Regional Groundwater Flow Direction Research Articles

Geology and hydrothermal resources in the northern Lake Abaya area (Ethiopia)

The northern Lake Abaya area located in the southern part of the Main Ethiopian Rift (MER) evidently contains an advective hydrothermal system. Regional extensional tectonics and subsidence that began in Late Miocene was followed by rift margin rhyolitic volcanism which produced extensive ignimbrite succession and rift shoulder trachytic volcanism in Pliocene. The extensional axis of the MER became the locus of volcanic activity in the Quaternary with bimodal basalt – rhyolitic volcanic products in the rift floor. Shallow crustal magma chambers feeding the axial volcanic complexes provide heat for the hydrothermal system which reside in Tertiary volcanic succession and is capped by lacustrine and volcanoclastic graben infill sediments. Duguna Fango rhyolitic volcanic complex which has extruded as much volume of volcanic products as all the other felsic centers put together is the most important heat source for the hydrothermal system and related manifestations. The closed drainage basin centered at Lake Abaya with adjacent plateau receiving over 1500 mm annual precipitation maintains a stable recharge for the prevailing hydrothermal system. Regional ground water flow direction is to the southwest following Bilate River which is the major river draining into the lake flowing parallel to the major NNE-SSW structural pattern of MER. Analyses of remote sensing data provided some insight into how the structural fabric had a control on the distribution of the groups of hydrothermal manifestations identified as hydrothermal fields. Thermal infrared image showed distinct thermal signature over the rift floor where hydrothermal fields are situated as compared to the plateau and areas covered by the cap-rock. A hydrothermal field around Duguna Fango volcanic complex has volcanological favorable setting despite lower inferred reservoir equilibrium geothermometer temperatures on the thermal springs. About 30 km to the south two other hydrothermal fields are located at a latitudinal distance of less than 15 km between each other. The near boiling point spring #6 of Northwest Abaya discharge mature water with highest geothermal fluid – host rock equilibrium geothermometer temperatures which together with a nearby fumarolic activity may be explained as an outflow. Across Chewkare graben the Bolcho and northeast Abaya hydrothermal fields located between Bilate and Gidabo rivers geochemically appear to be from a different high-enthalpy geothermal reservoir probably related to the cluster of felsic volcanic centers such as Chericho, Kilisa, Donga and Werencha. The high temperature springs indicate the existence of at least two geochemically distinct hydrothermal reservoirs in the study area. It remains to be proven weather an up-flow of an advective hydrothermal system centered to the north around Duguna Fango have long distance concealed out flows to the south on the northern shores of Lake Abaya which is the hydrologic depocenter.

Modeling on the Performance of Standing Column Wells During Continuous Operation Under Regional Groundwater Flow

Coupled hydrogeological–thermal simulation of the standing column well (SCW) system is essential to provide an optimized configuration and operation schedule for boreholes on the site. This paper presents numerical investigations and thermohydraulic evaluation of SCW system operating under continuous flow regime. A three-dimensional numerical model for groundwater flow and heat transport is used to analyze the heat exchange in the ground. The model includes the effects of convection and conduction heat transfer, heat loss to the adjacent confining strata, and hydraulic anisotropy. The operation scenario consists of continuous injection and recovery, and four periods per year to simulate the seasonal temperature conditions. For different parameters of the system, performances have been evaluated in terms of variations in the recovery temperature. The calculated temperatures at the producing pipe were relatively constant within a certain range through the year and fluctuating quarterly a year. Pipe-to-pipe distance, injection/production rate, ground thickness, and permeability considered in the model are shown to impact the predicted temperature profiles at each stage and the recovery water temperature. The influence of pressure gradient, which determines the direction and velocity of regional groundwater flow, is also substantial.

Numerical Simulation on the Continuous Operation of an Aquifer Thermal Energy Storage System Under Regional Groundwater Flow

A three-dimensional numerical model for groundwater flow and heat transport is used to analyze an aquifer thermal energy storage system operating under a continuous flow regime. This study emphasizes the influence of regional groundwater flow on the performance of the system under various operation scenarios. The pressure gradient across the system, which determines the direction and velocity of regional groundwater flow, has a substantial influence on the aquifer thermal storage. Injection/production rate and geometrical size of the aquifer used in the model also impact the predicted temperature distribution at each stage and the recovery water temperature. The hydrogeological-thermal simulation is shown to be an integral part in the prediction of performance for a process as complicated as aquifer thermal energy storage systems.

Combined use of frequency-domain electromagnetic and electrical resistivity surveys to delineate near-lake groundwater flow in the semi-arid Nebraska Sand Hills, USA

A frequency-domain electromagnetic (FDEM) survey can be used to select locations for the more quantitative and labor-intensive electrical resistivity surveys. The FDEM survey rapidly characterized the groundwater-flow directions and configured the saline plumes caused by evaporation from several groundwater-dominated lakes in the Nebraska Sand Hills, USA. The FDEM instrument was mounted on a fiberglass cart and towed by an all-terrain vehicle, covering about 25 km/day. Around the saline lakes, areas with high electrical conductivity are consistent with the regional and local groundwater flow directions. The efficacy of this geophysical approach is attributed to: the high contrast in electrical conductivity between various groundwater zones; the shallow location of the saline zones; minimal cultural interference; and relative homogeneity of the aquifer materials.

Simulation on the cyclic operation of an open borehole thermal energy storage system under regional groundwater flow

Coupled hydrogeological-thermal simulation is performed to analyze the effect of the configuration of boreholes and operation schedule on the performance of the borehole thermal energy storage (BTES) system. This paper presents numerical investigations and thermohydraulic evaluation on the cyclic flow regime operation of open borehole thermal energy storage system under the effects of regional groundwater flow. A three-dimensional numerical model for groundwater flow and heat transport in the ground is used to determine the annual variation of recovery temperature from the thermal energy storage. The model includes the effects of convective and conductive heat transfer, heat loss to the adjacent confining strata, and hydraulic anisotropy. The operation scenario consists of cyclic injection and recovery after holding interval and four periods per year to simulate the seasonal temperature conditions. For different parameters of the system, performances were compared in terms of the variation of extraction temperature. The calculated water temperature at the producing pipe remains relatively constant within a certain range throughout the simulation period. Heat loss, injection/production rate, aquifer thickness, and permeability ratio used in the model are shown to impact the predicted temperature profiles at each stage and the recovery water temperature. The influence of pressure gradient, which determines the direction and velocity of regional groundwater flow, is substantial for all cases considered.

Variation of groundwater salinity in the partially irrigated Amudarya River delta, Uzbekistan

The Amudarya delta region contains surface and groundwater resources that discharge into the shrinking Large Aral Sea and ultimately control its future fate. These freshwater resources are prerequisites for sustaining the population of the region. However, salinization and pollution caused by agricultural irrigation is a key problem for these water systems. Here, we report results from a recent field measurement campaign conducted during April 2005 which included 24 monitoring wells located in an irrigated region of the Amudarya delta, thereby extending the historical data set of groundwater levels and salinity measurements. This data set is combined with corresponding data from a downstream, non-irrigated region that was formerly irrigated (together covering 16,100km 2 between the Uzbek cities of Nukus and Muynak). This comparison shows that in the downstream region, which is currently not irrigated, shallow groundwaters are far more saline (average 23g l − 1) than the currently irrigated region (average 3g l − 1). We estimate that the unconfined aquifer within the 13,500km 2 non-irrigated zone of study area contains 9billion tons of salt, or almost as much salt as the entire Aral Sea (containing 11billion tons of salt and covering an area of 20,000km 2 in year 2000). Within the non-irrigated zone, there are statistically significant large-scale spatial correlations between groundwater salinity and distance to the Amudarya River, irrigation canals and surface water bodies when distance is measured along the modelled regional groundwater flow direction. Generally, groundwater salinities are lower downstream of surface water bodies in the non-irrigated zone. Annual fluctuations in groundwater salinity are too large to be explained by input from surface water (Amudarya) or wind-blown salt from the dried Aral Sea sediments. Salt transport by groundwater is the only plausible remaining explanation for these changes.

Hydrochemical framework of the aquifer in and around East Kolkata Wetlands, West Bengal, India

The area lies between Hugli river in the northwest and Bidyadhari river in the east and includes the East Kolkata Wetlands. The East Kolkata Wetlands is included in the List of Wetlands of International Importance (“Ramsar List”), as per the Convention on Wetlands signed in Ramsar, Iran, in 1971. This wetland has been declared as a Ramsar site on the 19th August 2002 (Ramsar site no. 1208) and therefore has acquired an international status. The area is a part of the lower deltaic plain of the Bhagirathi–Ganga river system and is generally flat in nature. The sub-surface geology of the area is completely blanketed by the Quaternary fluviatile sediments comprising a succession of clay, silty clay, sand and sand mixed with occasional gravel. The Quaternary aquifer is sandwiched between two clay sequences. The confined aquifer is made up of moderately well sorted sand and reflects fluviatile environment of deposition. The regional groundwater flow direction is from east to west. Detailed geochemical investigations of 40 groundwater samples along with statistical analysis (for example, correlation and principal component analysis) on these chemical data reveal: (i) four types of groundwater quality, for example, good, poor, very poor and water unsuitable for drinking purpose, (ii) four hydrochemical facies which may be assigned to three broad types such as “fresh”, “blended”, and “brackish” waters, (iii) the evolution of the “blended” water is possibly due to hydraulic mixing of “fresh” and “brackish” waters within the aquifer matrix and/or in well mixing, and (iv) absence of Na–Cl facies indicates continuous flushing of the aquifer.

Major ions composition of the groundwater and surface water systems and their geological and geochemical controls in the Ethiopian volcanic terrain

Broad hydrochemical survey has been carried out to study the spatial variation of the major ions composition of the surface and groundwater systems in the Ethiopian volcanic terrain and associated Plio-Quaternary sediments. The result revealed wide hydrochemical variations controlled by geological, geochemical, geomorphological and climatological factors. The total ionic concentration increases dramatically from the highlands towards the rift valley following the regional groundwater flow directions to low-lying regions characterized by low annual rainfall and high evapotranspiration. In the rift the total dissolved solids (TDS) variation is dramatic (in places more than 50 fold). The highland waters are predominantly fresh with low total ionic concentrations, TDS varying often from 50 to 1200 mg/l; with dominant ions of Ca, Mg and HCO 3 . The rift valley waters have very high TDS (varying in a wide range between 200 and 73,015 mg/l) characterized by high Na and HCO3, in places highly enriched with F. Waters of the southern and central Main Ethiopian Rift (MER) are dominated by HCO 3 tending to SO 4 and Cl type in northern and central Afar mainly due to changes in lithology related to evaporite deposits and due to the natural chemical evolution of waters as they move from highland recharge to low-lying rift discharge areas. Deep geothermal waters, alkaline terminal lakes of the MER and small highly evaporated saline and brackish lakes of the Afar have the highest ionic concentrations. Keywords : Ethiopian rift, hydrogeology, ionic concentration, TDS, volcanic terrain SINET: Ethiopian Journal of Science Vol. 28(2) 2005: 171-188

Estimating Darcy flow velocities from correlated anomalies in temperature logs

Fluid flow in reservoirs can be identified by its characteristic signatures in temperature logs. The method presented here is based on the great sensitivity of the subsurface temperature field with respect to heat advected by fluid flow. This requires that all other temperature perturbations are eliminated. These may be caused, for instance, by heterogeneous thermal properties or paleoclimate transients, both of which may distort an otherwise linear temperature log. We propose a simple geothermal technique for quantifying regional strata-bound flow based on a detailed analysis of borehole temperatures. Depending on the spatial data distribution and the geothermal situation, this method yields results of varying quality. In a case study with data from southern Germany, the method yields a lower bound for the flow magnitude. Another case study from northern Germany yields the magnitude of the regional flow owing to a larger data set and information of the direction of flow from additional independent information. Under optimum conditions, this method also yields an estimate of the direction of regional strata-bound groundwater flow as well as the uncertainty in magnitude and direction. Finally, the full potential of the method is illustrated using a synthetic data set.

Large‐scale hydraulic conductivities inferred from three‐dimensional groundwater flow and 4He transport modeling in the Carrizo aquifer, Texas

Through a series of groundwater flow and 4He transport simulations, this study illustrates the conceptual and practical gains achieved by expanding a two‐dimensional (2‐D) model to a true 3‐D one through an application in the Carrizo aquifer and surrounding formations in southwestern Texas. The 3‐D simulations allow for a more detailed and accurate definition of the heterogeneities of the system by specifically identifying and differentiating processes that directly impact the three‐dimensional hydraulic conductivity field. It is shown that while hydraulic conductivity decreases exponentially along the regional groundwater flow direction, such decrease is better described as a function of depth rather than recharge distance. This relationship reflects the combined influences of differential compaction of the media as well as downdip lithological change. The intrinsic permeability derived from this relationship agrees with field information and with previous findings obtained for the continental crust for depths ≤2 km, suggesting that for large scales, decrease rate of permeability with depth is independent of the media. Results also suggest that the solution for groundwater flow simulations based on calibration of hydraulic heads depends on the ratio between hydraulic conductivities of different formations, indicating that an infinite number of solutions are available for calibration of 3‐D groundwater flow models. Understanding how geological processes directly affect the 3‐D hydraulic conductivity field at the regional scale is essential not only to hydrogeological applications but also at improving our understanding of the Earth's crust and mantle dynamics by allowing for a more accurate quantification of helium and heat fluxes.

The spatial distribution of groundwater flooding in a chalk catchment in southern England

AbstractGroundwater flooding occurred in the upper parts of many chalk rivers in the UK during the exceptionally wet winter of 2000–01. This provided a rare opportunity to investigate the spatial distribution of groundwater discharge and flooding along the normally dry intermittent headwaters of a chalk catchment. The extent of flooding along the River Pang, upstream of the seasonal head, was mapped using aerial photography, and point measurements of flow and water temperature were used to identify the contributing reaches of the river. The results are discussed in the context of the geological and groundwater conditions. The occurrence of flooding can largely be explained by the regional groundwater flow directions, but increased flow in some locations may be as a result of preferential groundwater flow along lines of geological structure. Published by John Wiley & Sons, Ltd.

Containment of groundwater contamination plumes: minimizing drawdown by aligning capture wells parallel to regional flow

Pump-and-treat systems that are installed to contain contaminated groundwater migration typically involve placement of extraction wells perpendicular to the regional groundwater flow direction at the down gradient edge of a contaminant plume. These wells capture contaminated water for above ground treatment and disposal, thereby preventing further migration of contaminated water down gradient. In this work, examining two-, three-, and four-well systems, we compare well configurations that are parallel and perpendicular to the regional groundwater flow direction. We show that orienting extraction wells co-linearly, parallel to regional flow, results in (1) a larger area of aquifer influenced by the wells at a given total well flow rate, (2) a center and ultimate capture zone width equal to the perpendicular configuration, and (3) more flexibility with regard to minimizing drawdown. Although not suited for some scenarios, we found orienting extraction wells parallel to regional flow along a plume centerline, when compared to a perpendicular configuration, reduces drawdown by up to 7% and minimizes the fraction of uncontaminated water captured.

Hydraulic containment: analytical and semi-analytical models for capture zone curve delineation

We present an efficient semi-analytical algorithm that uses complex potential theory and superposition to delineate the capture zone curves of extraction wells. This algorithm is more flexible than previously published techniques and allows the user to determine the capture zone for a number of arbitrarily positioned extraction wells pumping at different rates. The algorithm is applied to determine the capture zones and optimal well spacing of two wells pumping at different flow rates and positioned at various orientations to the direction of regional groundwater flow. The algorithm is also applied to determine capture zones for non-colinear three-well configurations as well as to determine optimal well spacing for up to six wells pumping at the same rate. We show that the optimal well spacing is found by minimizing the difference in the stream function evaluated at the stagnation points.

Application of isotope and water balance approaches for the study of the hydrogeological regime of the Bishoftu crater lakes, Ethiopia

The major Ethiopian rift lakes have been studied for various purposes in the last few decades. However, crater lakes situated elsewhere in the country are some of the poorly understood hydrologic systems. This study focuses on quantifying the groundwater fluxes and assessment of the hydrogeological regime of the Bishoftu Crater Lakes using conventional water balance and stable environmental isotope (2h and 18o) techniques. The convenient geological setting of these lakes has provided a suitable base for determining groundwater exchange rates using stable isotopes coupled with hydrologic balance. The result indicates that these lakes have highly variable groundwater fluxes. Evaporation and groundwater fluxes are the major components of the hydrologic balance. Unlike the other lakes, lake Hora appears to have extremely low groundwater outflow. The lakes are highly evaporated with respect to the present day precipitation. Other groundwater and surface water sources are relatively depleted in the heavy isotopes. The isotope and hydrochemical signature indicates that the circulation of groundwater and recharge is fast and the regional groundwater flow direction is from north to south. SINET: Ethiopian Journal of Science Volume 24, No. 2 (December 2001), pp. 151-166 Key words/phrases: Crater lakes, Ethiopian rift, groundwater, isotopes, water balance

Hydrogeological sections of the Pergamino basin (Buenos Aires province, Argentina): audio magnetotelluric and geochemical results

Aquifers in the northeastern zone of the Buenos Aires Province, mainly used for agricultural purposes, were studied regionally by several authors, but local studies are necessary because great variations may occur in different basins, and well data are limited. In this work, geophysical exploration by means of Audio-magnetotelluric soundings and geochemical analyses of some well data were carried out at the Pergamino rivulet basin. Depth and thicknesses of the upper hydrogeological section, the Epiparaniana, including the Pampeano and Puelches aquifers, were determined. Also, the hydro chemical evolution along the direction of regional groundwater flow was studied. The depth of the very conductive layer corresponding to the salty Puelches and the "Green Miocene", limiting the agricultural water usage, came out to be very variable depending on the zone. Geochemical analyses showed that the Pampeano aquifer has bicarbonate sodium water, with an increase of electrical conductivity, sodium and bicarbonates contents at a local recharge zone with impermeable and bad drainage soils and a topographical small slope.

Source-identification investigations of petroleum contaminated groundwater in the Missouri Ozarks

The growing population in the Missouri Ozarks rely on the abundant supply of potable groundwater found in the fractured bedrock underlying the region. Protection of this valuable resource from surface, or near-surface contamination is essential to the health and safety of the citizens of Missouri. Petroleum products stored in underground storage tanks pose a threat to groundwater if a release occurs. “Pools” of free product of petroleum can become trapped in voids commonly found on the karst bedrock of the Ozarks. These pools become underground sources of contamination by slowly dissolving into the groundwater. Although the free product rarely travels far from the surface source, dissolved-phase contamination can migrate thousands of meters through underground conduits. Dissolved phase petroleum degasses and partitions out of the groundwater in these conduits. Consequently, contaminated groundwater is not commonly found more than a kilometer or two (few thousand feet) from the source. The Missouri Department of Natural Resources, Division of Geology and Land Survey's Leaking Underground Storage Tank Unit (DGLS/LUST) investigates sites where petroleum-contaminated groundwater has been found in springs or private wells but a source of the contamination is unknown. These investigations start at the regional scale by determining the recharge area of the contaminated groundwater and by identifying the potentially responsible parties (PRPs). A review of DGLS documents and well logs supplemented by field measurements of water levels in private wells throughout the area is used to construct a potentiometric map and determine regional groundwater flow direction. Water tracing is used to confirm or deny the validity/accuracy of the potentiometric map and to establish hydrogeologic connections between the potential surface source of the contamination and the affected groundwater. Fluorescent dyes are injected into the subsurface, recovered in packets containing activated carbon and analyzed by a synchronously scanning spectrofluorometer. These results are compared to background fluorescence conditions established at the site prior to injection. The background conditions and dye recovery results must be submitted to the Missouri Water Trace Committee for approval as an independent quality assurance/quality control measure. Each of the PRPs found to be within the recharge area of the contaminated groundwater are investigated to determine the source, or sources, of the contamination. Initial field screening techniques are followed up by confirmatory sampling of soil and groundwater. Geophysical methods such as resistivity and electromagnetic conductivity are used to locate the bedrock surface, fracture patterns and areas of extensive petroleum contamination. On-site drilling activities are performed at each of the PRPs with on-site contamination screening methods such as analyzing soil gas within the borehole with a photoionization detector (PID) and colorimetric tubes. Soil samples are collected and can be analyzed on-site with immunoassay kits with confirmatory samples sent to the lab. Temporary monitoring wells are installed in boreholes to collect on-site groundwater data. Depth-to-water levels are measured, and the relative elevations of the temporary wells are surveyed so that a more detailed potentiometric map can be constructed. Water samples are collected from the temporary wells, and the boreholes are abandoned per Missouri Law. Petroleum fingerprinting by fluorescence is another tool used to investigate PRPs and determine the source of contamination. Petroleum is extracted from soil or water samples collected during on-site drilling and compared to the petroleum which is extracted from a sample collected at the contaminated spring or well.

Hydrogeologic Site Characterization Using Azimuthal Resistivity Surveys

The purpose of this work is to evaluate whether azimuthal resistivity surveys (ARS) provide diagnostic information for mapping preferred hydraulic conductivity directions in unconsolidated sediments as they do in fractures/jointed media. Nineteen azimuthal resistivity surveys (ARS) were conducted in three different places within a shallow, unconsolidated, sand and gravel aquifer located at the Michigan Integrated Remediation Technology Laboratory (MIRTL). Two of the three places where soundings were conducted lie along the flow path where a controlled tracer release using bromide took place. The ARS were conducted using a Wenner array, with electrode separations (AB/3) that ranged from 4.0–11.0 meters. Electrical anisotropy ratios gave a range of 1.08–1.37 for all 19 soundings. Ground water direction of the induced plume was along the regional flow path as determined for the period from July 1996, using continuous water level recorders. Thus, the observed electrical anisotropy ratios were not significant enough to represent changes in the regional ground water flow direction. However, the results from this work suggest that ARS may be used as part of an integrated hydrogeological study which may yield semi‐quantitative information of the electrical anisotropy with increasing electrode spacing.

Flow of river water into a Karstic limestone aquifer. 1. Tracing the young fraction in groundwater mixtures in the Upper Floridan Aquifer near Valdosta, Georgia

The quality of water in the Upper Floridan aquifer near Valdosta, Georgia is affected locally by discharge of Withlacoochee River water through sinkholes in the river bed. Data on transient tracers and other dissolved substances, including Cl −, 3H, tritiogenic helium-3 ( 3He), chlorofluorocarbons (CFC-11, CFC-12, CFC-113), organic C (DOC), O 2 (DO), H 2S, CH 4, δ 18O, δD, and 14C were investigated as tracers of Withlacoochee River water in the Upper Floridan aquifer. The concentrations of all tracers were affected by dilution and mixing. Dissolved Cl −, δ 18O, δD, CFC-12, and the quantity ( 3H+ 3He) are stable in water from the Upper Floridan aquifer, whereas DOC, DO, H 2S, CH 4, 14C, CFC-11, and CFC-113 are affected by microbial degradation and other geochemical processes occurring within the aquifer. Groundwater mixing fractions were determined by using dissolved Cl − and δ 18O data, recognizing 3 end-member water types in the groundwater mixtures: (1) Withlacoochee River water (δ 18O=−2.5±0.3‰, Cl −=12.2±2 mg/l), (2) regional infiltration water (δ 18O=−4.2±0.1‰, Cl −=2.3±0.1 mg/l), and (3) regional paleowater resident in the Upper Floridan aquifer (δ 18O=−3.4±0.1‰, Cl −=2.6±0.1 mg/l) (uncertainties are ±1σ). Error simulation procedures were used to define uncertainties in mixing fractions. Fractions of river water in groundwater range from 0 to 72% and average 10%. The influence of river-water discharge on the quality of water in the Upper Floridan aquifer was traced from the sinkhole area on the Withlacoochee River 25 km SE in the direction of regional groundwater flow. Infiltration of water is most significant to the N and NW of Valdosta, but becomes negligible to the S and SE in the direction of general thickening of post-Eocene confining beds overlying the Upper Floridan aquifer.

Evaluating the source and residence times of groundwater seepage to streams, New Jersey Coastal Plain

A conceptual model of the patterns and residence times of groundwater seepage to gaining streams indicates that groundwater seepage originates from sources that are both near and far from the stream. Consequently, the age of groundwater seepage across a stream‐channel transect increases from its banks to its center and becomes progressively older with distance downstream. A groundwater flow model and particle‐tracking analysis of the Cohansey River Basin in the New Jersey Coastal Plain supports this conceptual model and demonstrates that the orientation of the stream channels with respect to the regional groundwater flow direction, and the heterogeneities of the aquifer and stream‐channel patterns, can shift source area locations and distributions of groundwater residence time from those expected. Groundwater samples collected from stream transects were analyzed for nitrogen, representative of widespread agricultural land use in the basin in recent decades, and for chlorofluorocarbons, used to estimate groundwater ages. The patterns of nitrogen concentration and the age of groundwater entering the stream channel corroborate model inferences. The conceptual model of groundwater seepage to streams presented herein is relevant to unconfined aquifer systems with gaining streams and demonstrates how nonpoint‐source contaminants are transported to streams by groundwater. Results are useful for the design of programs needed to monitor stream‐water quality.

A single continuum approximation of the solute transport in fractured porous media

Solute transport in fractured porous media is described by the single continuum model, i.e., equivalent porous medium model. In this model, one-dimensional solute transport in the fracture and two-dimensional solute transport in the porous rock matrix is considered. The network of fractures embedded in the porous rock matrix is idealized as two orthogonally intersecting families of equally spaced, parallel fractures directed at 45° to the regional groundwater flow direction. Governing equations are solved by the finite element method, and upstream weighting technique is used in order to prevent the oscillation of solution in case of highly advection dominated transport. The breakthrough curves similar to those of the one-dimensional solute transport problem in ordinary porous media are obtained as a function of time according to volume or flux averaging of the concentration profile across the width of the flow region. The equivalent parameters, i.e., porosity and overall coefficient of longitudinal dispersivity are obtained by trial-and-error method. Analyses for the non-sorbing solute transport case show that within the range of considered parameters, and except for the region very close to the source, application of the single continuum model in the idealized fracture system is sufficient for modeling the solute transport in fractured porous media. And this numerical scheme is shown to be applicable to the sorbing solute and radionuclide transport.