Bromide Tracer Experiment Research Articles

Consistent Modeling of Transport Processes and Travel Times—Coupling Soil Hydrologic Processes With StorAge Selection Functions

AbstractUnderstanding the transport processes and travel times of pollutants in the subsurface is crucial for an effective management of drinking water resources. Transport processes and soil hydrologic processes are inherently linked to each other. In order to account for this link, we couple the process‐based hydrologic model RoGeR with StorAge Selection (SAS) functions. We assign to each hydrological process a specific SAS function (e.g., power law distribution function). To represent different transport mechanisms, we combined a specific set of SAS functions into four transport model structures: complete‐mixing, piston flow, advection‐dispersion and advection‐dispersion with time‐variant parameters. In this study, we conduct modeling experiments at the Rietholzbach lysimeter, Switzerland. All modeling experiments are benchmarked with HYDRUS‐1D. We compare our simulations to the measured hydrologic variables (percolation and evapotranspiration fluxes and soil water storage dynamics) and the measured water stable isotope signal (18O) in the lysimeter seepage for a period of ten years (1997–2007). An additional virtual bromide tracer experiment was used to benchmark the models. Additionally, we carried out a sensitivity analysis and provided Sobol indices for hydrologic model parameters and SAS parameters. Our results indicate that the advection‐dispersion transport model produces the best results. And thus, advective‐dispersive transport processes play a dominant role at Rietholzbach lysimeter. Our modeling approach provides the capability to test hypotheses of different transport mechanisms and to improve process understanding and predictions of transport processes. Overall, the combined model allows a very effective simulation of combined flux and transport processes at various temporal and spatial scales.

Bromide reactivity in topsoil: Implications for use as a “conservative” tracer in assessing quantity and quality of water

AbstractBromide is a frequently used conservative tracer in soil leaching studies, including studies on contaminant leaching from arable topsoils. However, bromide often does not behave conservatively. Biogeochemists have known for many years that in natural soils, bromide is converted into organic bromine in a process known as bromination. However, bromination is seldom used as an explanation of non‐conservative leaching behavior by soil hydrologists. In a controlled small‐scale lysimeter study with arable soil we demonstrate such nonconservative behavior of bromide in opposition to control columns with fine gravel/coarse sand. By combining a literature review with the lysimeter study, we demonstrate the potential importance of bromination in topsoil and that bromination cannot be ignored, when interpreting bromide tracer experiments in arable soils. We also highlight the need for further studies on the processes of bromination and remineralization, to be able to account for these when conducting bromide leaching assessments.

Mineral Nanoparticle Aggregation Alters Contaminant Transport under Flow.

Iron oxyhydroxide nanoparticle reactivity has been widely investigated, yet little is still known on how particle aggregation controls the mobility and transport of environmental compounds. Here, we examine how aggregates of goethite (α-FeOOH) nanoparticle deposited on 100-300 μm quartz particles (GagCS) alter the transport of two emerging contaminants and two naturally occurring inorganic ligands-silicates and phosphates. Bromide tracer experiments showed no water fractionation into mobile and immobile water zones in an individual goethite-coated sand (GCS) column, whereas around 10% of the total water was immobile in a GagCS column. Reactive compounds were, in contrast, considerably more mobile and affected by diffusion-limited processes. A new simulation approach coupling the mobile-immobile equation with surface complexation reactions to surface reactive sites suggests that ∼90% of the binding sites were likely within the intra-aggregate zones, and that the mass transfer between mobile and immobile fractions was the rate-limited step. The diffusion-controlled processes also affected synergetic and competitive binding, which have otherwise been observed for organic and inorganic compounds at goethite surfaces. These results thereby call for more attention on transport studies, where tracer or conservative tests are often used to describe the reactive transport of environmentally relevant molecules.

Are Visible Fractures Accurate Predictors of Flow and Mass Transport in Fractured Till?

Tracer experiments conducted in the laboratory on undisturbed core samples (<7.3-cm-diameter) have been a standard method for estimating hydraulic and transport properties of fractured till since the 1980s. This study assesses the relationship between visible fractures on the top and bottom of core samples and the resulting hydraulic and mass transport properties of the core. We hypothesized that more visible fractures would indicate the presence of a well-connected fracture network, leading to greater hydraulic conductivity (K) values and earlier chemical breakthrough times. To test this hypothesis, water flow and bromide (Br-) tracer experiments were performed on 10, 16-cm diameter, 16-cm-tall samples of fractured Dows Formation till from central Iowa. Visually identifiable fractures were present on the top and bottom of every sample. Results indicate that the visual identification of fractures does not predict a connected fracture network, as some samples produced breakthrough curves showing rapid first arrival times and shapes characteristic of solute transport in a fractured medium, while others appeared similar to an unfractured medium. No correlation was found between the number of visible fractures and K (Pearson's r=0.25), or Br- first arrival time (r=-0.33), but a strong negative correlation between K and first arrival time (r=-0.92). Results indicate that the sample volume was not large enough to reliably contain a connected fracture network. Thus, testing large volumes of till at the field scale coupled with fracture-flow modeling likely represents the best approach for estimating hydraulic and mass transport properties for fractured till.

Continuous and High Transport of Particles and Solutes by Benthos in Coastal Eutrophic Sediments of the Pomeranian Bay

We present results on bioirrigation and reworking of sediments by benthic macrofauna in sediments of the Pomeranian Bay (southern Baltic Sea), that were obtained ~22 years ago. The investigation took place at four stations ranging from 9 – 19 m water depth, which we observed between 1993 and 1995 over the course of 30 months. In order to assess exchange of solutes across and particles from the sediment-water interface with the underlying sediments, we used bromide as ex situ tracer for bioirrigation and chlorophyll a equivalents as in situ tracer for particle reworking. Using models to interpret tracer distributions in the sediment, we compare the magnitudes of small scale, diffusion – like, versus relatively large-scale, non-local, transport modes. Our results indicate a spatial differentiation of the bay: the coastal station most heavily impacted by eutrophication close to the Oder River mouth showed medium reworking and intense bioirrigation combined with lower chlorophyll concentrations throughout the sediment. This contrasts with high surface pigment concentrations at the shallow Oder Bank station, indicating benthic primary production and intense particle mixing.Medium local particle reworking at the northwestern station and medium mean solute transport rates characterized the two deeper stations in the northern Bay. The bromide tracer experiments, which exclusively depict animal activity, showed significant biological solute transport to ~10 cm sediment depth within 3 days. Non-biological transport mechanisms in the field (e.g. resuspension, fishing activity) might additionally affect the in situ tracer chlorophyll a - depth distributions. This tracer, too, indicates mixing within the uppermost 10 cm of the sediment within ~2-3 months. In general, experimentally obtained solute transport constants were higher than most values reported previously (surface : 155 yr-1) and particle reworking was at the high end of reported values as well. Thus, benthic fauna is responsible for an intense bioturbation in Pomeranian Bay. Specifically, high bioirrigation rates were associated with high density and biomass of deep burrowing polychaetes. Recently published rates of particle reworking in the area are on the same order of magnitude as ours obtained two decades ago.This finding is consistent with the species composition which generally remained the same

FracKfinder: A MATLAB Toolbox for Computing Three-Dimensional Hydraulic Conductivity Tensors for Fractured Porous Media.

Fractures in porous media have been documented extensively. However, they are often omitted from groundwater flow and mass transport models due to a lack of data on fracture hydraulic properties and the computational burden of simulating fractures explicitly in large model domains. We present a MATLAB toolbox, FracKfinder, that automates HydroGeoSphere (HGS), a variably saturated, control volume finite-element model, to simulate an ensemble of discrete fracture network (DFN) flow experiments on a single cubic model mesh containing a stochastically generated fracture network. Because DFN simulations in HGS can simulate flow in both a porous media and a fracture domain, this toolbox computes tensors for both the matrix and fractures of a porous medium. Each model in the ensemble represents a different orientation of the hydraulic gradient, thus minimizing the likelihood that a single hydraulic gradient orientation will dominate the tensor computation. Linear regression on matrices containing the computed three-dimensional hydraulic conductivity (K) values from each rotation of the hydraulic gradient is used to compute the K tensors. This approach shows that the hydraulic behavior of fracture networks can be simulated where fracture hydraulic data are limited. Simulation of a bromide tracer experiment using K tensors computed with FracKfinder in HGS demonstrates good agreement with a previous large-column, laboratory study. The toolbox provides a potential pathway to upscale groundwater flow and mass transport processes in fractured media to larger scales.

Inverse Estimation of Soil Hydraulic and Transport Parameters of Layered Soils from Water Stable Isotope and Lysimeter Data

Core Ideas To determine the water retention curve from inverse modeling, θ and ψ need to be monitored. δ18O ratios contained information to inversely estimate soil hydraulic parameters. Different observation types should be combined in a single OF to estimate parameters. Averages of local measurement could be described using effective parameters. Accurate measurement of fluxes at lysimeter boundaries improved model parameterization. Accurate estimates of soil hydraulic parameters and dispersivities are crucial to simulate water flow and solute transport in terrestrial systems, particularly in the vadose zone. However, parameters obtained from inverse modeling can be ambiguous when identifying multiple parameters simultaneously and when boundary conditions are not well known. Here, we performed an inverse modeling study in which we estimated soil hydraulic parameters and dispersivities of layered soils from soil water content, matric potential, and stable water isotope (δ18O) measurements in weighable lysimeter systems. We used different optimization strategies to investigate which observation types are necessary for simultaneously estimating soil hydraulic and solute transport parameters. Combining water content, matric potential, and tracer (e.g., δ18O) data in one objective function (OF) was found to be the best strategy for estimating parameters that can simulate all observed water flow and solute transport variables. A sequential optimization, in which first an OF with only water flow variables and subsequently an OF with transport variables was optimized, performed slightly worse indicating that transport variables contained additional information for estimating soil hydraulic parameters. Hydraulic parameters that were obtained from optimizing OFs that used either water contents or matric potential could not predict non‐measured water flow variables. When a bromide (Br−) tracer experiment was simulated using the optimized parameters, the arrival time of the bromide pulse was underestimated. This suggested that Br− sorbed onto clay minerals and amorphous oxides under the prevailing geochemical conditions with low pH values. When accounting for anion adsorption in the simulation, Br− concentrations were well predicted, which validated the dispersivity parameterization.

Solute transport and nitrate removal in full-scale subsurface flow constructed wetlands of various designs treating agricultural drainage water

Subsurface flow constructed wetlands (SSF-CWs) consisting of woodchip based filter matrixes are promising measures targeting agricultural N loss via subsurface tile drains. Optimization of these systems may include the selection of appropriate hydraulic designs (i.e. horizontal and vertical flow), which may affect the solute residence time. In this study a bromide tracer experiment was performed in three full scale SSF-CWs, consisting of a woodchip-based filter matrix. Three different hydraulic designs (horizontal (H), vertical upward (Vup) and vertical down flow (Vdown)) and two flow rates were investigated (0.49 and 1.83Ls−1). Additionally, batch experiments investigating the intra-granular diffusion into woodchips using two tracer solutes (tritium and bromide) were carried out. Non-equilibrium solute transport, including a mass exchange between a mobile and an immobile domain, was found in all SSF-CWs. The Vup demonstrated the most pronounced non-equilibrium and the lowest N removal rate. In contrast the largest N removal rate was observed in the Vdown. The higher NO3 removal rates were attributed to a longer solute residence time. Tailing of the tracer BTC indicated the influence of diffusive exchange in solute residence time, and this was further supported by the intra-granular diffusion of tracer solutes. Generally, the results suggested the vertical downwards SSF-CW as the best performing SSF-CW in terms of solute transport behaviour and N removal efficiency.

Monitoring and engineering geology analysis of the Zhangmu landslide in Tibet, China

The Zhangmu landslide is a large ancient landslide located on the China–Nepal border. Many local slope failure events occurred at this site in the past, posing a direct threat to people and property. One hundred borehole logs (total drilling length of almost 5500 m) were collected to investigate the inner structure of the landslide. Electron spin resonance (ESR) dating and width measurement of annual tree rings were used to determine the reactivation time of different parts of the landslide. A bromide tracer experiment and measurements of the distribution of borehole groundwater were combined to analyze the flow field in the study area. The alluvial stratum in the study area indicates a long-term stable deposition environment. Based on the distribution of the alluvium stratum, we identified the area least prone to landslide damage. Both the surface and underground deformation characteristics were analyzed based on displacement monitoring data from 2006, 2011, and 2013. Combining the monitoring result with the lithology and precipitation, we found that landslide reactivation is closely related to precipitation, suggesting that the reactivation began in the old shear zone.

Variation in surface water–groundwater exchange with land use in an urban stream

Summary A suite of methods is being utilized in the Baltimore metropolitan area to develop an understanding of the interaction between groundwater and surface water at multiple space and time scales. As part of this effort, bromide tracer experiments were conducted over two 10-day periods in August 2007 and May 2008 along two sections (each approximately 900 m long) of Dead Run, a small urban stream located in Baltimore County, Maryland, to investigate the influence of distinct zones of riparian land cover on surface–subsurface exchange and transient storage under low and high baseflow conditions. Riparian land cover varied by reach along a gradient of land use spanning parkland, suburban/residential, commercial, institutional, and transportation, and included wooded, meadow, turf grass, and impervious cover. Under summer low baseflow conditions, surface water-groundwater exchange, defined by gross inflow and gross outflow, was larger and net inflow (gross inflow minus gross outflow) had greater spatial variability, than was observed under spring high baseflow conditions. In addition, the fraction of nominal travel time attributable to transient storage ( F med ) was lower and was more spatially variable under high baseflow conditions than under low baseflow conditions. The influence of baseflow condition on surface water–ground water exchange and transient storage was most evident in the subreaches with the least riparian forest cover and these effects are attributed to a lack of shading in reaches with little riparian forest cover. We suggest that under summer low baseflow conditions, the lack of shading allowed excess in-channel vegetation growth which acted as a transient storage zone and a conduit for outflow (i.e. uptake and evapotranspiration). Under spring high baseflow conditions the transient storage capacity of the channel was reduced because there was little in-channel vegetation.

Identification of Lateral Macropore Flow in a Forested Riparian Wetland through Numerical Simulation of a Subsurface Tracer Experiment

Understanding wetland hydrogeology is important as it is coupled to internal geochemical and biotic processes that ultimately determine the fate of potential contaminant inputs. Therefore, there is a need to quantitatively understand the complex hydrogeology of wetlands. The main objective of this study was to improve understanding of saturated groundwater flow in a forested riparian wetland located on a golf course in the Lower Pee Dee River Basin in South Carolina, USA. Field observations that characterize subsurface wetland flow critical to solute transport originating from storm-generated runoff are presented. Monitoring wells were installed, and slug tests were performed to measure permeabilities of the wetland soil. A field-scale bromide tracer experiment was conducted to mimic the periodic loading of nutrients caused by storm runoff. This experiment provided spatial and temporal data on solute transport that were analyzed to determine travel times in the wetland. Furthermore, a 3-D numerical, steady-state flow model (MODFLOW) was developed to simulate subsurface flow in the wetland. A particle tracking model was subsequently used to calculate solute travel times from the wetland inlet to the outlet based on flow modeling results. It was evident that observed tracer breakthrough times were not typical of these measured wetland soil matrix conductivity values. Based on surface water sampling results at the wetland outlet, tracer arrival time was about 9 h after the injection of the tracer. These results implied an apparent mean K value of 2,050 m/day, which is 152 times larger than the mean of the measured values using slug tests (13.4 m/day). Modeling efforts clearly demonstrated this implied preferential flow behavior; particle travel times resulting from the calibrated flow model were in the order of hundreds of days, while actual travel times in the wetland were in the order of hours to a few days. This significant difference in travel times was attributed to the presence of macropores in the form of dead root channels and cavities forming a pipe-flow network. The analyses presented in this study resulted in an estimate of the ratio of matrix permeability to matrix plus macropore permeability of approximately 1/150. Eventually, the tracer test and resulting travel times between various points in the wetland were critical to understanding the true wetland flow dynamics. The final conceptual model of the hydraulic properties of the wetland soils comprised a low permeability matrix containing a web of high K macropores. Simulation of tracer transport in this system was possible using a flow model with significantly elevated K values.

Change in flow and transport patterns in horizontal subsurface flow constructed wetlands as a result of biological growth

In order to investigate the influence of biological growth on flow and transport patterns in horizontal subsurface flow constructed wetlands, bromide tracer experiments were conducted in two 3 m long tanks. The tanks were filled with light weight aggregates and shell sand, respectively. The hydraulic conductivity, drainable porosity and dispersivity of the two filter media were determined before and after biological growth. The obtained breakthrough curves of bromide tracer showed that biological growth caused a pronounced reduction in drainable porosity, mainly for shell sand, whereas its effect on saturated hydraulic conductivity was negligible. The spatial distribution of the bromide after biological growth in the two filter media showed that the flow occurred preferentially along certain paths. However, in the light–weight aggregates filter medium, biological growth with possible clogging effects seems to reduce the efficiency of some preferential flow paths, increase the retention time, and hence, improves the system treatment efficiency. This study showed that selecting a filter medium including coarse fractions to some extent can mitigate the potential negative effects of bioclogging.

Groundwater recharge and evapotranspiration for two natural ecosystems covered with oak and heather

The evapotranspiration and groundwater recharge from two natural areas with high (oak) and low (heath) vegetation were estimated by calibrating a semi-physical numerical soil water and heat model to fit 8 and 7 years of TDR-measurements of water content, respectively. The measurements were made between the surface and 7 m depth. For the oak stand, the estimated annual recharge for the years 1992–1999 is 390 mm, the evaporation from soil and interception is 205 mm, and the transpiration is 285 mm. For the heath area estimation was carried out for the years 1993–1999. However, the heath was struck by a heavy beetle attack in 1994, which strongly affected the vegetation and thus the water balance for the following 3 years. For years not affected, the estimated recharge is 733 mm (about 50% larger than for the oak stand for the same years), the evaporation is 316 mm, and the transpiration is 128 mm. The estimated recharge values compare fairly well to estimates obtained from bromide tracer experiments. However, the recharge estimates obtained from the tracer experiments are very uncertain. The uncertainty is mainly due to spatial heterogeneity making the three replicate samples taken here for each time and depth insufficient. The analyses of TDR-measurements and tracer data showed that water front movement depends on the antecedent soil water content. Some layers are bypassed, especially at low water contents, and at high soil water contents preferential flow was observed at the heath site.

Fate of volatile organic compounds in constructed wastewater treatment wetlands.

The fate of volatile organic compounds was evaluated in a wastewater-dependent constructed wetland near Phoenix, AZ, using field measurements and solute transport modeling. Numerically based volatilization rates were determined using inverse modeling techniques and hydraulic parameters established by sodium bromide tracer experiments. Theoretical volatilization rates were calculated from the two-film method incorporating physicochemical properties and environmental conditions. Additional analyses were conducted using graphically determined volatilization rates based on field measurements. Transport (with first-order removal) simulations were performed using a range of volatilization rates and were evaluated with respect to field concentrations. The inverse and two-film reactive transport simulations demonstrated excellent agreement with measured concentrations for 1,4-dichlorobenzene, tetrachloroethene, dichloromethane, and trichloromethane and fair agreement for dibromochloromethane, bromodichloromethane, and toluene. Wetland removal efficiencies from inlet to outlet ranged from 63% to 87% for target compounds.

Infiltration and solute transport under a seasonal wetland: bromide tracer experiments in Saskatoon, Canada

AbstractIn the northern glaciated plain of North America, the duration of surface water in seasonal wetlands is strongly influenced by the rate of infiltration and evaporation. Infiltration also plays important roles in nutrient exchange at the sediment–water interface and groundwater recharge under wetlands. A whole‐wetland bromide tracer experiment was conducted in Saskatchewan, Canada to evaluate infiltration and solute transport processes. Bromide concentrations of surface water, groundwater, sediment pore water and plant tissues were monitored as the pond water‐level gradually dropped until there was no surface water. Hydraulic head gradients showed strong lateral flow from under the wetland to the treed riparian zone during the growing season. The bromide mass balance analysis showed that in early spring, almost 50% of water loss from the wetland was by infiltration, and it increased to about 70% in summer as plants in and around the wetland started to transpire more actively. The infiltration contributed to recharging the shallow, local groundwater under the wetland, but much of it was taken up by trees without recharging the deeper groundwater system. Emergent plants growing in the wetlands incorporated some bromide, but overall uptake of bromide by vegetation was less than 10% of the amount initially released. After one summer, most of the subsurface bromide was found within 40–80 cm of the soil surface. However, some bromide penetrated as deep as 2–3 m, presumably owing to preferential flow pathways provided by root holes or fractures. Copyright © 2004 Crown in the Right of Canada. Published by John Wiley &amp; Sons, Ltd.

Hydraulic and transport properties of the plant–soil system estimated by inverse modelling

SummaryModelling soil water flow and solute transport under field conditions requires the knowledge of many parameters that are difficult to determine directly. Values determined on small isolated samples in the laboratory are often not representative of field situations. We investigated the applicability of inverse modelling to a soil–plant system in lysimeter experiments. We also tested whether parameters obtained from one experiment could be applied to another with the same soil. In a lysimeter planted with young trees, we first did a multistep drainage experiment and then a long‐term bromide tracer experiment with atmospheric boundary conditions at the soil surface. To estimate the unsaturated hydraulic properties, we linked the inverse program SUFI (Sequential Uncertainty FItting) to the flow and transport model HYDRUS5. A comparison of several scenarios showed that the resulting values of parameters depended strongly on the data used for calibration and the formulation of the objective function. The results suggested that inverse modelling could be used to identify important processes. Inversely obtained parameters gave better predictions for a second experiment when more variables were considered in the objective function and when the range of hydraulic conditions was wider. Furthermore, with retention curves directly fitted to measured water retention data we achieved acceptable results. Despite some limitations, the inverse approach was found to be a sound and useful procedure for estimating parameters of a complex system involving water uptake by roots, solute transport and unsaturated flow.

Using Simple Bucket Models to Analyze Solute Export to Subsurface Drains by Preferential Flow

Leaching of fertilizer and pesticide compounds from agricultural lands into ground and surface waters is a major environmental problem. To study this phenomenon, a bromide tracer experiment was performed on a 1.6-ha area of a tile-drained arable field to determine the contribution of preferential flow to solute leaching, and to investigate the factors triggering the onset of such events. During the 2 yr following the application of Br − , concentration of the drainage effluent exhibited a pattern of peaks coinciding with the discharge peaks. The concentration peaks, on top of a smooth base flow, were interpreted as being the contribution of preferential flow. The separation of peak and base flow indicated that 73% of the Br − leached during the 2 yr of study was exported through preferential flow. A simple leaching bucket model was able to accurately predict the occurrence of solute peaks in the drain discharge. The analyses indicated that preferential flow originated primarily at the boundary between top- and subsoil as soon as the topsoil became sufficiently saturated. Thus, low intensity, high duration events could also trigger preferential flow.

Using Simple Bucket Models to Analyze Solute Export to Subsurface Drains by Preferential Flow

Leaching of fertilizer and pesticide compounds from agricultural lands into ground and surface waters is a major environmental problem. To study this phenomenon, a bromide tracer experiment was performed on a 1.6‐ha area of a tile‐drained arable field to determine the contribution of preferential flow to solute leaching, and to investigate the factors triggering the onset of such events. During the 2 yr following the application of Br−, concentration of the drainage effluent exhibited a pattern of peaks coinciding with the discharge peaks. The concentration peaks, on top of a smooth base flow, were interpreted as being the contribution of preferential flow. The separation of peak and base flow indicated that 73% of the Br− leached during the 2 yr of study was exported through preferential flow. A simple leaching bucket model was able to accurately predict the occurrence of solute peaks in the drain discharge. The analyses indicated that preferential flow originated primarily at the boundary between top‐ and subsoil as soon as the topsoil became sufficiently saturated. Thus, low intensity, high duration events could also trigger preferential flow.

Using Simple Bucket Models to Analyze Solute Export to Subsurface Drains by Preferential Flow

Leaching of fertilizer and pesticide compounds from agricultural lands into ground and surface waters is a major environmental problem. To study this phenomenon, a bromide tracer experiment was performed on a 1.6-ha area of a tile-drained arable field to determine the contribution of preferential flow to solute leaching, and to investigate the factors triggering the onset of such events. During the 2 yr following the application of Br−, concentration of the drainage effluent exhibited a pattern of peaks coinciding with the discharge peaks. The concentration peaks, on top of a smooth base flow, were interpreted as being the contribution of preferential flow. The separation of peak and base flow indicated that 73% of the Br− leached during the 2 yr of study was exported through preferential flow. A simple leaching bucket model was able to accurately predict the occurrence of solute peaks in the drain discharge. The analyses indicated that preferential flow originated primarily at the boundary between top- and subsoil as soon as the topsoil became sufficiently saturated. Thus, low intensity, high duration events could also trigger preferential flow.

Hydrogeologic assessment of in situ natural attenuation in a controlled field experiment

An experiment to investigate the natural attenuation of three volatile organic compounds, toluene, carbon tetrachloride, and tetrachloroethene (∼1–10 mg L−1) was performed in a 3 m deep, sandy aquifer isolated within a 24 m long, 2 m wide, three‐sided sheet pile alleyway (hereafter referred to as the gate). A constant flow was maintained in the test volume by pumping a well at the closed end of the gate at 130 mL min−1. The test compounds were introduced to the aquifer using diffusive emitters installed inside 25 cm diameter wells located at the open end of the gate. Monitoring was performed by sampling along six multilevel fences (consisting of 12 sampling points each) ranging in distance from 1 to 22 m from the source wells. A bromide tracer experiment established that there were no significant hydraulic leaks, nor was there any continuous channeling through the gate. Degradation of the test compounds was assessed by mass balance calculations between fences located 1 and 7 m from the source, and the results were compared with degradation rate estimates from snapshot analyses and the analysis of fluxes. There was reasonably good agreement between rates estimated by these different methods. Toluene degraded with a half‐life of 58–62 days, carbon tetrachloride degraded with a half‐life of ∼11–13 days, and tetrachloroethene degraded too slowly for a reliable estimate of rate to be made. Transformation products identified in the gate included acetate, possibly from toluene degradation, chloroform, trichloroethene, and cis‐1,2, dichloroethene. The latter two compounds only appeared in trace quantities and could not be assessed for continuing degradation. However, chloroform degradation was assessed with the snapshot data and using the flux estimates and was found to degrade with a half‐life in the range of 10–34 days. No additional chlorinated methanes were detected in the gate, suggesting that the carbon tetrachloride was completely dechlorinated by natural processes within 10 m of the source wells. This experiment demonstrated that degradation of chlorinated solvents occurs naturally at the Borden site but that the ethenes are more resistant to biodegradation than the methanes. In addition, the flux calculations were found to be the most robust in terms of estimating degradation rates.