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1.
A multi-layered aquifer, typical of riverbank alluvial deposits in Korea, was studied to determine the hydrologic properties. The geologic logging showed that the subsurface of the study site was comprised of four distinctive hydrogeologic units: silt, sand, highly weathered and fresh bedrock layers. The electrical resistivity survey supplied information on lateral extension of hydrogeologic strata only partially identified by a limited number of the geologic loggings. The laboratory column tracer test for the recovered core of the sand layer resulted in a hydraulic conductivity of 5.00×10−2 cm/s. The slug tests performed in the weathered rock layer yielded hydraulic conductivities of 4.32–7.72×10−4 cm/s. Hydraulic conductivities for the sand layer calculated from the breakthrough curves of bromide ranged between 2.08×10−3 and 2.44×10−2 cm/s with a geometric mean of 6.89×10−3 cm/s, which is 7 times smaller than that from the laboratory column experiment. The trend of increasing hydraulic conductivity with an increase in tracer travel length is likely a result of the increased likelihood of encountering a high conductivity zone as more of the aquifer is tested. The combined hydrogeologic site characterization using hydraulic tests, tracer tests, and column test with geologic loggings and geophysical survey greatly enhanced the understanding of the hydrologic properties of the multi-layered alluvial aquifer.  相似文献   

2.
The current study aimed to evaluate hydrogeologically the Nubian sandstone aquifer in El-Bahariya Oasis. It represents the main water-bearing horizon in the study area and consists of continental elastic sediments, mainly sandstone alternating with shale and clays. The general flow lines are directed from SW to NE direction, as detected from the constructed potentiometric head contour map. The piezometric surface reaches 149 m in El-Heiz area at the southern part, while it reaches 90 m at the northern, reflecting higher pressure head of the aquifer in the southern part. The map also illustrates that the southern part is considered as the most promising location for development. The structural elements play an important role in the deposition and distribution of the sedimentary succession of the Nubian sandstone sediments. Consequently, this sedimentary pattern affects the occurrences and movements of the groundwater within the aquifer system. Along the structurally high areas, in the study area, the piezometric head increases, while the reverse is recorded along the structurally low areas. The step-drawdown tests data were carried out by calculating the aquifer loss coefficient (B) and the well loss constant (C). The B values are smaller compared with C values, indicating that the aquifer under pressure has a behavior of leaky aquifer; therefore, it shows hydraulic connection with surrounding formation. The values of well efficiency range from 78.50% to 87.76%. Analysis of 12 pumping test data (constant discharge tests) was carried out in order to calculate the Nubian aquifer hydraulic parameters (transmissivity, hydraulic conductivity, and storage coefficient). The transmissivity values decrease from 3,045 m2/day in the southern part (El-Heiz area) to 236 m2/day in the northeastern part (El-Harra area). Accordingly, the aquifer classified as a high to moderate potentiality. Transmissivity contour map observes gradual increase of transmissivity values from the southern to northeastern direction. This may be due to the increase of shale or clay content in the concerned aquifer in that direction. The storage coefficient values range between 1.04 × 10?4 and 5.22 × 10?3, as obtained from the results of pumping test analysis, which ensure that the Nubian sandstone aquifer is classified as semi-confined to confined aquifer type. The S values show a decrease from southwest to northeast direction as detected from S-map. The hydraulic conductivity values vary from to 0.46 m/day in the northern part to 10.88 m/day in the southern part with an average of 5.67 m/day. According to the classification based on K values, the aquifer is mainly composed of coarse sand.  相似文献   

3.
Both spring-summer and fall-winter sand transport have been observed on the Long Island, New York, inner shelf at water depths of 20-22 m using a radio-isotope sand tracer system. The extent of dispersal of the tagged, fine sand was measured at 3 week intervals in two 70 day experiments. In the late spring and early summer, movement was primarily diffusive in nature, extending 100 m around the line of tracer injection, while late fall-winter patterns had strong advective features, including an ellipsoidal outline extending approximately 1500 m westward of the injection points after the passage of several storms with strong northeasterly winds. Near-bottom current observations made with Savonius rotor sensors identify the event responsible for the bulk of the transport over the 135 day observation period as a storm flow of 2 days duration. Tracer and current observations together suggest that westward winter storm flow along the Long Island shelf is the major mechanism of sand transport at these depths on a yearly time scale. A least-squares fit of several of the observed winter patterns with a plume model yields average sediment mass flux lower bounds of 3.2 × 10?3 gm/cm/sec and 1.7 × 10?1 gm/cm/sec for ‘typical’ and extreme winter storm activity.  相似文献   

4.
Several hydraulic techniques were used to estimate the flow depth (0.3 m) associated with the deposition of a tabular set (micro-delta) of cross-stratified sand in the Brampton esker. The competency of the flow, deduced from both the grain size and structural characteristics of the set, gave a value of approximately 0.65 m/sec for the palaeo-velocity of the flow. Estimates of palaeo-depth and velocity facilitated calculation of the Froude and Reynolds numbers, about 0.38 and 1.24 · 105, respectively. Extrapolation of other parameters included bed shear stress τ0 (4.50 N/m2), shear velocity U* (0.067 m/sec), dimensionless Chezy coefficient C/√g (9.7), slope of the energy gradient S(0.00153), Darcy-Weisbach friction factor f(0.085), Manning roughness coefficient n(0.027) and discharge of bed-material load (19 metric tons/day/m). The figures cited are reasonable estimates only. The occurrence of regressive ripples in the bottomset of the micro-delta aided in the hydraulic interpretation. These flow characteristics are only representative of the final stages of deposition at one location on the flank of the esker. The core of the esker was probably deposited under different hydraulic conditions.  相似文献   

5.
The groundwater abstracted at a well field near the Yamuna River in Central Delhi, India, has elevated ammonium (NH4 +) concentrations up to 35 mg/L and arsenic (As) concentrations up to 0.146 mg/L, constituting a problem with the provision of safe drinking and irrigation water. Infiltrating sewage-contaminated river water is the primary source of the NH4 + contamination in the aquifer, leading to reducing conditions which probably trigger the release of geogenic As. These conclusions are based on the evaluation of six 8–27-m deep drillings, and 13 surface-water and 69 groundwater samples collected during seven field campaigns (2012–2013). Results indicate that losing stream conditions prevail and the river water infiltrates into the shallow floodplain aquifer (up to 16 m thickness), which consists of a 1–2-m thick layer of calcareous nodules (locally known as kankar) overlain by medium sand. Because of its higher hydraulic conductivity (3.7 × 10?3 m/s, as opposed to 3.5 × 10?4 m/s in the sand), the kankar layer serves as the main pathway for the infiltrating water. However, the NH4 + plume front advances more rapidly in the sand layer because of its significantly lower cation exchange capacity. Elevated As concentrations were only observed within the NH4 + plume indicating a causal connection with the infiltrating reducing river water.  相似文献   

6.
This paper describes a natural-gradient field tracer test to characterise solute-transport properties in a sand and gravel aquifer in the Hebei Province, northern China. Some laboratory-scale column tests on aquifer material and a local-scale field borehole-dilution test have been conducted previously, but the field test reported herein represents the only large-scale tracer test in the aquifer, which is the sole water supply to the city of Shi Jiazhuang and which is threatened by urban pollution. The aim of the study was to quantify the transport behaviour of nonreactive pollutants in this aquifer. Little quantitative data are available concerning its solute-transport properties; thus, the results of the tracer test are significant and critical for understanding pollutant transport and fate. The in-situ tracer test was carried out in the aquifer using a slug injection of the geochemically conservative, radioactive iodine tracer 131I. The longitudinal (α L ) and transverse (α T ) hydrodynamic dispersivities for solute transport in the field are 1.72 and 0.0013 m, respectively. The ratio of longitudinal dispersivity α L and the flow length at the field scale is 1:10. The ratio between α L and α T from the in-situ test (~1,300:1) demonstrates a dominant longitudinal dispersion in this fluvial sand and gravel aquifer. The tracer test further indicates a relatively short transit time for the aquifer (linear velocities ~13 m/d) under natural-gradient conditions. Electronic Publication  相似文献   

7.
A transparent aqueous-saturated sand surrogate for use in physical modeling   总被引：3，自引：0，他引：3  
This paper presents the geotechnical properties of a new family of synthetic transparent soils made of fused quartz, saturated with a matched refractive index water-based sucrose solution, suitable for modeling the behavior of sand in small-scale model tests. The dry density ranged between 1,134 and 1,358 kg/m3. The peak angle of friction was found to range from 46° to 57°. The average hydraulic conductivity was 1.7 × 10?5 cm/s. The compressibility index (C c) ranged from 0.34 to 0.57. The main advantage of fused quartz over available sand surrogates made of silica gel is that its solid structure better models the behavior of natural sand. The matching pore fluids are inert and non-toxic, which facilitates their use in educational settings. The availability of a safe and easy-to-use transparent sand permits measurement of three-dimensional deformation patterns and flow characteristics in controlled research experiments. The introduction of an aqueous solution permits the use of two immiscible pore fluids, one made of mineral oil and the other made of a sucrose solution, for modeling multiphase flow problems, as well as coupled flow-deformation problems.  相似文献   

8.
Sorption is a well-known phenomenon that may cause the retardation effect of zinc in the subsurface environment. In this study, the governing process for zinc sorption during transport was investigated by conducting 2-D plume tests in a laboratory scale sand tank model using the time domain reflectometry (TDR) method. Tracer solutions of NaNO3 and ZnSO4 were applied at a constant flow rate as a pulse type to capture the plumes of both solutes based on TDR-measured resistance. It was revealed that the observed zinc sulfate plume showed no retardation relative to sodium nitrate with a retardation factor of R ≈ 1. Instead of retardation, a prominent reduction of zinc sulfate mass occurred during transport through the tank model due to the irreversible sorption as well as longitudinal dispersion. This indicates that the controlling factor for the sorption process of zinc sulfate in the sand tank model is kinetic rather than equilibrium. These hydrogeological parameters would provide valuable information on the prediction of the fate of zinc in sandy aquifer materials.  相似文献   

9.
Yang  Heejun  Tawara  Yasuhiro  Shimada  Jun  Kagabu  Makoto  Okumura  Azusa 《Hydrogeology Journal》2021,29(6):2091-2105

The hydraulic conductivity of an unconfined carbonate aquifer at the uplifted atoll of Minami-Daito, Japan, was evaluated by a combination of cross-spectral analysis, analytical solution, and density-dependent groundwater modeling based on observed groundwater levels in 15 wells and at sea level. The island area was divided into 10 subregions based on island morphology and on inland propagation of ocean tides. The hydraulic conductivity was obtained for each subregion using analytical solutions based on phase lags of M2 constituents of ocean tides at each well by assuming two aquifer thicknesses (300 and 1,800 m) and two effective porosities (0.1 and 0.3). The density-dependent groundwater model evaluated the hydraulic conductivity of the subregions by reproducing observed groundwater levels. The hydraulic conductivity in the subregions was estimated as 3.46?×?10?3 to 6.35?×?10?2 m/s for aquifer thickness of 300 m and effective porosity of 0.1, and as 1.73?×?10?3 to 3.17?×?10?2 m/s for aquifer thickness of 1,800 m and the effective porosity of 0.3. It was higher in southern and northern areas, and higher in interior lowland than in the western and eastern areas. Fissures and dolomite distributions on the island control differences of the omnidirectional ocean tidal propagation and cause these differences in hydraulic conductivity. The method used for this study may also be applicable to other small islands that have few or no data for hydraulic conductivity.

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10.
The hydrogeologic and hydraulic characteristics of a lateritic terrain in West Bengal, India, were investigated. Test drilling was conducted at ten sites and grain-size distribution curves (GSDCs) were prepared for 275 geologic samples. Performance evaluation of eight grain-size-analysis (GSA) methods was carried out to estimate the hydraulic conductivity (K) of subsurface formations. Finally, the GSA results were validated against pumping-test data. The GSDCs indicated that shallow aquifer layers are coarser than the deeper aquifer layers (uniformity coefficient 0.19–11.4). Stratigraphy analysis revealed that both shallow and deep aquifers of varying thickness exist at depths 9–40 and 40–79 m, respectively. The mean K estimates by the GSA methods are 3.62–292.86 m/day for shallow aquifer layers and 0.97–209.93 m/day for the deeper aquifer layers, suggesting significant aquifer heterogeneity. Pumping-test data indicated that the deeper aquifers are leaky confined with transmissivity 122.69–693.79 m2/day, storage coefficient 1.01?×?10?7–2.13?×?10?4 and leakance 2.01?×?10?7–34.56?×?10?2 day?1. Although the K values yielded by the GSA methods are generally larger than those obtained from the pumping tests, the Slichter, Harleman and US Bureau Reclamation (USBR) GSA methods yielded reasonable values at most of the sites (1–3 times higher than K estimates by the pumping-test method). In conclusion, more reliable aquifers exist at deeper depths that can be tapped for dependable water supply. GSA methods such as Slichter, Harleman and USBR can be used for the preliminary assessment of K in lateritic terrains in the absence of reliable field methods.  相似文献   

11.
The combined use of pumping and tracer test data enabled the derivation of equivalent average hydraulic conductivities (Kavg) for each test in a heterogeneous channelized alluvial aquifer, whereas K values of the preferential flow paths were two orders of magnitude higher. Greater and earlier drawdown was generally observed along preferential flow lines in a pumping test, within an array of 21 wells. The study aim was to characterize hydraulic properties of a channelized aquifer system in New Zealand by combining tracer and pumping test data. Estimates were able to be made of the percentage of highly permeable channels within the profile (～1.2%), effective porosity that reflected the maximum fraction of highly permeable channels within the aquifer (?eff–pc ～0.0038), and flows through highly permeable channels (～98%) and the sandy gravel matrix material (～2%). Using ?eff–pc, a tracer test Kavg value (～93 m/day) was estimated that was equivalent to pumping test values (～100 m/day), but two orders of magnitude smaller than K calculated solely from transport through permeable channels (Kpc ～8,400 m/day). Derived K values of permeable and matrix material were similar to values derived from grain size distribution using the Kozeny-Carman equation.  相似文献   

12.
Pokhrel  Pranisha  Zhou  Yangxiao  Smits  Frank  Kamps  Pierre  Olsthoorn  Theo 《Hydrogeology Journal》2023,31(5):1291-1309

Managed aquifer recharge (MAR) is increasingly used to secure drinking water supply worldwide. The city of Amsterdam (The Netherlands) depends largely on the MAR in coastal dunes for water supply. A new MAR scheme is proposed for the production of 10 × 106 m3/year, as required in the next decade. The designed MAR system consists of 10 infiltration ponds in an artificially created sandbank, and 25 recovery wells placed beneath the ponds in a productive aquifer. Several criteria were met for the design, such as a minimum residence time of 60 days and maximum drawdown of 5 cm. Steady-state and transient flow models were calibrated. The flow model computed the infiltration capacity of the ponds and drawdowns caused by the MAR. A hypothetical tracer transport model was used to compute the travel times from the ponds to the wells and recovery efficiency of the wells. The results demonstrated that 98% of the infiltrated water was captured by the recovery wells which accounted for 65.3% of the total abstraction. Other sources include recharge from precipitation (6.7%), leakages from surface water (13.1%), and natural groundwater reserve (14.9%). Sensitivity analysis indicated that the pond conductance and hydraulic conductivity of the sand aquifer in between the ponds and wells are important for the infiltration capacity. The temperature simulation showed that the recovered water in the wells has a stable temperature of 9.8–12.5 °C which is beneficial for post-treatment processes. The numerical modelling approach is useful and helps to gain insights for implementation of the MAR.

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13.
This study highlights the distribution of hydraulic conductivity (K) in the regional aquiferous Ajali Formation of SE-Nigeria on one hand and assesses the possible influences of textural and geochemical characteristics on the hydraulic conductivity on the other hand. The investigation approach involved field sampling and collection of 12 sandstone samples from different outcrop locations, followed by laboratory studies such as grain-size analysis (GSA), constant head permeameter test and geochemical analysis of major and trace elements using X-ray fluorescence method. GSA and textural studies show that the sandstones range from fine to medium sands, constituting about <75–99% sand fraction, with graphic mean grain size of 0.23–0.53 mm. Other parameters such as coefficient of uniformity (Cu) range from 1.58 to 5.25 (av. 2.75), while standard deviation (sorting) values of 0.56Ø–1.24Ø imply moderately well sorted materials. In addition, the order of the estimated K values is Kpermeameter>KBeyer>KHazen>KKozeny-Carmen>KFair-Hatch with average values of 1.4×10?3, 4.4×10?4, 3.8×10?4, 2.2×10?4 and 8.1×10?5m/s, respectively. These values fall within the range of 10?5 and 10?3m/s for fine to medium sands. However, multivariate factor analysis of the data revealed significant positive dependence of the empirically determined K values on graphic mean grain size and percentage sand content and much less dependence on sorting and total porosity. Geochemical profiles of the fresh samples are dominated by quartz with corresponding SiO2 content of 76.1–98.2% (av. 89.7%) while other major oxides are generally below 1.0wt.% in the fresh samples. However, the ferruginized samples exhibited elevated concentrations of Al2O3 (3.50–11.60wt.%) and Fe2O3 (1.80–3.60wt.%), which are clear indications of weathering/ferruginization processes with attendant trace metal release/enrichment (2.5mg/l Cu, 7.5mg/l Pb, 6.5mg/l Zn, 3.9mg/l Ni and 19.6mg/l Cr) call for concern in respect of the chemical quality of the groundwater system. The associated groundwater is generally soft, slightly acidic, and with low dissolved solids (EC=14–134μs/cm) dominated by silica; implying water from clean sandy aquifer devoid of labile and weatherable minerals. Nonetheless, most of the metals (with exception of Si, Fe and Mn) exhibited higher degree of mobility (2–12 folds), which can be attributed to reduction of Fe-/Mn-oxyhydroxides as sinks/hosts for trace metals. Consequently, infiltration-induced geochemical reactions (redox, ferruginization and leaching processes) signify potential environmental impact in terms of groundwater quality as well as borehole/aquifer management, especially under humid tropical environment of the study area.  相似文献   

14.
Landfills are one of the major sources of methane (CH4) emission which is a very potent greenhouse gas. The use of a natural process for microbial CH4 oxidation through biocovers provides a source reduction of CH4 emission. Previous studies have mostly focused on biochemical properties, and limited research has been conducted with regards to the geotechnical characterization of compost based biocovers. This paper presents the results of a comprehensive laboratory investigation on pure compost and compost–sand mixtures (with mix ratio of 3:1, 1:1, and 1:3 w/w) to determine the compaction, shear strength, compressibility, and hydraulic and thermal conductivity properties of compost based biocovers. Direct shear and ring shear tests have shown that the cohesion (c) and friction angle (?) are in the range of 2.1–19.7 kPa and 44.1°–54.7°, respectively. Based on the results of one dimensional consolidation tests, the coefficient of consolidation (Cv) values are in the range of 1.71–0.63 m2/year, which is a function of the moisture and organic contents of the samples. The lowest hydraulic conductivity ranges from 6.09 × 10?8 to 1.78 × 10?7 cm/s which occur at optimum moisture contents. Thermal conductivity is measured under various porosities and moisture contents. By increasing the dry density and sand content of the mixtures, thermal conductivity increases. The results presented in this paper will contribute to a better understanding of the geotechnical behaviour of compost based biocover, and thus to a more cost-effective design of biocovers.  相似文献   

15.
 Two multitracer tests performed in one of the major cross-fault zones of the Lange Bramke basin (Harz Mountains, Germany) confirm the dominant role of the fault zone in groundwater flow and solute transport. Tracers having different coefficients of molecular diffusion (deuterium, bromide, uranine, and eosine) yielded breakthrough curves that can only be explained by a model that couples the advective–dispersive transport in the fractures with the molecular diffusion exchange in the matrix. For the scale of the tests (maximum distance of 225 m), an approximation was used in which the influence of adjacent fractures is neglected. That model yielded nearly the same rock and transport parameters for each tracer, which means that the single-fracture approximation is acceptable and that matrix diffusion plays an important role. The hydraulic conductivity of the fault zone obtained from the tracer tests is about 1.5×10–2 m/s, whereas the regional hydraulic conductivity of the fractured rock mass is about 3×10–7 m/s, as estimated from the tritium age and the matrix porosity of about 2%. These values show that the hydraulic conductivity along the fault is several orders of magnitude larger than that of the remaining fractured part of the aquifer, which confirms the dominant role of the fault zones as collectors of water and conductors of fast flow. Received, April 1997 Revised, January 1998, August 1998 Accepted, August 1998  相似文献   

16.
Darcy’s law is the equation of reference widely used to model aquifer flows. However, its use to model karstic aquifers functioning with large pores is problematic. The physics occurring within the karstic conduits requires the use of a more representative macroscopic equation. A hydrodynamic model is presented which is adapted to the karstic aquifer of the Val d’Orléans (France) using two flow equations: (1) Darcy’s law, used to describe water flow within the massive limestone, and (2) the Brinkman equation, used to model water flow within the conduits. The flow equations coupled with the transport equation allow the prediction of the karst transfer properties. The model was tested by using six dye tracer tests and compared to a model that uses Darcy’s law to describe the flow in karstic conduits. The simulations show that the conduit permeability ranges from 5?×?10?6 to 5.5?×?10?5?m2 and the limestone permeability ranges from 8?×?10?11 to 6?×?10?10?m2. The dispersivity coefficient ranges from 23 to 53 m in the conduits and from 1 to 5 m in the limestone. The results of the simulations carried out using Darcy’s law in the conduits show that the dispersion towards the fractures is underestimated.  相似文献   

17.
The Guarani aquifer system (GAS) represents one of the biggest aquifers in the world and is the most relevant groundwater resource in South America. For the first time, by combining field and laboratory measurements, a high-resolution aquifer analog model of fluvial–aeolian sediments of the GAS in São Paulo State (Brazil) is constructed. Three parallel sections of frontal outcrops, 28 m × 5.8 m, and two parallel sections of lateral outcrops, 7 m × 5.8 m, are recorded during open-pit mining of sandy sediments and describe in detail the three-dimensional distribution of the local lithofacies and hydrofacies. Variations of hydraulic conductivity, K, and porosity, n, are resolved on the centimeter scale, and the most permeable units of the fluvial–aeolian facies association are identified. The constructed aquifer analog model shows moderate hydraulic heterogeneity and a mean K value of 1.36 × 10?4 m/s, which is greater than the reported range of K values for the entire GAS in São Paulo State. The results suggest that the examined sedimentary unit constitutes a relevant portion of the GAS in São Paulo State in the context of groundwater extraction and pollution. Moreover, the constructed aquifer analog is considered an ideal basis for future numerical model experiments, aiming at in-depth understanding of the groundwater flow and contaminant transport patterns at this GAS portion or at comparable fluvial–aeolian facies associations.  相似文献   

18.
In this study a multi-tracer test with fluorescent tracers was combined with time series analyses of natural tracers to characterize the dynamics of the solute transport through different recharge pathways and to study hydraulic behaviour of a binary karst system under low-flow conditions. Fluorescent tracer testing included the introduction of uranine, amidorhodamine G, or naphthionate at three injection points. Sampling and monitoring took place at two karst springs (Malenščica, Unica) and at two underground rivers (Pivka, Rak) recharging the Unica River at the Polje of Planina, SW Slovenia. Other monitored parameters included precipitation, spring or underground river discharge, water temperature, and electrical conductivity. Water samples were collected and analyzed for total organic carbon, Mg2+, SO4 2−, and NO3 − in the laboratory. In the study area, results of the tracer test suggest that contaminant transport in karst may be retarded for several weeks during low-flow conditions followed by increases in contaminant concentrations after subsequent rainfall events. Based on interpretation of tracer concentration breakthrough curves, low apparent dominant flow velocities (i.e., between 5.8 and 22.8 m/h through the well developed karst conduits, and 3.6 m/h through the prevailing vadose zone with a dominant influence of a diffuse recharge) were obtained. Together with analyses of hydro-chemographs the artificial tracing identified different origins of water recharging the studied aquifer. During prolonged low-water conditions the Malenščica spring is mainly recharged from the karst aquifer and the Unica spring by the sinking Pivka River. After more intensive rainfall events allogenic recharge from Cerknica prevails in the Malenščica spring, while the Unica spring drains mainly the allogenic water from the Pivka Valley. These findings of alternating hydraulic connections and drainage areas due to respective hydrological conditions are important and should be considered when monitoring water quality, implementing groundwater protection measures, and optimizing future water exploitation.  相似文献   

19.
Sulfate transport in a Coastal Plain confining unit, New Jersey, USA   总被引：1，自引：0，他引：1  
 A transient 1-D, two-pathway non-equilibrium deterministic advective dispersion model was used to examine the distribution of chloride (43–100 mg/L) and sulfate (57–894 mg/L) concentrations in the 35-m-thick section of the Lower confining unit, Atlantic Coastal Plain, New Jersey, USA. The model was used to constrain hypotheses about how pore-water chemistry changed over time. Explanations of the solute concentrations were explored by inverse and direct methods given a few known constraints, including concentrations of pore-water constituents from 12 core samples, reported simulated flow rates, and estimated hydrogeologic properties. The hypothesis that is best supported by the model results is that the distribution of chloride and sulfate concentrations in the confining unit reflect the history of the aquifer system since it was filled with seawater at the last eustatic high, about 84×103yr BP. The model simulates fresh-water flushing of the seawater-permeated silts at a steady upward pore-water flow velocity of 8.8×10–6 m/d, with a dispersion coefficient of 9.2×10–7 m2/d, a dimensionless partition expression for chloride, βCl=0.981, and a dimensionless exchange coefficient, ωCl=0.31×10–2. Sulfate concentrations were simulated over the flow path using flow and dispersion values calculated for chloride transport plus a retardation term. Parameters for sulfate transport include retardation coefficient=4.51, βSO4=0.994, and ωSO4=0.31×10–2. Sensitivity analysis indicates that the model is most sensitive to flow velocity, and that fresh-water flushing of the confining unit is best simulated by having seawater concentration levels at the inflow boundary of the confining unit exponentially decrease with a concentration half-life rate of 825 yr. Received, January 1997 / Revised, April 1998, October 1998, January 1999 / Accepted, January 1999  相似文献   

20.
An active and short-duration thermal tracer test (TTT) was conducted in a shallow sedimentary aquifer at the Lauswiesen test site, near Tübingen, Germany. By injecting 16  m3 of warm water at 22°C, a thermal anomaly was created, which propagated along the local groundwater flow direction. This was comprehensively monitored in five observation wells at a few meters distance. The purpose of this well-controlled experiment was to determine the practicability of such a TTT and its suitability to examine hydraulic characteristics of heterogeneous aquifers. The results showed that the thermal peak arrival times in the observation wells were consistent with previous observations from alternative field testing such as direct-push injection logging (DPIL). Combined analysis of depth-dependent temperatures and peak arrival times, and comparison with a numerical heat transport model, offers valuable insights into the natural flow field and spatial distribution of hydraulic conductivities. The study was able to identify vertical flow focusing and bypassing, which are attributed to preferential flow paths common in such sedimentary sand and gravel aquifers. These findings are fundamental for further development of experimental designs of active and short-duration TTTs and provide a basis for a more quantitative analysis of advective and conductive transport processes.  相似文献   

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