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The dynamic behavior of the freshwater-saltwater interface (FSI) in coastal aquifers can introduce unexpected lags between recharge and stream discharge, especially when recharge is forced by long-term cyclical precipitation patterns. This work seeks to assess these FSI impacts at the watershed scale. Recharge-discharge time lags were evaluated in 68 watersheds overlying the Floridan Aquifer System in the coastal region of the southeastern United States (Florida, Georgia, and South Carolina). Utilizing the strength of the Atlantic multidecadal oscillation (AMO) signal in this region, 10–20 year averaged recharge and discharge time series were used for the selected watersheds. Lags of 10–25 years between recharge and discharge were found in 16% of the basins considered, possibly induced by a dynamic FSI which responded slowly to the AMO-scale recharge signal. Freshwater storage coefficients (S) were estimated from time series of change-in-storage and groundwater level, with 11 basins showing S>1.5 indicating water storage well above that expected for unconfined aquifers. These 11 basins with both multidecadal recharge-discharge time lags and high S values showed a positive linear relationship between time lag and FSI depth with slope 0.016 yr/m (R-squared = 0.30). These large time lags may be directly impacting the management of these basins as they obscure water and solute mass balances in the southeastern US.

The effect of lowered groundwater surface tension using alcohols and surfactants, and increased viscosity using a thickener on the performance of the air sparging (AS) process was investigated, and...

Large volumes of per- and polyfluoroalkyl substances (PFAS)-contaminated wastewaters, such as municipal solid waste landfill leachates, pose a challenge for PFAS treatment technologies in practice today. In this study, the surfactant properties of PFAS were exploited to concentrate the compounds in foam produced via the bubble aeration of landfill leachate. The effectiveness of the foaming technique for concentrating PFAS varied by compound, with a mean removal percentage (the percent difference between PFAS in leachate before and after foam removal) of 69% and a median removal percentage of 92% among the 10 replicate foaming experiments. This technique appears to be similarly effective at sequestering sulfonates and carboxylate PFAS compounds and is less effective at concentrating the smallest and largest PFAS molecules. The results of this study suggest that for the pretreatment or preconcentration of landfill leachates, foaming to sequester PFAS may provide a practical approach that could be strategically coupled to high-energy PFAS-destructive treatment technologies. The process described herein is simple and could feasibly be applied at a relatively low cost at most landfills, where leachate aeration is already commonplace.

The surfactant-enhanced gas sparging process designed to specifically target the source zone of an organic contaminant in an aquifer with minimal usage of injected additives was investigated using a physical model. Aqueous solutions of the anionic surfactant Sodium dodecylbenzne sulfonate (SDBS) and/or the thickener Sodium carboxymethylcellulose (SCMC) were applied in a contaminated horizontal layer in the simulated laboratory aquifer model followed by gas sparging. Fluorescein sodium salt (FSS) was added to the SDBS/SCMC solutions and represented the organic contaminant. Air and ozone were injected to generate gas sparging. A modified surfactant-enhanced ozone sparging method was also tested by applying additional air venting ports installed in the aquifer above the gas injection zone. Both non-aqueous phase liquid (NAPL) and water-dissolved TCA were applied to the SDBS-applied region to evaluate the removal of contaminants during gas sparging. A significant expansion of the de-saturated zone for the SDBS-applied region was observed during air sparging. During ozone sparging, the fluorescence by FSS in the SDBS-applied layer disappeared over a much wider range than that of the control experiment. SCMC application enhanced the performance of the SDBS-applied gas sparging process. The TCA mass removed by volatilization during air sparging from the SDBS-applied layer was 2.3 times the application in the absence of SDBS. Among five regions of injected NAPL contamination located above the single gas injection port, and during 2 h of ozone sparging, with SDBS applied, more than 50% of fluorescence in the NAPL was removed, whereas under the same conditions with no SDBS applied, less than 30% was removed. Diverted gas flow through the venting ports installed in the aquifer model induced a horizontally expanded oxidative reaction zone during ozone sparging. This study demonstrates enhanced gas sparging performance for the removal of contaminants from the aquifer with limited usage of additives applied specifically to the source zone.

The effects of surface-tension and/or viscosity changes in groundwater on the remedial performance of air sparging for heterogeneous aquifers were investigated. The study used a one-dimensional (1-D) column and a two-dimensional (2-D) flow-chamber aquifer model. To introduce a heterogeneous setting, the middle part of the model was packed with a finer soil [LKZ, low hydraulic conductivity (K) zone]. Fluorescein sodium salt was used at 200 mg/L for all experiments as a surrogate contaminant. For the 1-D column experiments, the rate of fluorescence decay in the LKZ during surfactant-enhanced air sparging (SEAS) was significantly higher than during the standard air sparing (AS) process without additives; the area of fluorescence loss, measured after 17 h of air sparging (including ozone), was double and triple that of the conventional AS process, for SEAS without thickener (SEAS1) and SEAS with thickener (SEAS2), respectively. Experimental results using the 2-D chamber also confirmed the enhanced air intrusion into the LKZ during the SEAS process. The air fluxes through the LKZ increased by 47 and 103% for the SEAS1 and SEAS2 compared to AS, respectively; and 79 and 90% of fluorescence disappeared in the LKZ during ozone injection for SEAS1 and SEAS2, respectively, whereas only 10% disappeared for AS during the 3-h experimental period. The findings of this study indicate that the AS process, at low surface tension and increased groundwater viscosity may be a viable alternative to the conventional AS process for aquifers of heterogeneous hydrogeological formations.

This work enhances our understanding of catchment-scale N budgets by demonstrating the modification and application of a simple method for direct in situ measurements of vadose zone nitrate leaching and attenuation. We developed a soil passive flux meter (SPFM) to measure solute leaching based on a modified design of ion-exchange resin columns, and we tested the design in numerical simulations, laboratory experiments, plot-scale field experiments, and a catchment-scale field deployment. Our design minimized flow divergence around the resin column to attain nearly 100% capture of surface applied tracers in plot- and catchment-scale deployments. We found that mixing resin with native soil and extending the column height 10 cm above the resin layer minimized divergence of soil water around the column, resulting in a field-measured convergence factor (χ) of 1.3 that was consistent with numerical simulations. For catchment-scale testing, SPFMs were used at nine sites in three dominant land uses (crop, pasture, and turf) with known N inputs in two deployments, one during the 4-mo wet season and an additional set during the 8-mo dry season, to obtain integral annual measures of soil nitrate fluxes. In situ measured nitrate leaching determined from the SPFMs was positively correlated with known N inputs ( = 0.55, < 0.05) and attenuation averaged 67% (± 24% SD) of inputs across all sites. Although N inputs explain a large portion of the variability, our results emphasize the importance of both inter- and intra-land use variability in landscape-scale N budgets.

This paper introduces and tests the Sediment Bed Borehole Advection Method (SBBAM), a low cost, point-measurement technique which utilizes a push-point probe to quantify the vertical direction and magnitude of Darcy flux at the surface water&mdash, groundwater sediment interface. The Darcy flux measurements are derived from the residence-time analysis of tracer arrival calculated from measured tracer concentration time-series data. The technique was evaluated in the laboratory using a sediment bed simulator tank at eight flow rates (1&ndash, 90 cm/d). Triplicate test runs for each flow rate returned average errors between 4&ndash, 20 percent, r2 = 0.9977.

Source strength functions (SSF), defined as contaminant mass discharge or flux-averaged concentration from dense nonaqueous phase liquid (DNAPL) source zones as a function of time, provide a quantitative model of DNAPL source-zone behavior. Such information is useful for calibration of screening-level models to assist with site management decisions. We investigate the use of historic data collected during long-term monitoring (LTM) activities at a site in Rhode Island to predict the SSF based on temporal mass discharge measurements at a fixed location, as well as SSF estimation using mass discharge measurements at a fixed time from three spatially distributed control planes. Mass discharge based on LTM data decreased from ~300 g/day in 1996 to ~70 g/day in 2012 at a control plane downgradient of the suspected DNAPL source zone, and indicates an overall decline of ~80% in 16 years. These measurements were compared to current mass discharge measurements across three spatially distributed control planes. Results indicate that mass discharge increased in the downgradient direction, and was ~6 g/day, ~37 g/day, and ~400 g/day at near, intermediate, and far distances from the suspected source zone, respectively. This behavior was expected given the decreasing trend observed in the LTM data at a fixed location. These two data sets were compared using travel time as a means to plot the data sets on a common axis. The similarity between the two data sets gives greater confidence to the use of this combined data set for site-specific SSF estimation relative to either the sole use of LTM or spatially distributed data sets.

Transformation and retention of nitrogen and other biologically reactive solutes in the hyporheic zones of running water contribute to an essential ecosystem service. However, the synoptic impact of intense agricultural or urban land-uses, elevated nutrient loading, flow alterations, riparian clear-cutting, and channelization on the source-sink behavior of solutes in hyporheic zones remains largely uncharacterized and unquantified. Therefore, we studied nutrient dynamics in a hydromorphologically and chemically modified stream reach using a new monitoring approach allowing the simultaneous measurement of nutrient and water flux through a screened area in the subsurface of rivers (hyporheic passive flux meter, HPFM). With HPFMs we directly assessed time-integrated lateral hyporheic nitrate fluxes during early spring and midsummer covering different temperature and discharge regimes. Contrary to our expectations, higher stream discharge coincided with substantially lower hyporheic exchange rates. While in streams featuring a natural morphology, bed form induced exchange commonly increases with surface flow, the influence of groundwater level was dominant in this reach. Furthermore, in contrast to less impacted environments, where progressive substrate depletion with depths reduces metabolic rates in the subsurface, we identified not the upper, but the intermediate layer of the hyporheic zone as hot spot of nutrient turnover. Overall, the hyporheic zone at the study site functioned partly as nitrate source, partly as a sink. Neither of the commonly used determinants redox state and residence time could explain this source or sink function. Our results give clear evidence to carefully transfer the knowledge of hyporheic zone processes from “natural” systems to anthropologically modified streams.

This paper introduces the sediment bed passive flux meter (SBPFM) as a new tool for in situ measurements of vertical volumetric water and contaminant mass fluxes across hyporheic and hypolentic zones (i.e., stream or lake bed sediments that function as the contiguous zone between the overlying surface water body and the underlying aquifer). The device is a direct-push probe which contains a permeable internal sorbent located between two screened intervals. In the presence of a vertical hydraulic gradient the screens allow water flow through the SBPFM's internal sorbent matrix that is impregnated with resident tracers. These tracers are displaced from the sorbent at rates proportional to the water flux through the sorbent. At the same time, dissolved contaminants present in the intercepted sediment water are retained on the SBPFM sorbent at rates proportional to the ambient contaminant mass flux in the hyporheic zone. Potential flow theory is applied to convert observations of water and contaminant fluxes through the SBPFM into estimates of undisturbed ambient vertical water and contaminant fluxes in the sediment. To validate the theory and demonstrate the SBPFM as a potential site characterization tool, multiple bench-scale sediment bed experiments are performed. Results demonstrate that water and contaminant mass fluxes are accurately measured in the laboratory and that future field tests are warranted.

The exchange of solutes between aquifers and lower-permeability argillaceous formations is of considerable interest for solute and contaminant fate and transport. We present a synthesis of analytical solutions for solute diffusion between aquifers and single aquitard systems, validated in well-controlled experiments, and applied to several datasets from laboratory and field-scale problems with diffusion time and length scales ranging from 10−2-108 years and 10−2-102 m. One-dimensional diffusion models were applied using the method of images to consider the general cases of a finite aquitard bounded by two aquifers at the top and bottom, or a semi-infinite aquitard bounded by an aquifer. The simpler semi-infinite equations are appropriate for all domains with dimensionless relative diffusion length, ZD

A field demonstration was performed at Edwards Air Force Base to assess bioaugmentation for treatment of a well-characterized tetrachloroethene (PCE) dense nonaqueous phase liquid (DNAPL) source area in fractured rock. Groundwater recirculation was employed to deliver remedial amendments, including bacteria, to facilitate reductive dechlorination and enhance DNAPL dissolution. An active treatment period of 9 months was followed by a 10-month posttreatment rebound evaluation. Dechlorination daughter products were observed in both the shallow and deep fracture zones following treatment. In the shallow fracture zone, the calculated DNAPL mass removed was approximately equal to the DNAPL mass estimated using partitioning tracer testing, and no rebound in chlorinated ethenes or ethene was observed during the posttreatment period. A maximum DNAPL dissolution enhancement factor of 5 was observed in the shallow fracture zone. In the deep fracture zone, only approximately 45% of the DNAPL mass—as estimated via partitioning tracer testing—was removed and rebound in the total molar chlorinated ethenes + ethene was observed. The difference in behavior between the shallow and deep fracture zones was attributed to DNAPL architecture and the fracture flow field.

An innovative solution for groundwater velocity measurement in wells, the Direct Velocity Tool (DVT), is developed and tested. The DVT allows measurement of Darcy flux at centimeter scale vertical resolution requiring only a few minutes for each measurement (typically 5–10 min). Results are generated in real time through the use of an equation. The theoretical functional Darcy flux range is between 1 and 40 cm.day−1 for an aquifer with a hydraulic conductivity, K, less than 10−3 m.s−1. Laboratory tests showed a linear response between Darcy flux imposed and Darcy flux measured with the DVT, with a high correlation coefficient (R2 = 0.99). The DVT was tested for velocities ranging from 5 to 30 cm.day−1 where a higher standard deviation was observed for higher velocity (±6.46 cm.day−1 for 30 cm.day−1). At a hydraulically controlled field site, tracer tests, borehole dilutions, Passive Flux Meter (PFM) and DVT were tested under constant flow conditions. The PFM, tracer test and DVT technologies measured similar Darcy flux ranging from 10 to 17 cm.day−1. The DVT and PFM had uncertainties of 2.70 cm.day−1 and 2.50 cm.day−1, respectively. At a contaminated field site, velocities measured with the PFM range between 5 and 9 cm.day−1, while the DVT provided velocities of 4 to 8 cm.day−1. Velocity measurement showed similar results at the contaminated field site between PFM and DVT. The DVT offers an innovative solution for Darcy flux measurement and can be deployed easily. It provides the capability to characterize the vertical distribution of the horizontal velocity for a well in a period of 1–2 h. Keywords: Direct velocity tool, Darcy flux measurement, Laboratory and field comparison, Innovative

This paper presents the fundamental theory and laboratory test results on a new device that is deployed in boreholes in fractured rock aquifers to characterize vertical distributions of water and contaminant fluxes, aquifer hydraulic properties, and fracture network properties (e.g., active fracture density and orientation). The device, a fractured rock passive flux meter (FRPFM), consists of an inflatable core assembled with upper and lower packers that isolate the zone of interest from vertical gradients within the borehole. The outer layer of the core consists of an elastic fabric mesh equilibrated with a visible dye which is used to provide visual indications of active fractures and measures of fracture location, orientation, groundwater flux, and the direction of that flux. Beneath the outer layer is a permeable sorbent that is preloaded with known amounts of water soluble tracers which are eluted at rates proportional to groundwater flow. This sorbent also captures target contaminants present in intercepted groundwater. The mass of contaminant sorbed is used to quantify cumulative contaminant flux; whereas, the mass fractions of resident tracers lost are used to provide measures of water flux. In this paper, the FRPFM is bench tested over a range of fracture velocities (2–20 m/day) using a single fracture flow apparatus (fracture aperture = 0.5 mm). Test results show a discoloration in visible dye corresponding to the location of the active fracture. The geometry of the discoloration can be used to discern fracture orientation as well as direction and magnitude of flow in the fracture. Average contaminant fluxes were measured within 16% and water fluxes within 25% of known imposed fluxes.

The effects of controlled air flow paths during air sparging on the removal of volatile organic compounds were examined in this study using a two-dimensional bench-scale physical model. An aqueous solution of sodium carboxymethylcellulose (SCMC), which is a thickener, was used to increase the resistance of water to displacement by injected air in a region around the targeted zone. At the same time, an aqueous solution of sodium dodecylbenzene sulfonate (SDBS), which is a surfactant, was used to reduce the air entry pressure to enhance the air flow through the targeted region. Trichloroethene (TCE), dissolved in water, was used to represent an aqueous phase volatile organic compound (VOC). A binary mixture of perchloroethene (PCE) and n-hexane was also used as a nonaqeous phase liquid (NAPL). Controlled air flow through the source zone, achieved by emplacing a high viscosity aqueous solution into a region surrounding the TCE-impacted zone, resulted in increased TCE removal from 23.0% (control) to 38.2% during a 2.5h period. When the air flow was focused on the targeted source zone of aqueous phase TCE (by decreasing the surface tension within the source zone and its vicinity by 28 dyn/cm, no SCMC applied), the mass removal of TCE was enhanced to 41.3% during the same time period. With SCMC and SDBS applied simultaneously around and beneath a NAPL source zone, respectively, the NAPL components were found to be removed more effectively over a period of 8.2h than the sparging experiment with no additives applied; 84.6% of PCE and 94.0% of n-hexane were removed for the controlled air flow path experiments (with both SCMC and SDBS applied) compared to 52.7% (PCE) and 74.0% (n-hexane) removal for the control experiment (no additives applied). Based on the experimental observations made in this study, applying a viscous aqueous solution around the source zone and a surfactant solution in and near the source zone, the air flow was focused through the targeted contaminant zone, enhancing the removal of VOCs from either an aqueous phase or a NAPL phase.

Attenuation processes of chlorinated ethenes in complex near-stream systems result in site-specific outcomes of great importance for risk assessment of contaminated sites. Additional interdisciplinary and comprehensive field research is required to enhance process understanding in these systems. In this study, several methods were combined in a multi-scale interdisciplinary in-situ approach to assess and quantify the near-stream attenuation of a chlorinated ethene plume, mainly consisting of cis-dichloroethene (cis-DCE) and vinyl chloride (VC), discharging to a lowland stream (Grindsted stream, Denmark) over a monitoring period of seven years. The approach included: hydrogeological characterisation, reach scale contaminant mass balance analysis, quantification of contaminant mass discharge, streambed fluxes of chlorinated ethenes quantified using Sediment Bed Passive Flux Meters (SBPFMs), assessment of redox conditions, temporal assessment of contaminant concentrations, microbial analysis, and compound-specific isotope analysis (CSIA). This study site exhibits a special attenuation behaviour not commonly encountered in field studies: the conversion from an initially limited degradation case (2012-16), despite seemingly optimal conditions, to one presenting notable levels of degradation (2019). Hence, this study site provides a new piece to the puzzle, as sites with different attenuation behaviours are required in order to acquire the full picture of the role groundwater-surface water interfaces have in risk mitigation. In spite of the increased degradation in the near-stream plume core, the contaminant attenuation was still incomplete in the discharging plume. A conceptualization of flow, transport and processes clarified that hydrogeology was the main control on the natural attenuation, as short residence times of 0.5-37 days restricted the time in which dechlorination could occur. This study reveals the importance of: taking an integrated approach to understand the influence of all attenuation processes in groundwater - surface water interactions; considering the scale and domain of interest when determining the main processes; and monitoring sufficiently both spatially and temporally to cover the transient conditions.

Coastal aquifers are an important freshwater resource for significant portions of the global population. Understanding the dynamic relationship between aquifer recharge, storage and discharge is fundamental for sustainable groundwater management. Here we present an expanded freshwater balance equation that considers the dynamic interaction between fresh groundwater and underlying saltwater through a respective storage term, which may explain significant time lags in long-term trends between recharge, storage and discharge. We demonstrate the presence of such a time lag and its effect on freshwater budget calculations with data from a major karst spring in Florida. We develop and validate a parsimonious aquifer model and show that our expanded water balance is consistent with an observed time lag on the order of 15 years between multi-decadal averages of rainfall and spring discharge. Long-term trends in precipitation are known to occur in many regions of the world, and we expect our findings to have direct relevance for coastal aquifers at a global scale.

Aqueous solutions of micro-nano bubbles (MNBs) containing ozone gas were injected through a NAPL-contaminated glass bead column. The glass column (15 cm × 2.5 cm) was packed with glass beads: the first 12 cm was packed with coarse glass beads while much finer glass beads were used to pack the remaining 3.0 cm of the column. Decane was used as the representative NAPL, to which an oil-soluble fluorescence tracer was added. The fluorescence tracer was considered as a constituent of the NAPL that readily reacts with ozone. Air and ozone-containing oxygen were used to generate MNB solutions, and injected through the column. In addition, H2O2 was introduced to the O3-containing MNB (O3-MNB) solution to investigate the effect of hydroxyl free radicals on the NAPL removal. An ozone gas sparging experiment was also conducted for comparison. After 72 h of O3-MNB application, a significant mass of n-decane (27.6% of the initial mass applied) was removed from the column. H2O2 injection into the column during O3-MNB application was effective in increasing the n-decane mass removal by 22%, compared to the O3–MNB experiment. The rate of NAPL removal during O3-MNB flushing was significant, although slower than ozone sparging. During O3-MNB application, fast decay of fluorescence was observed; whereas, during co-injection of H2O2 and O3-MNB solutions, only a slight change in the fluorescence was observed. This indicates that oxidative degradation of NAPL during H2O2 and O3-MNB injection takes place only at the NAPL-water interface due to the reactivity of hydroxyl free radical, whereas ozone diffusion into NAPL induced the decay of the fluorescence tracer in the bulk NAPL. The removal characteristics during MNB application and ozone gas sparging were investigated based on the analysis of NAPL using mass spectrophotometer. When O3-MNB and H2O2 were co-injected, only n-decane was detected in the NAPL; while when O3-MNB was used for flushing, oxidative products were found in the NAPL. More hydrophilic compounds were found in the NAPL after ozone sparging. This implies different removal mechanisms depending on the kind of oxidation agent, and the state of oxidizing fluid. Based on the findings in this study, the application of O3-MNB could be a feasible option for cleaning up NAPL-contaminated aquifers.

Two important factors that affect groundwater contaminant persistence are the temporal pattern of contaminant source depletion and solute diffusion into and out of aquitards . This study provides a framework to evaluate the relative importance of these effects on contaminant persistence, with emphasis on the importance of thin aquitards. We developed one-dimensional (1D) analytical solutions for forward and back diffusion in a finite domain with a no flux boundary using the method of images and demonstrated their applicability to measured data from three well-controlled laboratory diffusion experiments with exponentially depleting sources. We used both in situ aquitard solute concentrations and aquifer breakthrough curves for sorbing and non-sorbing solutes. The finite-domain no flux boundary solutions showed better agreement with measured data than was available with semi-infinite approaches, with increasing discrepancy for dimensionless relative diffusion length scale beyond a critical threshold value ( Zd > 0.7). We also used a mass balance to demonstrate that the temporal pattern of contaminant source depletion controls the duration of solute mass accumulation in the aquitard, as well as the total solute mass release back into the aquifer. Lower rates of source depletion result in a longer period of mass accumulation in the aquitard and later back diffusion initiation time. The amount of solute mass stored in the aquitard increases with longer loading duration, thereby contributing to overall longer contaminant persistence in aquifers. This study entails widespread implications for anthropogenic waste and contamination sites, which are all dependent on efficient and cost-effective contaminant management strategies.

Passive flux meters (PFMs) have become invaluable tools for site characterization and evaluation of remediation performance at groundwater contaminated sites. To date, PFMs technology has been demonstrated in the field to measure midrange hydrophobic contaminants (e.g., chlorinated ethenes, fuel hydrocarbons, perchlorate) and inorganic ions (e.g., uranium and nitrate). However, flux measurements of low partitioning contaminants (e.g., 1,4-dioxane, hexahydro-1,3,5-trinitro-s-triazine (RDX)) and reactive ions-species (e.g., sulfate (SO42&minus, ), Chromium(VI) (Cr(VI)) are still challenging because of their low retardation during transport and quick transformation under highly reducing conditions, respectively. This study is the first application of PFMs for in-situ mass flux measurements of 1,4-dioxane, RDX, Cr(VI) and SO42&minus, reduction rates. Laboratory experiments were performed to model kinetic uptake rates and extraction efficiency for sorbent selections. Silver impregnated granular activated carbon (GAC) was selected for the capture of 1,4-dioxane and RDX, whereas Purolite 300A (Bala Cynwyd, PA, USA) was selected for Cr(VI) and SO42&minus, PFM field demonstrations measured 1,4-dioxane fluxes ranging from 13.3 to 55.9 mg/m2/day, an RDX flux of 4.9 mg/m2/day, Cr(VI) fluxes ranging from 2.3 to 2.8 mg/m2/day and SO42&minus, consumption rates ranging from 20 to 100 mg/L/day. This data suggests other low-partitioning contaminates and reactive ion-species could be monitored using the PFM.

This study assessed the long-term effectiveness of bioremediation as a remedial strategy for a chlorinated, ethene dense, non-aqueous phase liquid (DNAPL) source area, consisting of a higher- and a lower-permeability zone at Alameda Point, California. The evaluation was performed over 3.7 years after cessation of active source area bioremediation using passive flux meters (PFMs), push-pull tracer tests, and soil cores. PFMs showed that total chlorinated ethene molar discharge emanating from the source area remained relatively unchanged pre-and post-bioremediation, but molar discharge compositions shifted from trichloroethene (TCE) and cis -1,2-dichloroethene ( cis -DCE) to vinyl chloride (VC) and ethene dominated during post-remedial monitoring. First-order rate constants, derived from PFM data at the edge of the source area and describing the complete dechlorination of TCE at 3.7 years following active bioremediation, were approximately 1.05 yr −1 , which was over three times lower than the rate 3.6 yr −1 determined using compound stable isotope analysis (CSIA). Soil cores and push-pull tracer test data showed that DNAPL volume estimates were relatively unchanged pre- and post-bioremediation due to the remaining presence of DNAPL in the lower-permeability zone. These data suggest biotransformation processes are continuing in the higher-permeability zone, whereas DNAPL in the lower-permeability zone continues to serve as a significant source of groundwater contamination. The results suggest that it will take many years under current conditions to attain the United States Environmental Protection Agency (EPA) Maximum Contaminant Levels (MCLs) cleanup objectives.

A simple method, modified from White's method, was developed and verified for estimating seepage from two stormwater detention areas (SDAs) for 2 years, using night-time changes in surface water levels. The SDAs were located in warm sub-tropical Florida where the assumption of negligible night-time evaporation for White's method does not hold true. Daily seepage was estimated using the nocturnal water level fluctuations on no flow days during winter when evaporation losses were insignificant. Specific yield, rather than the composite specific yield, provided accurate seepage estimates. The average annual seepage from the two SDAs was 2.03 m/year. At almost 70% of surface flows, seepage is a significant contributor to regional surface and sub-surface flows. Comparison of seepage estimates from the night-time method (NM) and the water balance (WB) method showed that the NM-based estimates were within the range of the estimates from the WB method. At SDA1, the differences between the NM and WB estimates were 1% and 11%, for the 2 years. The discrepancy between the two estimates became higher (27% and 23%) at SDA2. Larger differences at SDA2 were because of higher error in quantifying pumped inflows for the WB method. Successful performance of NM combined with its low resource (single well monitoring) requirements will help quantify seepage from detention areas and other similar features (e.g. ponds, constructed wetlands) in warmer climates. A scale-up for the Caloosahatchee River basin showed that seepage from SDA's accounted for 15% of annual river flows indicating the importance of seepage in evaluating water and chemical balances. Copyright © 2015 John Wiley & Sons, Ltd.

The uncertainty of mass discharge measurements associated with point-scale measurement techniques was investigated by deriving analytical solutions for the mass discharge coefficient of variation for two simplified, conceptual models. In the first case, a depth-averaged domain was assumed, consisting of one-dimensional groundwater flow perpendicular to a one-dimensional control plane of uniformly spaced sampling points. The contaminant flux along the control plane was assumed to be normally distributed. The second case consisted of one-dimensional groundwater flow perpendicular to a two-dimensional control plane of uniformly spaced sampling points. The contaminant flux in this case was assumed to be distributed according to a bivariate normal distribution. The center point for the flux distributions in both cases was allowed to vary in the domain of the control plane as a uniform random variable. Simplified equations for the uncertainty were investigated to facilitate screening-level evaluations of uncertainty as a function of sampling network design. Results were used to express uncertainty as a function of the length of the control plane and number of wells, or alternatively as a function of the sample spacing. Uncertainty was also expressed as a function of a new dimensionless parameter, Ω, defined as the ratio of the maximum local flux to the product of mass discharge and sample density. Expressing uncertainty as a function of Ω provided a convenient means to demonstrate the relationship between uncertainty, the magnitude of a local hot spot, magnitude of mass discharge, distribution of the contaminant across the control plane, and the sampling density.

A novel approach using foam to manage hazardous waste was successfully demonstrated under active site conditions. The purpose of the foam was to divert groundwater flow, that would normally enter the source zone area, to reduce dissolved contaminant release to the aquifer . During the demonstration, foam was pre generated and directly injected surrounding the chlorinated solvent source zone. Despite the constraints related to the industrial activities and non-optimal position of the injection points, the applicability and effectiveness of the approach have been highlighted using multiple metrics. A combination of measurements and modelling allowed definition of the foam extent surrounding each injection point, and this appears to be the critical metric to define the success of the foam injection approach. Information on the transport of chlorinated solvents in groundwater showed a decrease of contaminant flux by a factor of 4.4 downstream of the confined area. The effective permeability reduction was maintained over a period of three months. The successful containment provides evidence for consideration of the use of foam to improve traditional flushing techniques, by increasing the targeting of contaminants by remedial agents.

Sorption is one of the most efficient and low cost strategies for volatile organic compound (VOC) removal, but VOC sorption by biochar has seen limited research. In this work, gas phase sorption experiments were conducted to determine the sorption potential and mechanisms of VOCs onto biochar. A total of 15 biochars produced from 5 common feedstocks at 300, 450, and 600°C were evaluated as sorbents. Three common VOCs (acetone, cyclohexane, and toluene) were chosen as sorbates. The results showed that all the tested biochars had VOC sorption capacity in the range of 5.58-91.2mgg-1. The sorption capacities were mainly influenced by both the surface area of biochar and its noncarbonized organic matter content. The vapor sorption process was exothermic, and the removal of VOCs by the biochars decreased with increasing feedstock temperature. Both the physical adsorption and partition mechanisms played important roles in controlling the VOC removal by the biochars. Biochar maintained its VOC removal ability after five consecutive sorption-desorption cycles, which indicated good reusability. Findings of this work suggest that biochar is a promising alternative sorbent for gaseous VOC removal.

The effect of groundwater viscosity control on the performance of surfactant-enhanced air sparging (SEAS) was investigated using 1- and 2-dimensional (1-D and 2-D) bench-scale physical models. The viscosity of groundwater was controlled by a thickener, sodium carboxymethylcellulose (SCMC), while an anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), was used to control the surface tension of groundwater. When resident DI water was displaced with a SCMC solution (500 mg/L), a SDBS solution (200 mg/L), and a solution with both SCMC (500 mg/L) and SDBS (200 mg/L), the air saturation for sand-packed columns achieved by air sparging increased by 9.5%, 128%, and 154%, respectively, (compared to that of the DI water-saturated column). When the resident water contained SCMC, the minimum air pressure necessary for air sparging processes increased, which is considered to be responsible for the increased air saturation. The extent of the sparging influence zone achieved during the air sparging process using the 2-D model was also affected by viscosity control. Larger sparging influence zones (de-saturated zone due to air injection) were observed for the air sparging processes using the 2-D model initially saturated with high-viscosity solutions, than those without a thickener in the aqueous solution. The enhanced air saturations using SCMC for the 1-D air sparging experiment improved the degradative performance of gaseous oxidation agent (ozone) during air sparging, as measured by the disappearance of fluorescence (fluorescein sodium salt). Based on the experimental evidence generated in this study, the addition of a thickener in the aqueous solution prior to air sparging increased the degree of air saturation and the sparging influence zone, and enhanced the remedial potential of SEAS for contaminated aquifers.

Understanding the effects of back diffusion of groundwater contaminants from low-permeability zones to aquifers is critical to making site management decisions related to remedial actions. Here, we combine aquifer and aquitard data to develop recommended site characterization strategies using a three-stage classification of plume life cycle based on the solute origins: aquifer source zone dissolution, source zone dissolution combined with back diffusion from an aquitard, and only back diffusion. We use measured aquitard concentration profile data from three field sites to identify signature shapes that are characteristic of these three stages. We find good fits to the measured data with analytical solutions that include the effects of advection and forward and back diffusion through low-permeability zones, and linearly and exponentially decreasing flux resulting from source dissolution in the aquifer. Aquifer contaminant time series data at monitoring wells from a mature site were well described using analytical solutions representing the combined case of source zone and back diffusion, while data from a site where the source had been isolated were well described solely by back diffusion. The modeling approach presented in this study is designed to enable site managers to implement appropriate remediation technologies at a proper timing for high- and low-permeability zones, considering estimated plume life cycle.

Aquitards can serve as long-term contaminant sources to aquifers when contaminant mass diffuses from the aquitard following aquifer source mass depletion. This study describes analytical and experimental approaches to understand reactive and nonreactive solute transport in a thin aquitard bounded by an adjacent aquifer. A series of well-controlled laboratory experiments were conducted in a two-dimensional flow chamber to quantify solute diffusion from a high-permeability sand into and subsequently out of kaolinite clay layers of vertical thickness 15 mm, 20 mm, and 60 mm. One-dimensional analytical solutions were developed for diffusion in a finite aquitard with mass exchange with an adjacent aquifer using the method of images. The analytical solutions showed very good agreement with measured breakthrough curves and aquitard concentration distributions measured in situ by light reflection visualization. Solutes with low retardation accumulated more stored mass with greater penetration distance in the aquitard compared to high-retardation solutes. However, because the duration of aquitard mass release was much longer, high-retardation solutes have a greater long-term back diffusion risk. The error associated with applying a semi-infinite domain analytical solution to a finite diffusion domain increases as a function of the system relative diffusion length scale, suggesting that the solutions using image sources should be applied in cases with rapid solute diffusion and/or thin clay layers. The solutions presented here can be extended to multilayer aquifer/low-permeability systems to assess the significance of back diffusion from thin layers.

Light reflection visualization (LRV) experiments were performed to investigate solute diffusion in low-permeability porous media using a well-controlled two-dimensional flow chamber with a domain composed of two layers (one sand and one clay). Two different dye tracers (Brilliant Blue FCF and Ponceau 4R) and clay domains (kaolinite and montmorillonite) were used. The images obtained through the LRV technique were processed to monitor two-dimensional concentration distributions in the low-permeability zone by applying calibration curves that related light intensity to equilibrium concentrations for each dye tracer in the clay. One dimensional experimentally-measured LRV concentration profiles in the clay were found to be in very good agreement with those predicted from a one-dimensional analytical solution, with coefficient of efficiency values that exceeded 0.97. The retardation factors ( R ) for both dyes were relatively large, leading to slow diffusive penetration into the clays. At a relative concentration C / C 0 = 0.1, Brilliant Blue FCF in kaolinite ( R = 11) diffused approximately 10 mm after 21 days of source loading, and Ponceau 4R in montmorillonite ( R = 7) diffused approximately 12 mm after 23 days of source loading. The LRV experimentally-measured two-dimensional concentration profiles in the clay were also well described by a simple analytical solution. The results from this study demonstrate that the LRV approach is an attractive non-invasive tool to investigate the concentration distribution of dye tracers in clays in laboratory experiments.

Groundwater flow discharging to springs from carbonate aquifers is governed by the interaction of slow matrix flow and fast fracture/conduit flow, which creates highly complex flow and transport conditions. An important unknown is the relative contribution of matrix and conduit flow to the total discharge. This study experimentally investigated groundwater fluxes in the Floridan aquifer within the springshed of Silver Springs, FL, one of the largest freshwater springs in the world with mean discharge of approximately 20 m3/s. Using in situ passive flux meters (PFMs, n = 48 at 16 wells) and a new karstic borehole dilution (KBHD, n = 21 at 7 wells) technique we measured groundwater fluxes in rock matrix and non-matrix (conduit and fracture) zones of 0.06 ± 0.003 m/day and 3.05 ± 1.8 m/day (mean ± standard error). These data, combined with previously conducted tracer tests (n = 12 at 3 sites), were coupled with simple analytical and numerical solutions to identify the proportion of the aquifer that contributes most significantly to water flow to the spring with three different modeling scenarios: single domain, dual domain including matrix and non-matrix zones, and triple domain including matrix, fracture, and conduit zones. The analytical and numerical models coupled with the in situ measured fluxes for the dual and triple domain scenarios showed good agreement with measured head profiles (Nash-Sutcliffe E > 0.90), when compared to the homogeneous porous domain scenario (E = −1.84). Conduit and fracture zones were estimated to represent between 2% and 22% of the aquifer cross-sectional area (at radial distance of 3 km from the spring outlet), yet these zones contributed between 75 and 96% of the total groundwater flow. The results of this study offer field-measured hydrogeologic data that can be used for active resource management in springsheds, and the simple modeling approach presented here may be applicable to other springsheds with fairly simple geometry to estimate the relative contributions of fast and slow water flow and solute transport pathways to the spring outlet.

The equilibrium streamtube model (EST) has demonstrated the ability to accurately predict dense nonaqueous phase liquid (DNAPL) dissolution in laboratory experiments and numerical simulations. Here the model is applied to predict DNAPL dissolution at a tetrachloroethylene (PCE)-contaminated dry cleaner site, located in Jacksonville, Florida. The EST model is an analytical solution with field-measurable input parameters. Measured data from a field-scale partitioning tracer test were used to parameterize the EST model and the predicted PCE dissolution was compared to measured data from an in-situ ethanol flood. In addition, a simulated partitioning tracer test from a calibrated, three-dimensional, spatially explicit multiphase flow model (UTCHEM) was also used to parameterize the EST analytical solution. The EST ethanol prediction based on both the field partitioning tracer test and the simulation closely matched the total recovery well field ethanol data with Nash–Sutcliffe efficiency E = 0.96 and 0.90, respectively. The EST PCE predictions showed a peak shift to earlier arrival times for models based on either field-measured or simulated partitioning tracer tests, resulting in poorer matches to the field PCE data in both cases. The peak shifts were concluded to be caused by well screen interval differences between the field tracer test and ethanol flood. Both the EST model and UTCHEM were also used to predict PCE aqueous dissolution under natural gradient conditions, which has a much less complex flow pattern than the forced-gradient double five spot used for the ethanol flood. The natural gradient EST predictions based on parameters determined from tracer tests conducted with a complex flow pattern underestimated the UTCHEM-simulated natural gradient total mass removal by 12% after 170 pore volumes of water flushing indicating that some mass was not detected by the tracers likely due to stagnation zones in the flow field. These findings highlight the important influence of well configuration and the associated flow patterns on dissolution.

[1] Estimation of water and contaminant discharges is an important hydrological problem. Fractured rock aquifers are recognized as highly complex flow and transport systems, and the fractured rock passive flux meter (FRPFM) is a recently tested device to simultaneously measure cumulative water and contaminant mass fluxes in fractures intersecting an observation well (boring). Furthermore, the FRPFM is capable of indicating orientations and directions of flow in hydraulically active (“flowing”) fractures. The present work develops a discharge estimator for when FRPFM measurements of fracture fluxes in the direction perpendicular to a transect (control plane) along one or more observation wells are available. In addition, estimation uncertainty in terms of a coefficient of variation is assessed based on a Monte Carlo approach under normalized conditions. Sources of uncertainty considered are spatially random fracture trace locations, random trace lengths, and orientations as well as variability of trace average fluxes (including smooth spatial trends), variability of local fluxes within traces, and flux measurement errors. Knowledge about the trace length distribution, which is commonly not available from borehole surveys, is not required for discharge estimation. However, it does affect the uncertainty assessment, and equations for upper uncertainty bounds are given as an alternative. In agreement with general statistical inference, it is found that discharge uncertainty decreases proportionally with the number of fluxes measured. Results are validated, and an example problem illustrates practical application and performance.

Mass depletion–mass flux relationships usually applied to a groundwater plume were established at field scale for groundwater pumped from within the source zone of a dense non-aqueous phase liquid (DNAPL). These were used as part of multiple lines of evidence in establishing the DNAPL source mass and architecture. Simplified source mass-dissolved concentration models including those described by exponential, power, and error functions as well as a rational mass equation based on the equilibrium stream tube approach were fitted to data from 285 days of source zone pumping (SZP) from a single well which removed 152 kg of dissolved organics from a multi-component, reactive brominated solvent DNAPL. The total molar concentration of the source compound, tetrabromoethane and its daughter products was used as a single measure of contaminant concentration to relate to source mass. A partitioning inter-well tracer test (PITT) conducted prior to the SZP provided estimates of groundwater travel times, enabling parameterisation of the models. After accounting for capture of the down-gradient dissolved plume, all models provided a good fit to the observed data. It was shown that differentiation between models would only emerge after appreciably more pumping from the source zone. The model fits were not particularly sensitive to the exponent parameters and variance of groundwater travel time. In addition, the multi-component nature of the DNAPL did not seem to affect the utility of the models for the period examined. Estimates of the DNAPL mass prior to the start of SZP from the models were greatest where the log of the variance of travel time was used explicitly in the source depletion models (mean 295 kg) compared to where the associated power exponent and variance was fitted freely (mean 258 kg). The estimates of source mass were close to that of 220 kg determined from the PITT. In addition to the PITT, multi-level groundwater sampling from within the source zone provided important supporting information for developing the conceptual model of the source zone. It is concluded that SZP may be an effective and relatively simple means for characterising DNAPL source zones.

The hyporheic zone is a hotspot of biogeochemical turnover and nutrient removal in running waters. However, due to methodological constraints, our quantitative knowledge on nutrient fluxes to those reactive zones is still limited. In groundwater systems passive flux meters, devices which simultaneously detect water and nutrient flows through a screen well in the subsurface, proofed to be valuable tools for load estimates. Here we present adaptations to this methodology and a smart deployment procedure which allow its use for investigating water and solute fluxes in river sediments. The new hyporheic passive flux meter (HPFM) delivers time integrating values of horizontal hyporheic nutrient fluxes for periods of several days up to weeks. Especially in highly heterogeneous environments like the hyporheic zone, measuring flow and nutrient concentration in a single device is preferable when compared to methods that derive flux estimates from separate measurements of water flows and chemical compounds. We constructed HPFMs of 50 cm length, separated in 5–7 segments which allowed for vertical resolution of horizontal nutrient and water transport in the range of 10 cm. The results of a seven day long field test, which included simultaneous measurements of oxygen and temperature profiles and manual sampling of pore water, revealed further advantages of the method: While grab sampling of pore water could not account for the high temporal variability of nitrate fluxes in our study reach, HPFMs accumulatively captured reliable values for the complete deployment time. Mass balances showed that more than 50 % of the nitrate entering the hyporheic zone was removed in the assessed area. Being low in costs and labor effective, many flux meters can be installed in order to capture larger areas of river beds. The extended application of passive flux meters in hyporheic studies has therefore the potential to deliver the urgently needed quantitative data which is required to feed into realistic models and lead to a better understanding of nutrient cycling in the hyporheic zone.

Solute diffusive exchange between low-permeability aquitards and high-permeability aquifers acts as a significant mediator of long-term contaminant fate. Aquifer contaminants diffuse into aquitards, but as contaminant sources are depleted, aquifer concentrations decline, triggering back diffusion from aquitards. The dynamics of the contaminant source depletion, or the source strength function, controls the timing of the transition of aquitards from sinks to sources. Here, we experimentally evaluate three archetypical transient source depletion models (step-change, linear, and exponential), and we use novel analytical solutions to accurately account for dynamic aquitard-aquifer diffusive transfer. Laboratory diffusion experiments were conducted using a well-controlled flow chamber to assess solute exchange between sand aquifer and kaolinite aquitard layers. Solute concentration profiles in the aquitard were measured in situ using electrical conductivity. Back diffusion was shown to begin earlier and produce larger mass flux for rapidly depleting sources. The analytical models showed very good correspondence with measured aquifer breakthrough curves and aquitard concentration profiles. The modeling approach links source dissolution and back diffusion, enabling assessment of human exposure risk and calculation of the back diffusion initiation time, as well as the resulting plume persistence.

While aquifer storage and recovery (ASR) is becoming widely accepted as a way to address water supply shortages, there are concerns that it may lead to release of harmful trace elements such as arsenic (As). Thus, mechanisms of As release from limestone during ASR operations were investigated using 110-day laboratory incubations of core material collected from the Floridan Aquifer, with treatment additions of labile or refractory dissolved organic matter (DOM) or microbes. During the first experimental phase, core materials were equilibrated with native groundwater lacking in DO to simulate initial non-perturbed anaerobic aquifer conditions. Then, ASR was simulated by replacing the native groundwater in the incubations vessels with DO-rich ASR source water, with DOM or microbes added to some treatments. Finally, the vessels were opened to the atmosphere to mimic oxidizing conditions during later stages of ASR. Arsenic was released from aquifer materials, mainly during transitional periods at the beginning of each incubation stage. Most As released was during the initial anaerobic experimental phase via reductive dissolution of Fe oxides in the core materials, some or all of which may have formed during the core storage or sample preparation period. Oxidation of As-bearing Fe sulfides released smaller amounts of As during the start of later aerobic experimental phases. Additions of labile DOM fueled microbially-mediated reactions that mobilized As, while the addition of refractory DOM did not, probably due to mineral sorption of DOM that made it unavailable for microbial utilization or metal chelation. The results suggest that oscillations of groundwater redox conditions, such as might be expected to occur during an ASR operation, are the underlying cause of enhanced As release in these systems. Further, ASR operations using DOM-rich surface waters may not necessarily lead to additional As releases.

Aquifer microbial communities can be investigated using Bio-traps(®) ("bio-traps"), passive samplers containing Bio-Sep(®) beads ("bio-beads") that are deployed in monitoring wells to be colonized by bacteria delivered via groundwater flow through the well. When bio-beads are "baited" with organic contaminants enriched in (13)C, stable isotope probing allows assessment of the composition and activity of the microbial community. This study used an ex situ system fed by groundwater continuously extracted from an adjacent monitoring well within an experimentally-created aerobic zone treating a tert-butyl alcohol (TBA) plume. The goal was to evaluate aspects of bio-trap performance that cannot be studied quantitatively in situ. The measured groundwater flow through a bio-trap housing suggests that such traps might typically "sample" about 1.8 L per month. The desorption of TBA or methyl tert-butyl ether (MTBE) bait from bio-traps during a typical deployment duration of 6 weeks was approximately 90% and 45%, respectively, of the total initial bait load, with initially high rate of mass loss that decreased markedly after a few days. The concentration of TBA in groundwater flowing by the TBA-baited bio-beads was estimated to be as high as 3400 mg/L during the first few days, which would be expected to inhibit growth of TBA-degrading microbes. Initial inhibition was also implied for the MTBE-baited bio-trap, but at lower concentrations and for a shorter time. After a few days, concentrations in groundwater flowing through the bio-traps dropped below inhibitory concentrations but remained 4-5 orders of magnitude higher than TBA or MTBE concentrations within the aquifer at the experimental site. Desorption from the bio-beads during ex situ deployment occurred at first as predicted by prior sorption analyses of bio-beads but with apparent hysteresis thereafter, possibly due to mass transfer limitations caused by colonizing microbes. These results suggest that TBA- or MTBE-baited bio-traps could be baited at lower initial total mass loading with no detriment to trapping ability. The bio-traps were able to collect detectable amounts of microbial DNA and thus allow some insight into the sparse microbial community present in the aquifer during remediation of the low concentration plume.

Aquifer storage and recovery (ASR) is an aquifer recharge technique in which water is injected in an aquifer during periods of surplus and withdrawn from the same well during periods of deficit. It is a critical component of the long-term water supply plan in various regions, including Florida, USA. Here, the viability of ASR as a safe and cost-effective water resource is currently being tested at a number of sites due to elevated arsenic concentrations detected during groundwater recovery. In this study, we developed a process-based reactive transport model of the coupled physical and geochemical mechanisms controlling the fate of arsenic during ASR. We analyzed multicycle hydrochemical data from a well-documented affected southwest Floridan site and evaluated a conceptual/numerical model in which (i) arsenic is initially released during pyrite oxidation triggered by the injection of oxygenated water (ii) then largely complexes to neo-formed hydrous ferric oxides before (iii) being remobilized during recovery as a result of both dissolution of hydrous ferric oxides and displacement from sorption sites by competing anions.

Innovative remediation studies were conducted between 1994 and 2004 at sites contaminated by nonaqueous phase liquids (NAPLs) at Hill and Dover AFB, and included technologies that mobilize, solubilize, and volatilize NAPL: air sparging (AS), surfactant flushing, cosolvent flooding, and flushing with a complexing-sugar solution. The experiments proved that aggressive remedial efforts tailored to the contaminant can remove more than 90% of the NAPL-phase contaminant mass. Site-characterization methods were tested as part of these field efforts, including partitioning tracer tests, biotracer tests, and mass-flux measurements. A significant reduction in the groundwater contaminant mass flux was achieved despite incomplete removal of the source. The effectiveness of soil, groundwater, and tracer based characterization methods may be site and technology specific. Employing multiple methods can improve characterization. The studies elucidated the importance of small-scale heterogeneities on remediation effectiveness, and fomented research on enhanced-delivery methods. Most contaminant removal occurs in hydraulically accessible zones, and complete removal is limited by contaminant mass stored in inaccessible zones. These studies illustrated the importance of understanding the fluid dynamics and interfacial behavior of injected fluids on remediation design and implementation. The importance of understanding the dynamics of NAPL-mixture dissolution and removal was highlighted. The results from these studies helped researchers better understand what processes and scales are most important to include in mathematical models used for design and data analysis. Finally, the work at these sites emphasized the importance and feasibility of recycling and reusing chemical agents, and enabled the implementation and success of follow-on full-scale efforts.

An automated on-line instrument has been developed to rapidly measure groundwater velocity within a screened well. The instrument consists of a carbon dioxide gas tracer that is periodically delivered to a permeable chamber located within a screened well. The rate of diffusion of the tracer through the wall of the permeable chamber was rapid and the effective diffusion into the groundwater was controlled by the mass transfer limitations at the groundwater/chamber interface with gas entrainment proportional to the groundwater velocity past the chamber. By periodically delivering the gas tracer and monitoring the reduction in concentration of the tracer from the permeable chamber, the groundwater velocity was determined multiple times daily. Laboratory experiments undertaken within a calibrated flow chamber have demonstrated that the instrument can be used to accurately and reliably determine groundwater flow velocities at 3h intervals for flow rates between 25 and 300 m y(-1). Field testing of the velocity probe at multiple well locations in a sandy aquifer gave velocities consistent with another monitoring technique (passive flux meter) and site modelling.

A partitioning interwell tracer test (PITT) was performed in the Borden sand aquifer to characterize an aged heterogeneous DNAPL source zone. This zone evolved during 5 years of natural groundwater flow following the infiltration of 50 L chlorinated solvents DNAPL. To assess the lateral variability of remaining DNAPL mass and morphology, four sweepzones were analyzed. The low saturation residual nature of the source zone required correction of tracer breakthrough data for natural background sorption. Corrected estimates of the DNAPL percentage remaining (total 13.2–16.6%), average saturation (0.05–0.18%) and distribution across the sweepzones were in good agreement with previous findings based on detailed transect monitoring, core analyses and ground-penetrating radar reflection. Using a newly defined metric “average spherical radius equivalent (ASRE)”, sweepzone estimates of the average size of DNAPL presence indicated the dominance of single pore DNAPL blobs and suggested the absence of DNAPL pools. Tracer tests indicated that DNAPL presence in the most DNAPL depleted sweepzone was potentially overestimated due to increased sediment sorption by residualized Sudan IV that was added to the DNAPL infiltrate. As hydrophobic compounds are normally present in spent solvent DNAPL, this suggests that additional sorption needs to be considered when using PITTs to characterize aged DNAPL source zones.

Standard methods for determining pollutant loads in streams typically require the collection of separate instantaneous measurements of water velocities and solute concentrations at discrete points in space and time. A recently developed device, the passive surface water flux meter (PSFM), has been introduced as an alternate method for the measurement of time-integrated surface water flux (velocity) and solute mass flux. This paper extends PSFM development by evaluating and comparing two PSFM designs in laboratory flumes, as well as reporting on initial steady-state field testing. The shape of the PSFM body determines the velocity with which water passes through the device, and different designs may thus be preferred for different applications. Experiments compared the accuracy of flux measurement by the previously introduced hydrofoil-shaped PSFM and nitrate-sorbing cartridge with that of a newly designed cylindrical-shaped PSFM and phosphate-sorbing cartridges. Testing was performed in a laboratory flume...

Key attributes of the source zone and the expanding dissolved plume at a trichloroethene (TCE) site in Australia were evaluated using trends in groundwater monitoring data along with data from on-line volatile organic compound (VOC) samplers and passive flux meters (PFMs) deployed in selected wells. These data indicate that: (1) residual TCE source mass in the saturated zone, estimated using two innovative techniques, is small ( approximately 10 kg), which is also reflected in small source mass discharge ( approximately 3 g/day); (2) the plume is disconnecting, based on TCE concentration contours and TCE fluxes in wells along a longitudinal transect; (3) there is minimal biodegradation, based on TCE mass discharge of approximately 6 g/day at a plume control plane approximately 175 m from source, which is also consistent with aerobic geochemical conditions observed in the plume; and (4) residual TCE in the vadose zone provides episodic inputs of TCE mass to the plume during infiltration/recharge events. TCE flux data also suggest that the small residual TCE source mass is present in the low-permeability zones, thus making source treatment difficult. Our analysis, based on a synthesis of the archived data and new data, suggests that source treatment is unwarranted, and that containment of the large TCE plume (approximately 1.2 km long, approximately 0.3 km wide; 17 m deep; approximately 2000-2500 kg TCE mass) or institutional controls, along with a long-term flux monitoring program, might be necessary. The flux-based site management approach outlined in this paper provides a novel way of looking beyond the complexities of groundwater contamination in heterogeneous domains, to make intelligent and informed site decisions based on strategic measurement of the appropriate metrics.

Surfactant-enhanced air sparging (SEAS) was evaluated in this laboratory-scale study to assess: (i) the removal efficiency of volatile contaminant from an aquifer model contrasted to conventional air sparging; and (ii) the effect of mass removal of dense non-aqueous phase liquid (DNAPL) during air sparging on the changes in aqueous flux of dissolved DNAPL. We conducted sparging experiments to remove perchloroethene (PCE) sources from laboratory flow chambers packed with sand. PCE was emplaced in rectangular zones at three locations within the flow chamber. The resident water was supplemented with the anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), to reduce the surface tension of water, and then sparged with nitrogen gas at a constant flow rate of 0.12 L/min. It was found that SEAS was significantly more efficient than conventional air sparging for removing PCE. For SEAS, about 78% and 75% of total PCE mass was depleted from the flow chamber at a surface tension of 52.2 dynes/cm (350 mg/L SDBS) and 63.1 dynes/cm (150 mg/L SDBS), respectively, whereas only 38% was removed at 72.5 dyne/cm (no SDBS added). Before and after sparging, PCE mass flux in the aqueous phase during steady water flow through the chamber was measured in the flow chambers. Post-SEAS PCE fluxes were reduced, but not in direct proportion to the reduction in PCE mass.