Laboratory Sand Tank Research Articles

Investigating the occurrence of flow instabilities in coastal unconfined aquifers using sand tank experiments

Subterranean estuaries form the land-sea transition zone in coastal aquifers and are relevant for coastal freshwater management and coastal ecology. Tidal fluctuations typically lead to the formation of upper saline plumes, where denser saltwater circulates on top of the discharging, less dense freshwater. Extensive numerical simulations and only a few experimental trials previously demonstrated that these saline plumes may become unstable and exhibit salt fingering flow. This work used laboratory sand tank experiments to study the formation and evolution of tide induced flow instabilities in homogenous, unconfined coastal aquifers in a systematic manner, addressing the combined effects of relevant parameters such as sediment type, beach slope, fresh groundwater inflow, tidal amplitude, and tidal period variation. Some of these parameters have never been targeted in laboratory-based studies before and a comparable systematic and comprehensive approach has so far not been conducted. The resulting flow patterns were studied visually through the application of a colour tracer to the saltwater. The results show that the upper saline plume may be stable, unstable, or transitional, whereby the beach slope predominantly defines its state. When salt fingers form, they amplify the mixing zone between ground- and saltwater and force the freshwater to discharge at multiple locations along the beach. The processes were best visible in the coarsest sediment (1.02 × 10-2 m s−1). The finer grained the sediment was, the smaller the upper saline plume that formed. The present study further reveals the possibility of three-dimensional fingering flow, even though the physical model was intended to represent a two-dimensional cross-section. The application of a previously proposed non-dimensional stability diagram yielded reasonably accurate predictions of stability behaviour in coastal unconfined aquifers under laboratory settings. This further supports the plausibility of flow instabilities in subterranean estuaries within specific ranges of influencing hydrogeological factors.

Remediation of aniline-contaminated aquifer by combining in-well Rhizobium borbori and circulated groundwater electrolysis

Aniline has become a common groundwater contaminant due to its wide use as a raw material in agriculture and pharmaceutical products. The current technologies for in situ remediation of aniline in groundwater are limited by the strains deficient in bacterial species, limited oxygen supply, excessive waste gas load and cost. Accordingly, we conducted a laboratory sand tank experiment to remediate groundwater contaminated with aniline by combining circulated groundwater electrolysis and in-well Rhizobium borbori, which was isolated from activated sludge. The results of the experiment indicated that the optimum concentration of aniline for Rhizobium borbori is about 5 mg/L, beyond which the maximum cell density and the highest specific growth rate decreases as the aniline concentration increases. The optimized duration for immobilizing the Rhizobium borbori into the bioreactor is 4–5 days. Though the Rhizobium borbori was strongly inhibited by the high-concentration of aniline, the immobilized bioreactor in the 350 mg/L aniline solution successfully formed biofilm. The aniline volatilization had limited influence on the observation of bioremediation performance, and the combination of circulated groundwater and in-well Rhizobium borbori supplied a steady dose of oxygen to the bioreactor efficiently degrading the entire region between the injection and extraction well. In addition, a numerical model for the sand tank remediation experiment was used to estimate the yield coefficient of oxygen to be 0.484 g/g, which indicates the presence of ammonia nitrogen as by-products; accordingly, a smaller wellbore size as well a higher circulation flow rate and intensity of current are recommended to improve the water quality. Despite the positive outcomes and potential of the newly developed technology to degrade subsurface aniline, parallel experiments should be conducted to estimate the environmental risk of the by-products and explore the controlling mechanisms of each component in this comprehensive system.

Scaling of groundwater flow subject to managed aquifer recharge using injection boreholes: Physical experiments and upscaled numerical models

Managed aquifer recharge (MAR) holds great promise for solving the issue of groundwater resource depletion and quality deterioration in urban areas, but the scaling of groundwater flow patterns under MAR influence from controlled laboratory experiments to complex field scale conditions has not been fully resolved. This study applied physical and numerical groundwater flow experiments at the laboratory scale (100 m) along with ten numerical models at increasing spatial scales (100–103 m) for porous media representative of unconsolidated sand aquifers. We considered several spatial and temporal features that characterize groundwater flow patterns, such as the water table elevation, the thickness of the capillary fringe, the MAR lens geometry (defined as formed by the plume of artificially recharged water), the locations of stagnation points (defined as the points of convergence between ambient groundwater flow and injected water flow), and the groundwater travel times, which were examined at relative scales for comparison. The analysis showed that small-scale sandbox experiments overall underestimate the magnitude of the effects of MAR on groundwater flow pathways. The trends with increasing scales for these parameters were non-linear and could be fitted to power law relationships. These trends were more prominent at scales <20 times the size of the laboratory sand tank. Importantly, the heterogeneity of the sand material, as reflected by non-uniform spatial distributions of hydraulic conductivities (K) also had a major effect on the scaling of groundwater flow patterns, even for the relatively low degree of heterogeneity considered. Specifically, an increase in the average K and its standard deviation further exacerbates the scale dependency. This study provides insights into the appropriate scaling techniques to be used when applying small scale experimental results (e.g., from laboratory) to predict MAR-induced groundwater flow dynamics at larger field scales in moderately heterogeneous, unconsolidated sands.

Modeling Dissolution of Soluble Compounds from Multi-Component NAPL Using a Desorption Approximation.

Groundwater professionals require methods to estimate the potential time required to achieve remedial goals, including locations within and downgradient of zones containing non-aqueous phase liquids (NAPLs). NAPLs have long been recognized as persistent contaminant sources to groundwater. Dissolution of multi-component NAPLs is particularly complex, and numerical models that explicitly simulate it are not widely available. This study introduces an equilibrium partitioning approximation to simulate the dissolution of the most soluble chemical components from multi-component NAPL containing a significant fraction of relatively insoluble mass. The effective distribution coefficient that describes depletion of a specific compound from NAPL is estimated based on the properties of the NAPL and the porous medium. This study also presents numerical modeling results that support the utility of the method, with verification using published empirical data collected during dissolution of residual coal tar in a controlled laboratory sand tank experiment. The numerical modeling method uses equilibrium partitioning as an approximation and matched the concentrations of the two most soluble NAPL components in and downgradient of the NAPL zone with reasonable accuracy. The results suggest that the method should be useful for screening level assessments and can be adapted to compare relative groundwater restoration timeframes of select NAPL components for various remedial alternatives.

Evaluation of a wheel-based seismic acquisition system for a planetary rover

Prior to using in-situ planetary resources, efficient mapping of geochemical and physical characteristics of the near surface will be required. As part of an integrated geophysical instrument suite on exploration and prospecting vehicles, we investigated the suitability of seismic piezo-sensors rigidly mounted on the interior of a generic rover wheel. Factors that can compromise proper data acquisition for this system include the natural mechanical resonance of the wheel and wheel-to-ground coupling. We characterized the natural resonance frequency bands of a generic wheel with an electromagnetic shaker. We also collected seismic shot gathers for subsequent seismic surface-wave analysis using a wheel in both a dry, laboratory sand tank (1.8 × 1.8 × 0.6 m) and in frozen loess soils within the United States Army Corps of Engineers Permafrost Tunnel in Alaska. For our wheel, self-weighted coupling to the ground was found to be adequate, although suitable wheelbase dimensions can constrain field acquisition geometries. In unconsolidated sediments, represented by medium sand (0.25–0.5 mm), wheel resonance of 1 kHz does not affect the fundamental mode used in shear-wave-velocity-to-depth inversion. When analyzed, shot-gather data collected from both wheel-mounted sensors and sand-planted sensors, in loose dry sand, effectively captured similar fundamental surface-wave modes. This is evident in frequency-versus-phase velocity dispersion images. Because narrow frequency bands of wheel resonance exhibit a high signal-to-noise ratio, they also readily detect lateral attenuation changes. Thus, wheel resonance can also be used to capture soil attenuation changes, including those produced when pore H2O ice acts to cement the regolith or loose-grained soils.

Heat tracing of embankment dam leakage: Laboratory experiments and 2D numerical modelling

Recently, the use of heat as a tracer to evaluate the process of leakage in embankment dams has attracted wide attention. A more accurate flow-heat coupling model of embankment dams could help us to better understand the patterns of water flow and heat transfer in embankment dams and provide a scientific basis for the seepage prevention and repair of these dams. In this paper, combined with thermal conductivity empirical models (TCEMs), the saturated-unsaturated flow-heat coupling model of embankment dams was established. Through laboratory sand tank experiments of concentrated leakage in embankment dams, the accuracy of the flow-heat coupling model under 10 types of TCEMs were tested and compared. The results show that the performance of the flow-heat coupling model varies under different types of TCEMs, and the Chung and Horton (1987) model shows better simulation effects, with a coefficient of determination (R2), root mean square error (RMSE) and relative error (Re) ranging from 0.916 to 0.980, 0.266–0.467℃ and 1.370–2.442%, respectively. Therefore, this model could better reflect the dynamic temperature variations in embankment dams. Finally, the flow-heat coupling model was improved by modifying the COMSOL built-in equation, i.e. built-in COMSOL model was replaced by the Chung and Horton (1987) model, which further improved the accuracy of the flow-heat coupling model in the numerical simulation of seepage heat monitoring. Based on the improved model, the concentrated leakage of embankment dams under dynamic water levels was simulated numerically. Under the condition of a dynamic water level, the flow velocity and pressure at the leakage passage are positively correlated with the water level change, and the temperature field also shows the same change trend.

Effects of River Partial Penetration on the Occurrence of Riparian Freshwater Lenses: Experimental Investigation

AbstractThis study examines the occurrence of riparian lenses adjacent to partially penetrating, gaining rivers under the controlled conditions of a laboratory sand tank. Laboratory experiments and numerical modeling of the freshwater lens extent are used to provide physical verification (in light of limited examples of well‐characterized field cases) of the analytical methodology, thereby evaluating the underlying assumptions. Parameter calibration and uncertainty analysis are applied to assess both the experimental conditions and the benefit of lens observations in applying the analytical approach. The experimental freshwater lens was reproduced by both analytical and numerical models, with the exception of small mismatches (between analytical results and measured data) in the lens thickness in the near‐river region. These are most likely due to vertical flow effects that arise from the partial river penetration and saltwater inflows to the river bottom, and that are only partly accounted for in the analytical approach. Uncertainty analysis highlighted that accurate lens predictions based on the analytical method requires calibration to direct lens measurements; a similar finding from earlier studies of fully penetrating river conditions. Sensitivity analysis highlighted that the saltwater head boundary, freshwater and saltwater densities, and the aquifer depth below the riverbed (in descending order of sensitivity) are the most important factors in controlling freshwater lens occurrence and saltwater discharge. The results provide the first physical verification of the occurrence of stable riparian lenses adjacent to partially penetrating, gaining rivers, and verify a recent analytical solution for lens extent and saltwater discharge.

Laboratory treatability of hydrocarbon contaminated groundwater using engineered nanotechnology

The paper focuses on the laboratory treatability of hydrocarbon contaminated groundwater in a laboratory 3 Dimensional (3D) Sand tank model with the used of nanomaterial (NM). NM utilized is nanoscale zerovalent iron (nZVI) which is metal oxides. The NM was first used without modification with oxidizing agent (nZVI + 0% KMnO4) and later modified with potassium permanganate (KMnO4) at two unique percentages (15% and 30%), which is indicated as nZVI + 15% KMnO4 and nZVI + 30% KMnO4 for the remediation procedure in a 3 Dimensional (3D) laboratory Sand Tank Model. Physicochemical parameters and Total Petroleum Hydrocarbon (TPH) was monitored and tested with YSI 650 MDS-6600 probe (V2-4 Sonde, YSI Inc.) and Petroleum Hydrocarbon Analyzer (PHA) petrosense respectively during monitoring period. The results obtained showed that the nZVI + 15% KMnO4 reduced contaminants of concern by 89%. The results from the experiment in the laboratory sand tank model exhibited that nanoparticles can potentially remain in suspension for extended period.

An Automated Seepage Meter for Streams and Lakes

AbstractWe describe a new automatic seepage meter for use in soft bottom streams and lakes. The meter utilizes a thin‐walled tube that is inserted into the streambed or lakebed. A hole in the side of the tube is fitted with an electric valve. Prior to the test, the valve is open and the water level inside the tube is the same as the water level outside the tube. The test starts with closure of the valve, and the water level inside the tube changes as it moves toward the equilibrium hydraulic head that exists at the bottom of the tube. The time rate of change of the water level immediately after the valve closes is a direct measure of the seepage rate (q). The meter utilizes a precision linear actuator and a conductance circuit to sense the water level to a precision of about ±0.1 mm. The meter can also provide an estimate of vertical hydraulic conductivity (K v ) if data are collected for a characteristic time. The detection limit for q depends on the vertical hydraulic head gradient. For K v = 1 m/day, q of about 2 mm/day can be measured. Results from a laboratory sand tank show excellent agreement between measured and true q, and results from a field site are similar to values from calculations based on independent measurements of K v and vertical head gradients. The meter can provide rapid (30 min) q measurements for both gaining and losing systems and complements other methods for quantifying surface water groundwater interactions.

Comparative analysis of microbial loads of molecular potential chain disintegrator (MPCD) and potassium permanganate (PP) in remediation of hydrocarbon contaminated soil/groundwater in three dimensional (3D) laboratory sand tank model

This paper evaluates the comparative analysis of bacterial loads of molecular potential chain disintegrator (MPCD) and potassium permanganate (PP) in remediation of artificial hydrocarbon contaminated soil and groundwater in 3D laboratory sand tank. Alluvial sand was packed in the sand tank model, and 20 cm chamber was packed with prepared artificial diesel contaminated soil. Free flow of water was allowed through the soil medium from the inlet chamber which was being fed continuously from the storage elevated tank with the discharge port opened at flow rate of 58 ml/min. The remediating agents (MPCD and PP) were prepared differently and introduced through the injection wells (IW) to the sand medium at different experimental design stages. Microbial analysis was carried using standard techniques. The microbial loads of unpolluted soil samples collected from the sand tank model was 3.5 x 104 ± 1.5 Cfu/g while the oil-degraders from the diesel contaminated soil was 9.5 x 104 ± 2 Cfu/g. It could be observed that the population of oil-degraders in the MPCD amended soils and water decreased gradually during the incubation period of 15 days from 36 x 104 ± 4 Cfu/g on day 0 to 12.5 x 104 ± 3.5 Cfu/g for soil samples and from 58.5 x 102 ± 1.35 Cfu/mL to 17.5 x 102 ± 0.5 Cfu/mL for water samples. The decrease in the population of oil-degraders could be attributed to the reduction in the concentration of diesel in the soil and water samples undergoing bioremediation. The composition of the two nutrients (MPCD and PP) can be responsible for the ability of the nutrients to enhance the degradation process.

A comparison of numerical and Lu modeling of water flow and heat transport with laboratory experiments

Reservoirs are considered to result in significant changes to river water temperature. Discharge of deep water has a large impact on aquatic ecosystems downstream of dam and on both river banks. A laboratory sand tank test investigation was conducted to simulate water flow and thermal dynamics in the riparian zone. The sand temperature (ST) data generated were used to validate and compare HYDRUS-2D, a physically based numerical model, with Lu et al.’s (Soil Sci Soc Am J 71(1):8–14, 2007) soil thermal conductivity model under different water temperature, hydraulic head and radiation temperature conditions. The Richards model and the heat conduction model were coupled through the Horton thermal conductivity model and the Lu et al. (Soil Sci Soc Am J 71(1):8–14, 2007) model, respectively. The results demonstrated the success of model coupling and its application for investigating water flow and thermal dynamics in the riparian zone. The Lu et al. (Soil Sci Soc Am J 71(1):8–14, 2007) model based on COMSOL and the Horton thermal conductivity model based on HYDRUS each had their own advantages. Global analysis showed that the Lu et al. (Soil Sci Soc Am J 71(1):8–14, 2007) model was better able to simulate the riparian zone temperature field under the investigated experimental conditions. The sensitivity analysis results showed that the parameters nv, T and H had a considerable influence on the temperature field in the model, of which nv was the most sensitive, whereas the parameters ks, α, θs, and θr were relatively less sensitive to the temperature field.

Study on the Effect of Sulfate on the Degradation of BTEX in Leakage Area of Gasoline by Using Numerical Simulation

In order to explore the migration of sulfate electron acceptors injected into aqueous media, and study the specific attenuation of BTEX with sulfate injecting. In this study, the laboratory sand tank was used for the injecting and natural attenuation monitoring experiments. The combination of MODFLOW and RT3D was used to simulate the attenuation process of sulfate and natural degradation of BTEX. The results showed that the concentration of sulfate in the source area was significantly lower than that of the surrounding area at the 600th day after injected on the 416th day and BTEX concentration also gradually tends to zero. The injection experiments were conducted for a total of 46 days with the injected sulfate quality of 1488.24 g, and the calculated sulfate output of 1320.52 g. It can be seen that the input was significantly higher than the output and the sulfate loss could be judged as acting on Redox reactions .Sulfate promoted the degradation of BTEX. The numerical simulation was carried out for the water flow model and water quality model calibration, the normalized RMS of water flow model and water quality model are 9.8% and 8% respectively, which are less than 10% of the conventional limit. The model can accurately simulate the migration of sulfate and BTEX attenuation and predict the further pollution of BTEX. And can be applied to the actual site contaminated by BTEX that has important practical significance.

Finite element modeling of Rayleigh wave propagation in a laboratory sand tank

Rayleigh waves were measured in a laboratory sand tank using an array of accelerometers. This tank allows for the measurement of interface waves for various sediment and soil arrangements in a well-controlled laboratory setting. Rayleigh waves were excited by simple impulsive sources including dropped balls and hammer impacts. In this study, a finite element modeling tool developed by Sandia National Laboratories was used to simulate the sand tank during these excitations. Different modeling scenarios were investigated. Time series of the Rayleigh wave arrivals were modeled at the sensor locations using time domain analysis. A calibrated bender element system was also developed to provide direct measurements of the shear wave speed profile at selected depths. These data were used to develop the material property input parameters for the finite element analysis. The simulated time series at the sensor locations are compared to the accelerometer data. [Work supported by Army Research Laboratory and BOEM.]

In-situ falling-head test for hydraulic conductivity: Evaluation in layered sediments of an analysis derived for homogenous sediments

Summary The hydraulic conductivity (K) of streambeds is a critical variable controlling interaction of groundwater and surface water. The Hvorslev analysis for estimating K from falling-head test data has been widely used since the 1950s, but its performance in layered sandy sediments common in streams and lakes has not previously been examined. Our numerical simulations and laboratory experiments show that the Hvorslev analysis yields accurate K values in both homogenous sediment (for which the analysis was originally derived) and layered deposits with low-K sand over high-K sand. K from the Hvorslev analysis deviated significantly from true K only when two conditions were present together: (1) high-K sand was present over low-K sand, and (2) the bottom of the permeameter in which K was measured was at or very near the interface between high-K and low-K. When this combination of conditions exists, simulation and laboratory sand tank results show that in-situ Hvorslev K underestimates the true K of the sediment within a permeameter, because the falling-head test is affected by low-K sediment outside of (below the bottom of) the permeameter. In simulation results, the maximum underestimation (occurring when the bottom of the permeameter was at the interface of high K over low K) was by a factor of 0.91, 0.59, and 0.12 when the high-K to low-K ratio was 2, 10, and 100, respectively. In laboratory sand tank experiments, the underestimation was by a factor of about 0.83 when the high-K to low-K ratio was 2.3. Also, this underestimation of K by the Hvorslev analysis was about the same whether the underlying low-K layer was 2 cm or 174 cm thick (1% or 87% of the domain thickness). Numerical model simulations were useful in the interpretation of in-situ field K profiles at streambed sites with layering; specifically, scaling the model results to the maximum measured K at the top of the field K profiles helped constrain the likely ratio of high K to low K at field locations with layered heterogeneity. Vertical K values are important in field studies of groundwater–surface water interaction, and the Hvorslev analysis can be a useful tool, even in layered media, when applied carefully.

Remediation of Nonaqueous Phase Liquid Polluted Sites Using Surfactant‐Enhanced Air Sparging and Soil Vapor Extraction

A two-dimensional laboratory sand tank was installed to study the remediation efficiency of surfactant-enhanced air sparging (-SEAS) coupled with soil vapor extraction (SVE) in nonaqueous phase liquid (NAPL) polluted sites. During initial stages of remediation, it was more reasonable to use conventional air sparging coupled with SVE. When most free NAPLs were removed and contaminant removal rate was maintained at a relatively low level, surfactant was added to the groundwater. During enhanced remediation, lower interfacial tension caused residual NAPLs in the porous media to slightly migrate, making the downstream contaminant concentration somewhat higher. The polluted area, however, was not more enlarged than before. The decrease in surface tension resulted in increased air saturation in the groundwater and the extent of the air influence zone. After 310 hours, 78.7% of the initial chlorobenzene mass had volatilized, 3.3% had migrated out of the sand profile, 17.5% was in the vadose zone, and 0.5% remained in the groundwater, thus revealing that SEAS/SVE can effectively improve the remediation of NAPL polluted sites.

Measurement of LNAPL Flux Using Single-Well Intermittent Mixing Tracer Dilution Tests

The stability of subsurface Light Nonaqueous Phase Liquids (LNAPLs) is a key factor driving expectations for remedial measures at LNAPL sites. The conventional approach to resolving LNAPL stability has been to apply Darcy's Equation. This paper explores an alternative approach wherein single-well tracer dilution tests with intermittent mixing are used to resolve LNAPL stability. As a first step, an implicit solution for single-well intermittent mixing tracer dilution tests is derived. This includes key assumptions and limits on the allowable time between intermittent mixing events. Second, single-well tracer dilution tests with intermittent mixing are conducted under conditions of known LNAPL flux. This includes a laboratory sand tank study and two field tests at active LNAPL recovery wells. Results from the sand tank studies indicate that LNAPL fluxes in wells can be transformed into formation fluxes using corrections for (1) LNAPL thicknesses in the well and formation and (2) convergence of flow to the well. Using the apparent convergence factor from the sand tank experiment, the average error between the known and measured LNAPL fluxes is 4%. Results from the field studies show nearly identical known and measured LNAPL fluxes at one well. At the second well the measured fluxes appear to exceed the known value by a factor of two. Agreement between the known and measured LNAPL fluxes, within a factor of two, indicates that single-well tracer dilution tests with intermittent mixing can be a viable means of resolving LNAPL stability.

In Situ Quantification of Atmospheric Methane Oxidation in Near‐Surface Soils

The turnover of greenhouse gases in soils is largely mediated by the activity of microorganisms. In situ quantification of these processes is important to improve estimates on global budgets for many greenhouse gases, including CH4 In this study, we assessed the utility of a modified version of the gas push–pull test (GPPT) to derive in situ apparent first‐order rate coefficients for atmospheric CH4 oxidation in near‐surface soil. An ordinary GPPT consists of the controlled injection and subsequent extraction of reactants (i.e., CH4 and O2) and tracer gases (e.g., Ar, Ne, or He) into and out of the vadose zone at a location of interest. In the modified GPPT, injection and extraction takes place inside a cylinder previously placed in the soil and temporarily closed at the top. Using numerical simulations and tests in a laboratory sand tank, we found gas recovery to remain low (3–17%) and decrease with decreasing injection–extraction depth during ordinary GPPTs of∼1‐h duration. Under similar test conditions, modified GPPTs resulted in high gas recovery (64–90%) of all gases used. This allowed the modified GPPTs to be prolonged up to 12 h in simulations and 6 h in laboratory experiments, while gas recovery still remained substantial (19–57%). The modified GPPT was successfully applied to quantify atmospheric CH4 oxidation in situ in a sandy soil in Zurich, Switzerland. Calculated first‐order rate coefficients ranged from 0.7 to 1.6 h−1 and agreed with literature values and estimates derived from a diffusion–consumption model fitted to CH4 concentration profiles. Our modification extends the applicability of GPPTs to study microbially mediated gas turnover in near‐surface soils.

Numerical Implementation and Computer Simulation of Tracer Experiments in a Physical Aquifer Model

The numerical model Sand Model was developed and used to replicate the laboratory sand tank model. The plots of the TBCs for the twelve (12) multilevel observation points within the model domain show a reasonable match of the simulated and observed tracer breakthrough curves. The calculated t value (Paired-t test statistic) for each well is less than the critical value of t ( tcritical = 2.16 for df=13), and the calculated P value for each well is greater than the chosen significance level (P=0.05), therefore the null (no difference) between observed and simulated data sets hypothesis is accepted. The analysis also reveals high to very high correlation between the observed and simulated values (Average R2 = 0.86). It took eight (8) and Twelve (12) hours respectively for the tracer to reach its peak concentrations of 3.81mS/cm and 1.55mS/cm respectively for the middle (observation point (BM) [x,y,z] [70,15,24] and down gradient observation point (EM) [x,y,z] [30,15,24] within the sand tank model. A sensitivity analysis showed that the time required for complete source depletion, was most dependent on the source definition and the hydraulic conductivity K of the porous medium. The 12000mg/l chloride tracer source was almost completely dispersed within 34 hours.Keywords: Replication, Numerical simulation, Model validation, Solutetransport parameters, Tropical soil

The interaction between a manmade lake and groundwater: an example site in the Aurku area, Chiayi County, Taiwan

AbstractThe objectives of this study are (1) to understand the subsurface hydrology in the Aurku area, Chiayi County, southern Taiwan, and (2) to determine the interaction between the manmade lake and groundwater level through the recharge produced by infiltration by on‐site investigation and laboratory sand tank simulation. The manmade lake was selected as the field site for groundwater recharge effect so as to assess the role of infiltration from the aquaculture ponds in this area. These results can be used as reference for future application of constructing a series of manmade lakes. The field experiment was performed to measure the infiltration rate of the manmade lake by using the water balance method and double‐ring infiltration test. The results demonstrated that the manmade lake had helped the recharge of the groundwater. Raising or maintaining a higher water level of the manmade lake can promote higher infiltration. When the groundwater level is equal to or higher than the bottom of the manmade lake, infiltration will slow or cease. The field experiment and laboratory sand tank simulation demonstrated that the infiltration rate increased with the higher storage depth of the manmade lake. The laboratory simulation also indicated that while the groundwater level was lower than the bottom of manmade lake (i.e. the reference level) and the initial water depth (3 cm) was equal to or greater than 50% of the full water storage depth, the infiltration depth increased with time. However, the infiltration depth would be very small or nearly zero when the groundwater level was higher than the bottom of the manmade lake. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Interpreting DNAPL saturations in a laboratory‐scale injection using one‐ and two‐dimensional modeling of GPR data

AbstractGround‐penetrating radar (GPR) is used to track a dense non–aqueous phase liquid (DNAPL) injection in a laboratory sand tank. Before modeling, the GPR data provide a qualitative image of DNAPL saturation and movement. One‐dimensional (1D) GPR modeling provides a quantitative interpretation of DNAPL volume within a given thickness during and after the injection. DNAPL saturation in sublayers of a specified thickness could not be quantified because calibration of the 1D GPR model is nonunique when both permittivity and depth of multiple layers are unknown. One‐dimensional GPR modeling of the sand tank indicates geometric interferences in a small portion of the tank. These influences are removed from the interpretation using an alternate matching target. Two‐dimensional (2D) GPR modeling provides a qualitative interpretation of the DNAPL distribution through pattern matching and tests for possible 2D influences that are not accounted for in the 1D GPR modeling. Accurate quantitative interpretation of DNAPL volumes using GPR modeling requires (1) identification of a suitable target that produces a strong reflection and is not subject to any geometric interference; (2) knowledge of the exact depth of that target; and (3) use of two‐way radar‐wave travel times through the medium to the target to determine the permittivity of the intervening material, which eliminates reliance on signal amplitude. With geologic conditions that are suitable for GPR surveys (i.e., shallow depths, low electrical conductivities, and a known reflective target), the procedures in this laboratory study can be adapted to a field site to delineate shallow DNAPL source zones.