Showing papers on "Groundwater flow published in 1989"

Radon in groundwater: A tool to assess infiltration from surface waters to aquifers

Abstract: We measured the concentrations of natural 222Rn (half-life 3.8 days) in groundwater at three sites in Switzerland; here groundwater is recharged mainly by river water. Upon infiltration and movement in the ground, the radon concentration in the water increases by more than two orders of magnitude to reach a steady state. This increase was found at two of the three sites. At the site of main interest, we used the ingrowth of radon between the river and nearby observation wells to estimate groundwater residence times of up to about four half lives. We assumed that the ingrowth of radon can be described by the growth law of radioactivity, that the progenitors of radon (226Ra, 238U) are homogeneously distributed in the aquifer, and that the freshly infiltrated water is not mixed significantly with older groundwater. A linear regression through the data at the site of main interest yielded an average flow velocity of 4.6 m −1, which confirms earlier tracer observations. Radon accumulates to higher concentrations, when the top soil layer is frozen or exhibits a high moisture content. During these conditions the radon data cannot be used for dating purposes.

Applying groundwater flow models in vapor extraction system design

Abstract: Vapor extraction systems have recently been used in a variety of environmental engineering applications, including controlling methane migration and remediating problems associated with spills of volatile and semi-volatile organic compounds. The differential equations that govern pressure flow of gas and vapor in soil are nonlinear in that gas density depends upon gas pressure. However, if the maximum pressure difference between any two points in the flow field is less than approximately 0.5 atmospheres, the differential equations developed to model groundwater flow provide good approximations to gas transport. Vapor extraction systems generally operate under pressure differences on the order of 0.2 atmospheres. Analytical and numerical groundwater flow models can therefore be used to model vapor and gas transport if the proper set of input variables is defined. Analytical groundwater flow models are used to evaluate the results of a field gas extraction test. The results of these evaluations indicate groundwater models can provide an efficient and readily-accessible tool to aid in designing vapor extraction systems.

Reliable aquifer remediation in the presence of spatially variable hydraulic conductivity: From data to design

Abstract: Most optimization models for groundwater quality management have ignored the effects of uncertainty due to spatial variability of hydraulic conductivity. Here we explicitly incorporate this uncertainty into a procedure for the optimal design of aquifer remediation strategies. Local hydraulic conductivity and head data are used to quantify the uncertainty which is traced through to target a reliable remediation design. The management procedure is based on the stochastic approach to groundwater flow and contaminant transport modeling, in which the log-hydraulic conductivity is represented as a random field. The remediation design procedure has two steps. The first is solution of the stochastic inverse model. Maximum likelihood and Gaussian conditional mean estimation are used to characterize the random conductivity field based on the hydraulic conductivity and hydraulic head measurements. Based on this statistical characterization, conditional simulation is used to generate numerous realizations (maps) of spatially variable hydraulic conductivity that honor the head and conductivity data. The second step is solution of the groundwater quality management model. Two management model formulations are presented. The first, termed the multiple realization management model, simultaneously solves the nonlinear simulation-optimization problem for a sampling of hydraulic conductivity realizations. It is shown that reclamation design based on as few as 30 conductivity realizations can provide reliable (over 90%) remediation strategies. The second model, termed the Monte Carlo management model, solves the nonlinear simulation-optimization problem individually for a sampling of hydraulic conductivity realizations. This provides a relationship between pumping (cost) and reliability. Each of the management models is linked with the stochastic inverse model, and each is demonstrated for two cases: (1) the available data are limited to hydraulic conductivity measurements and (2) both hydraulic conductivity and hydraulic head measurements are used.

The influence of groundwater flow on thermal regimes in mountainous terrain: A model study

Abstract: Groundwater flow in high-relief mountainous terrain is modeled to examine how geology, surface topography, climate, and regional heat flow influence the pattern and magnitude of an advective thermal disturbance. A numerical procedure is used to estimate the position of the water table within constraints provided by the available infiltration rate and the permeability of the mountain massif. Results show that where the water table is located at depth within the mountain massif, the rate of groundwater recharge, rather than permeability, is the appropriate factor to characterize the potential for an advective disturbance. Thermal disturbances should be assessed within the framework of the two-dimensional character of the flow system because interpretations based on one-dimensional models are prone to significant error. Modeling of site-specific systems can exploit the existence of a permeability “window,” where temperatures in discharge areas and in springs discharging from fracture zones reach peak values. Temperatures of thermal springs reflect the complex interaction between flow within the mountain massif and flow through permeable fracture zones. Consequently, simple calculations relating the geothermal gradient, depth of maximum groundwater circulation, and discharge temperature improperly represent the physics of the process.

Simulation of calcite dissolution and porosity changes in saltwater mixing zones in coastal aquifers

Abstract: Thermodynamic models of aqueous solutions have indicated that the mixing of seawater and calcite-saturated fresh groundwater can produce a water that is undersaturated with respect to calcite. Mixing of such waters in coastal carbonate aquifers could lead to significant amounts of limestone dissolution. The potential for such dissolution in coastal saltwater mixing zones is analyzed by coupling the results from a reaction simulation model (PHREEQE) with a variable density groundwater flow and solute transport model. Idealized cross sections of coastal carbonate aquifers are simulated to estimate the potential for calcite dissolution under a variety of hydrologic and geochemical conditions. Results show that limestone dissolution in mixing zones is strongly dependent on groundwater flux and nearly independent of the dissolution kinetics of calcite. The amount of dissolution varies within a mixing zone, depending on the properties, physical dimensions, and boundary conditions of the aquifer system. Nearly all of the dissolution occurs in the fresher side of the mixing zone, with the maximum dissolution occurring in water that is fresher than that predicted solely by geochemical reaction models. The greatest porosity and permeability development occur at the toe and at the top of the mixing zone. If permeability increases as porosity increases, asymmetry in the dissolution causes the mixing zone to migrate landward over time. Dissolution rates indicated by the model show that this mechanism can produce significant increases in porosity and permeability over time spans on the order of tens of thousands of years. Given the comparatively long span of geologic time, this process may be largely responsible for porosity and permeability development observed in those carbonate rocks through which a freshwater-saltwater mixing zone had at one time migrated.

Spatially Distributed Modeling: Conceptual Approach, Coupling Surface Water And Groundwater

Abstract: This chapter deals with the joint modeling of surface and groundwater flows by presenting and describing the MC model. The purpose of this deterministic physically-based model is to simulated the behavior of available water resources for one or several watershed. The model integrates surface flow, streamflow, flow in the unsaturated zone, groundwater flow and the interactions between rivers and water tables. Its formulation and its structure, especially its nested square meshes of variable sizes, give a great deal of flexibility to the model; this facilitates adaptation to variable modeling scales and to a wide range of geological geographical and climatological conditions. An application of the MC model on the Caramy watershed (France) is presented.

Ground-water flow in the Central Valley, California

Abstract: The agricultural productivity of the Central Valley is dependent on the availability of water from irrigation. About 7.3 million acres of cropland in the Central Valley receives about 22 million acre-feet of irrigation water annually. One half of this irrigation water is supplied by ground water, which amounts to about 20 percent of the Nation's ground-water pumpage. Ground water is important as a stable supply of irrigation water because of the high variability of surface-water supplies in the Central Valley. This large ground-water development during the past 100 years has had major impacts on the aquifer system, such as decline in water levels, land subsidence, depletion of the aquifer storage, and increase in recharge. The flow conditions before and during development were simulated on a regional scale using a three-dimensional finite-difference flow model. The concept presented in this report considers the entire thickness of the continental deposits as one aquifer system which has varying vertical leakance that depends on several factors, including amount of fine-grained sediments. The average horizontal hydraulic conductivity is about 6 feet per day, and the average thickness of the continental deposits is about 2,400 feet. The simulation results are shown on maps for comparison with observedmore » hydrologic data. A description of the computer-tape file, which contains estimates of recharge/discharge, and the aquifer properties used in the simulation are included in appendix A and B, respectively. The theoretical basis of calculating borehole hydraulic conductance of multilayer wells which cause increases in vertical leakance during the post-development period is discussed in appendix C.« less

Bias in Groundwater Samples Caused by Wellbore Flow

Abstract: Design of physical installations and sampling procedures for ground-water monitoring networks, particularly for detection and analysis of possible contaminants, is a topic of great scientific and practical interest at the present time. Recent practice in the design of monitoring networks associated with known contaminant sources sometimes includes an array of monitoring wells with long well screens (up to 50 feet or more). Numerical experiments with a detailed three-dimensional ground-water flow model indicate that significant wellbore flow can occur in contaminant monitoring wells with long well screens that are embedded in homogeneous aquifers with very small vertical head differences in the aquifer. This ``short circuiting'' of flow through boreholes should exist at some level on all scales. Consideration of the general flow pattern within the borehole, the flow pattern in the aquifer adjacent to the borehole, and the process of obtaining water samples from the borehole suggests that in many situations...

Mapping Recharge Areas Using a Ground-Water Flow Model – A Case Study

Abstract: We have developed a method to calculate ground- water recharge rates using the mass-balance equation, water- table elevation data, estimates of hydraulic conductivity, and aquifer thickness data, and have applied this method to produce a map of the recharge and discharge patterns for a ground-water basin in central Wisconsin. This recharge mapping method is simplified using a modified computer program, the USGS Modular Groundwater Flow Model (McDonald and Harbaugh, 1984). The modeled recharge pattern compares favorably with a recharge map based on field observations. Because recharge rates are extremely sensitive to hydraulic conductivity, the magnitudes of the calculated rates are less reliable than the patterns of recharge and discharge areas. However, introducing stream discharge data constrains the model to produce net recharge rates averaged over the basin which agree with estimates of the basin yield. Because the method is insensitive to the position of lateral boundaries, it can be used to map recharge over areas within basins that are not physically bounded. Recharge maps made with this method can be used to design ground-water monitoring networks and as frameworks for interpreting geochemical or potentiometric data.

Groundwater and surface water hydrology of a small groundwater-fed fen

Abstract: The hydrology of a small quaking fen was investigated by measuring all components of the water budget. Water-level measurements indicated that the fen is a focus for groundwater discharge, and that there is a lateral (subsurface) flow from the fen toward surrounding areas during most of the year. The hydrology, however, is strongly influenced by man, as water tables are maintained during dry periods by pumping water into the area. This action results in a reversal of flow patterns as surface water from a ditch infiltrates the fen and groundwater recharge occurs. Two approaches for calculation of groundwater and surface water terms were compared. The two procedures differ in the way that vertical and horizontal hydraulic conductivity of the soil is calculated. Hydraulic conductivity estimated from in situ tube experiments was one to two orders of magnitude lower than hydraulic conductivity estimates from numerical water budget analyses. Four one-month periods were selected for calculations of complete water budgets. Results of calculations of water flow were compared in respect to the overall error in the water budget (calculated as percent difference between inputs and outputs). Water budgets calculated from data on hydraulic conductivity gave high residual errors (56.% ± 16.3%). When hydraulic conductivity was derived from numerical analyses, the errors were very small (2.8% ± 3.1%) and the difference between inputs and outputs over a period of 127 days was only 2.6 mm (0.45%).

An isotopic investigation of groundwater in the Central San Juan Basin, New Mexico: Carbon 14 dating as a basis for numerical flow modeling

Abstract: Carbon 14 dating of groundwater can be used to help determine the transmissivity distributions of aquifers. This method may offer significant advantages, in certain respects, over traditional aquifer pumping test techniques. We have applied 14C dating to a hydraulic analysis of a multilayer aquifer system in the central San Juan Basin of New Mexico. After corrections for geochemical evolution of the solutes, the influence of dispersive processes on the 14C distribution was investigated. A model incorporating stochastic dispersion theory indicated that macroscopic dispersion exerted only a small influence on the measured 14C activities. The 14C-derived transmis sivity distribution was used to construct a numerical flow model which was applied to an analysis of interaquifer leakage. The model showed that even though vertical flow between aquifers was significant, in this case it did not cause the 14C distribution to differ significantly from that predicted by a simple piston flow model. Carbon 14 can be used as a basis for detailed hydraulic evaluations of groundwater flow in areas where traditional well hydraulics methods are not practical.

Measuring groundwater transport through lake sediments by advection and diffusion

Abstract: A method for estimating low rates of groundwater inflow and outflow through the bottom sediments of surface waters was developed and tested. A one-dimensional advection-diffusion model was fitted to measured pore water profiles of two nonreactive solutes, tritiated water and chloride, and the advection rate was calculated by a nonlinear least squares technique. Using 3H profiles measured 0–0.5 m below the sediment-water interface, rates of groundwater advection into a lake through interbedded sands and gyttja were estimated to be about 1.0 m/year. In midlake locations underlain by soft organic gyttja, rates of advection were much lower (<0.1 m/year). Knowledge of the rate and direction of groundwater flow substantially altered the interpretation of pore water profiles within the sediments and the fluxes of solutes. This technique can be used to estimate flow rates less than 2 m/annum with minimal disturbance, without enclosing the sediments in a container, in a diversity of systems.

Estimation of the groundwater resources of the Berkshire Downs supported by mathematical modeling

Abstract: This paper describes a study of the Chalk aquifer of the Berkshire Downs. Extensive fieldwork provided sufficient information for the development of a mathematical model of the aquifer system. Particular emphasis was placed on the representation of the variation of changing transmissivity with saturated depth and the simulation of streams and rivers; flows from the Chalk to the Upper Greensand were also found to be important in certain locations. Comparisons between field and modelled values of both groundwater flow and groundwater head showed good agreement. The model was then used for predictive purposes with particular emphasis on the use of groundwater for river augmentation.

Conjunctive Use of Geophysical and Geological Data in the Study of an Alluvial Aquifer

Abstract: A geoelectric survey, composed of vertical electrical soundings, was conducted over part of the lower Platte Valley alluvial aquifer of east-central Nebraska. Survey results, in conjunction with seismic-survey data and test- hole-log information, provided the basis to map the areal extent and thickness of a near-surface, low-permeability clay unit. Because of the low hydraulic conductivity of the clay, the unit forms a confining layer over a significant part of the aquifer. Generally, the approach to evaluating the lower Platte Valley alluvial aquifer for sustained yield has been to assume that the entire flow system is under water-table conditions. However, numerical-model simulations with and without the clay unit indicate that this confining layer has a significant effect on the hydraulic behavior of the groundwater flow system under pumping stresses. An estimate of the bulk porosity of the aquifer was made by using geoelectric data and applying the Archie equation. Values of porosity ranged from 0·27 to 0·43; although reasonable for fluvial sand and gravel, the range is higher than previous estimates.

Studies of geology and hydrology in the Basin and Range Province, Southwestern United States, for isolation of high-level radioactive waste; characterization of the Death Valley region, Nevada and California

Abstract: The Death Valley region, Nevada and California, in the Basin and Range province, is an area of about 80,200 sq km located in southern Nevada and southeastern California. Precambrian metamorphic and intrusive basement rocks are overlain by a thick section of Paleozoic clastic and evaporitic sedimentary rocks. Mesozoic and Cenozoic rocks include extrusive and intrusive rocks and clastic sedimentary rocks. Structural features within the Death Valley indicate a long and complex tectonic evolution from late Precambrian to the present. Potential repository host media in the region include granite and other coarse-grained plutonic rocks, ashflow tuff, basaltic and andesitic lava flows, and basin fill. The Death Valley region is composed largely of closed topographic basins that are apparently coincident with closed groundwater flow systems. In these systems, recharge occurs sparingly at higher altitudes by infiltration of precipitation or by infiltration of ephemeral runoff. Discharge occurs largely by spring flow and by evaporation and transpiration in the playas. Death Valley proper, for which the region was named, is the ultimate discharge area for a large, complex system of groundwater aquifers that occupy the northeastern part of the region. The deepest part of the system consists of carbonate aquifers that connect closed topographicmore » basins at depth. The discharge from the system occurs in several intermediate areas that are geomorphically, stratigraphically, and structurally controlled. Ultimately, most groundwater flow terminates by discharge to Death Valley; groundwater is discharged to the Colorado River from a small part of the region.« less

Hydrogeologic assessment—Figeh Spring, Damascus, Syria

Abstract: Hydrogeological studies at Figeh Springs were directed to determine groundwater flow paths, research, storage and discharge units, and the maximum reliable yield. The project was designed to provide information upon which to base pumpage to augment low-season flows from the spring which is the major water supply for the city of Damascus, Syria. As a basis for conclusions and recommendations, work included extensive surface geologic mapping, air photographic interpretation, a detailed well and spring inventory, and a quality of water sampling program. Geologic structural work included mapping and jointing, faulting, and folding, and an analysis of their impact on groundwater movement.

Rehabilitation of clogged discharge wells in the Netherlands

Abstract: During the last ten years much research has been conducted by KIWA, in close cooperation with various waterworks, into the rehabilitation of clogged production wells. During this research it became apparent that various types of clogging could be distinguished. One type of clogging is caused by the mixing of groundwaters with incompatible chemical compositions in the well itself, due to the vertical chemical heterogeneity of groundwater in the aquifer. Another type of clogging is caused by the increased velocity of groundwater flow near the borehole, which increases the intensity of various biological and chemical processes. The selection of the correct method of rehabilitation, the rehabilitation results, and the costs, all depend on the type of cloging that has occurred.

Landscape ecology of uncultivated lowlands in Central Spain

Abstract: The relationship between groundwater and landscape in representative areas of the Spanish plateaux is discussed, with special attention given to the Douro River basin. The study focusses on the transference of water and matter that is conditioned by groundwater flow systems, and also on water bodies, wet meadows, marshes, saline soils under their influence. These factors are of great importance in semiarid areas. Using photointerpretation, interviews, vegetation plots, water samples from wells and springs and soil samples, and the results of data processing (principal component analysis, shared information) we find that hydrological processes are the main controlling factor in the ecological function and variation of uncultivated lowlands. These processes include the alternation of recharge and discharge areas, the geochemical evolution of groundwater and the independent flows of the regional system. The landscapes in recharge and discharge areas are compared, as well as the influence of the evolutionary stage of the groundwater in the latter areas (glycophyte or halophyte vegetation). After observing the ecological importance of these aquifer discharges systems, the causes of their accelerated transformation are analyzed.

A discrete kernel method of characteristics model of solute transport in water table aquifers

Abstract: A simulation model of two-dimensional solute transport in water table aquifers is developed in this paper. The linearity of the governing partial differential equation of groundwater flow is exploited by using the discrete kernel approach that has been successfully employed previously in groundwater flow modeling. With this approach, the flow velocities resulting from different pumping and injection schemes can be efficiently computed. The conventional implementation of the method of characteristics is improved using an influence area particle tracking scheme that avoids the oscillations and step jumps of breakthrough curves. The concept of a porous medium consisting of mobile and immobile zones is introduced into the solution of the two-dimensional advection dispersion equation. Model results are compared with those generated with an analytical solution and with a conventional method of characteristics model.

Urban groundwater rise control: case study

Abstract: Seepage of wastewater from cesspools, and of leachate from green areas and parks, results in an annual groundwater rise of 0.5 m in Jeddah, Saudi Arabia. The rise causes several environmental and structural problems. A network of 110 observation wells is built to collect information regarding groundwater conditions, and to estimate soil settlement rates under different groundwater lowering conditions, and soil drainage rates. The hydraulic conductivity values range from 0.3 to 0.4 m/day. The groundwater recharge is mainly from cesspools and irrigated areas and its rate ranges from 0.6 to 1.2 mm/day. Three alternates, namely sanitary sewer, vertical drainage, and horizontal drains are evaluated. The horizontal drainage system is selected and a procedure is presented for its design in urban areas. Computer programs are developed for the analysis of steady and nonsteady flows into drains. Vitrified clay pipe material is selected because of its local availability and resistance to corrosion. The drained water is not suitable for irrigation because of its high salinity and sodium hazard. However, the quality of the drained water meets the local sea-discharge standards, and is therefore discharged into the Red Sea.

Hydrogeology of the South-Central Kentucky Karst

Abstract: The hydrogeologic setting provides the physical constraints determining the flow characteristics of an aquifer. Stratigraphy, lithology, and structure determine the type of aquifer, the location of recharge and discharge areas, and the nature of the porosity and permeability. If groundwater flow and the evolution of the conduit system are described by rate equations, the hydrogeologic setting defines the boundary conditions.