution will likely be greatly increased by sand dredging in the river channels and estuary. Although the ammonium in PRD groundwater occurred in the largest concentrations and mass reported globally, the literature shows no reports of other delta aquitards having been examined for ammonium occurrence and therefore abundant ammonium formed in aquitards rich in organic matter may not be uncommon and this "geologic" source of ammonium may present a large and hitherto unappreciated source of nitrogen discharging to surface waters.

A downhole passive sampling system to avoid bias and error from groundwater sample handling.

A new downhole groundwater sampler reduces bias and error due to sample handling and exposure while introducing minimal disturbance to natural flow conditions in the formation and well. This "In Situ Sealed", "ISS", or "Snap" sampling device includes removable/lab-ready sample bottles, a sampler device to hold double end-opening sample bottles in an open position, and a line for lowering the sampler system and triggering closure of the bottles downhole. Before deployment, each bottle is set open at both ends to allow flow-through during installation and equilibration downhole. Bottles are triggered to close downhole without well purging; the method is therefore "passive" or "nonpurge". The sample is retrieved in a sealed condition and remains unexposed until analysis. Data from six field studies comparing ISS sampling with traditional methods indicate ISS samples typically yield higher volatile organic compound (VOC) concentrations; in one case, significant chemical-specific differentials between sampling methods were discernible. For arsenic, filtered and unfiltered purge results were negatively and positively biased, respectively, compared to ISS results. Inorganic constituents showed parity with traditional methods. Overall, the ISS is versatile, avoids low VOC recovery bias, and enhances reproducibility while avoiding sampling complexity and purge water disposal.

Human enteric viruses in groundwater from a confined bedrock aquifer.

Confined aquifers are overlain by low-permeability aquitards that are commonly assumed to protect underlying aquifers from microbial contaminants. However, empirical data on microbial contamination beneath aquitards is limited. This study determined the occurrence of human pathogenic viruses in well water from a deep sandstone aquifer confined by a regionally extensive shale aquitard. Three public water-supply wells were each sampled 10 times over 15 months. Samples were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) for several virus groups and by cell culture for infectious enteroviruses. Seven of 30 samples were positive by RT-PCR for enteroviruses; one of these was positive for infectious echovirus 18. The virus-positive samples were collected from two wells cased through the aquitard, indicating the viruses were present in the confined aquifer. Samples from the same wells showed atmospheric tritium, indicating water recharged within the pastfew decades. Hydrogeologic conditions support rapid porous media transport of viruses through the upper sandstone aquifer to the top of the aquitard 61 m below ground surface. Natural fractures in the shale aquitard are one possible virus transport pathway through the aquitard; however, windows, cross-connecting well bores, or imperfect grout seals along well casings also may be involved. Deep confined aquifers can be more vulnerable to contamination by human viruses than commonly believed.

Groundwater-surface water interaction and its role on TCE groundwater plume attenuation.

A field investigation of a TCE plume in a surficial sand aquifer shows that groundwater-surface water interactions strongly influence apparent plume attenuation. At the site, a former industrial facility in Connecticut, depth-discrete monitoring along three cross-sections (transects) perpendicular to groundwater flow shows a persistent VOC plume extending 700 m from the DNAPL source zone to a mid-size river. Maximum TCE concentrations along a transect 280 m from the source were in the 1000s of microg/L with minimal degradation products. Beyond this, the land surface drops abruptly to a lower terrace where a shallow pond and small streams occur. Two transects along the lower terrace, one midway between the facility and river just downgradient of the pond and one along the edge of the river, give the appearance that the plume has strongly attenuated. At the river, maximum TCE concentrations in the 10s of microg/L and similar levels of its degradation product cis-DCE show direct plume discharge from groundwater to the river is negligible. Although degradation plays a role in the strong plume attenuation, the major attenuation factor is partial groundwater plume discharge to surface water (i.e. the pond and small streams), where some mass loss occurs via water-air exchange. Groundwater and stream mass discharge estimates show that more than half of the plume mass discharge crossing the first transect, before surface water interactions occur, reaches the river directly via streamflow, although river concentrations were below detection due to dilution. This study shows that groundwater and surface water concentration measurements together provide greater confidence in identifying and quantifying natural attenuation processes at this site, rather than groundwater measurements alone.