Showing papers in "Hydrogeology Journal in 2002"

Interactions between groundwater and surface water: the state of the science

Abstract: The interactions between groundwater and surface water are complex. To understand these interactions in relation to climate, landform, geology, and biotic factors, a sound hydrogeoecological framework is needed. All these aspects are synthesized and exemplified in this overview. In addition, the mechanisms of interactions between groundwater and surface water (GW–SW) as they affect recharge–discharge processes are comprehensively outlined, and the ecological significance and the human impacts of such interactions are emphasized. Surface-water and groundwater ecosystems are viewed as linked components of a hydrologic continuum leading to related sustainability issues. This overview concludes with a discussion of research needs and challenges facing this evolving field. The biogeochemical processes within the upper few centimeters of sediments beneath nearly all surface-water bodies (hyporheic zone) have a profound effect on the chemistry of the water interchange, and here is where most of the recent research has been focusing. However, to advance conceptual and other modeling of GW–SW systems, a broader perspective of such interactions across and between surface-water bodies is needed, including multidimensional analyses, interface hydraulic characterization and spatial variability, site-to-region regionalization approaches, as well as cross-disciplinary collaborations.

Choosing appropriate techniques for quantifying groundwater recharge

Abstract: DOI 10.1007/s10040-0010176-2 Abstract Various techniques are available to quantify recharge; however, choosing appropriate techniques is often difficult. Important considerations in choosing a technique include space/time scales, range, and reliabili- ty of recharge estimates based on different techniques; other factors may limit the application of particular tech- niques. The goal of the recharge study is important be- cause it may dictate the required space/time scales of the recharge estimates. Typical study goals include water-re- source evaluation, which requires information on re- charge over large spatial scales and on decadal time scales; and evaluation of aquifer vulnerability to contam- ination, which requires detailed information on spatial variability and preferential flow. The range of recharge rates that can be estimated using different approaches should be matched to expected recharge rates at a site. The reliability of recharge estimates using different tech- niques is variable. Techniques based on surface-water and unsaturated-zone data provide estimates of potential recharge, whereas those based on groundwater data gen- erally provide estimates of actual recharge. Uncertainties in each approach to estimating recharge underscore the need for application of multiple techniques to increase

Using groundwater levels to estimate recharge

Abstract: Accurate estimation of groundwater recharge is extremely important for proper management of groundwater systems. Many different approaches exist for estimating recharge. This paper presents a review of methods that are based on groundwater-level data. The water-table fluctuation method may be the most widely used technique for estimating recharge; it requires knowledge of specific yield and changes in water levels over time. Advantages of this approach include its simplicity and an insensitivity to the mechanism by which water moves through the unsaturated zone. Uncertainty in estimates generated by this method relate to the limited accuracy with which specific yield can be determined and to the extent to which assumptions inherent in the method are valid. Other methods that use water levels (mostly based on the Darcy equation) are also described. The theory underlying the methods is explained. Examples from the literature are used to illustrate applications of the different methods.

Evaluation of graphical and multivariate statistical methods for classification of water chemistry data

Abstract: A robust classification scheme for partitioning water chemistry samples into homogeneous groups is an important tool for the characterization of hydrologic systems. In this paper we test the performance of the many available graphical and statistical methodologies used to classify water samples including: Collins bar diagram, pie diagram, Stiff pattern diagram, Schoeller plot, Piper diagram, Q-mode hierarchical cluster analysis, K-means clustering, principal components analysis, and fuzzy k-means clustering. All the methods are discussed and compared as to their ability to cluster, ease of use, and ease of interpretation. In addition, several issues related to data preparation, database editing, data-gap filling, data screening, and data quality assurance are discussed and a database construction methodology is presented.

Artificial recharge of groundwater: hydrogeology and engineering

Abstract: Artificial recharge of groundwater is achieved by putting surface water in basins, furrows, ditches, or other facilities where it infiltrates into the soil and moves downward to recharge aquifers. Artificial recharge is increasingly used for short- or long-term underground storage, where it has several advantages over surface storage, and in water reuse. Artificial recharge requires permeable surface soils. Where these are not available, trenches or shafts in the unsaturated zone can be used, or water can be directly injected into aquifers through wells. To design a system for artificial recharge of groundwater, infiltration rates of the soil must be determined and the unsaturated zone between land surface and the aquifer must be checked for adequate permeability and absence of polluted areas. The aquifer should be sufficiently transmissive to avoid excessive buildup of groundwater mounds. Knowledge of these conditions requires field investigations and, if no fatal flaws are detected, test basins to predict system performance. Water-quality issues must be evaluated, especially with respect to formation of clogging layers on basin bottoms or other infiltration surfaces, and to geochemical reactions in the aquifer. Clogging layers are managed by desilting or other pretreatment of the water, and by remedial techniques in the infiltration system, such as drying, scraping, disking, ripping, or other tillage. Recharge wells should be pumped periodically to backwash clogging layers. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s10040-001-0182-4.

Groundwater recharge: an overview of processes and challenges

Abstract: Since the mid-1980s, a relative explosion of groundwater-recharge studies has been reported in the literature. It is therefore relevant to assess what is now known and to offer further guidance to practitioners involved in water-resource development. The paper summarizes current understanding of recharge processes, identifies recurring recharge-evaluation problems, and reports on some recent advances in estimation techniques. Emphasis is accorded to (semi-)arid regions because the need for information is greatest in those areas – groundwater is often the only water source, is vulnerable to contamination, and is prone to depletion. Few studies deal explicitly with groundwater recharge in temperate and humid zones, because recharge is normally included in regional groundwater investigations as one component of the water balance. The resolution of regional water-balance studies in (semi-)arid areas is, in contrast, often too low to quantify the limited recharge component with sufficient precision.

Groundwater recharge and agricultural contamination

Abstract: Agriculture has had direct and indirect effects on the rates and compositions of groundwater recharge and aquifer biogeochemistry Direct effects include dis- solution and transport of excess quantities of fertilizers and associated materials and hydrologic alterations relat- ed to irrigation and drainage Some indirect effects in- clude changes in water-rock reactions in soils and aqui- fers caused by increased concentrations of dissolved oxi- dants, protons, and major ions Agricultural activities have directly or indirectly affected the concentrations of a large number of inorganic chemicals in groundwater, for example NO 3 - , N 2 , Cl, SO 4 2- , H + , P, C, K, Mg, Ca, Sr, Ba, Ra, and As, as well as a wide variety of pesti- cides and other organic compounds For reactive con- taminants like NO 3 - , a combination of chemical, isoto- pic, and environmental-tracer analytical approaches might be required to resolve changing inputs from subse- quent alterations as causes of concentration gradients in groundwater Groundwater records derived from multi- component hydrostratigraphic data can be used to quan- tify recharge rates and residence times of water and dis- solved contaminants, document past variations in re- charging contaminant loads, and identify natural contam- inant-remediation processes These data indicate that many of the world's surficial aquifers contain transient records of changing agricultural contamination from the last half of the 20th century The transient agricultural groundwater signal has important implications for long- term trends and spatial heterogeneity in discharge

Aquifer overexploitation: what does it mean?

Abstract: Groundwater overexploitation and aquifer overexploitation are terms that are becoming common in water-resources management. Hydrologists, managers and journalists use them when talking about stressed aquifers or some groundwater conflict. Overexploitation may be defined as the situation in which, for some years, average aquifer abstraction rate is greater than, or close to the average recharge rate. But rate and extent of recharge areas are often very uncertain. Besides, they may be modified by human activities and aquifer development. In practice, however, an aquifer is often considered as overexploited when some persistent negative results of aquifer development are felt or perceived, such as a continuous water-level drawdown, progressive water-quality deterioration, increase of abstraction cost, or ecological damage. But negative results do not necessarily imply that abstraction is greater than recharge. They may be simply due to well interferences and the long transient period that follow changes in the aquifer water balance. Groundwater storage is depleted to some extent during the transient period after abstraction is increased. Its duration depends on aquifer size, specific storage and permeability. Which level of "aquifer overexploitation" is advisable or bearable, depends on the detailed and updated consideration of aquifer-development effects and the measures implemented for correction. This should not be the result of applying general rules based on some indirect data. Monitoring, sound aquifer knowledge, and calculation or modelling of behaviour are needed in the framework of a set of objectives and policies. They should be established by a management institution, with the involvement of groundwater stakeholders, and take into account the environmental and social constraints. Aquifer overexploitation, which often is perceived to be associated with something ethically bad, is not necessarily detrimental if it is not permanent. It may be a step towards sustainable development. Actually, the term aquifer overexploitation is mostly a qualifier that intends to point to a concern about the evolution of the aquifer-flow system in some specific, restricted points of view, but without a precise hydrodynamic meaning. Implementing groundwater management and protection measures needs quantitative appraisal of aquifer evolution and effects based on detailed multidisciplinary studies, which have to be supported by reliable data.

Identifying and quantifying urban recharge: a review

Abstract: The sources of and pathways for groundwater recharge in urban areas are more numerous and complex than in rural environments. Buildings, roads, and other surface infrastructure combine with man-made drainage networks to change the pathways for precipitation. Some direct recharge is lost, but additional recharge can occur from storm drainage systems. Large amounts of water are imported into most cities for supply, distributed through underground pipes, and collected again in sewers or septic tanks. The leaks from these pipe networks often provide substantial recharge. Sources of recharge in urban areas are identified through piezometry, chemical signatures, and water balances. All three approaches have problems. Recharge is quantified either by individual components (direct recharge, water-mains leakage, septic tanks, etc.) or holistically. Working with individual components requires large amounts of data, much of which is uncertain and is likely to lead to large uncertainties in the final result. Recommended holistic approaches include the use of groundwater modelling and solute balances, where various types of data are integrated. Urban recharge remains an under-researched topic, with few high-quality case studies reported in the literature.

Recharge and groundwater models: an overview

Abstract: Recharge is a fundamental component of groundwater systems, and in groundwater-modeling exercises recharge is either measured and specified or estimated during model calibration. The most appropriate way to represent recharge in a groundwater model depends upon both physical factors and study objectives. Where the water table is close to the land surface, as in humid climates or regions with low topographic relief, a constant-head boundary condition is used. Conversely, where the water table is relatively deep, as in drier climates or regions with high relief, a specified-flux boundary condition is used. In most modeling applications, mixed-type conditions are more effective, or a combination of the different types can be used. The relative distribution of recharge can be estimated from water-level data only, but flux observations must be incorporated in order to estimate rates of recharge. Flux measurements are based on either Darcian velocities (e.g., stream baseflow) or seepage velocities (e.g., groundwater age). In order to estimate the effective porosity independently, both types of flux measurements must be available. Recharge is often estimated more efficiently when automated inverse techniques are used. Other important applications are the delineation of areas contributing recharge to wells and the estimation of paleorecharge rates using carbon-14.

Main concepts of the "European approach" to karst-groundwater-vulnerability assessment and mapping

Abstract: In order to achieve some consistency in the establishment of groundwater intrinsic vulnerability maps in Europe, a new approach is proposed by Working Group 1 of the European COST Action 620 on "Vulnerability mapping for the protection of carbonate (karst) aquifers". A general procedure is offered which provides consistency while allowing the required flexibility for application to a continent and under conditions of varying geology, scale, information availability, time, and resources.

Estimating recharge at Yucca Mountain, Nevada, USA: comparison of methods

Abstract: Obtaining values of net infiltration, groundwater travel time, and recharge is necessary at the Yucca Mountain site, Nevada, USA, in order to evaluate the expected performance of a potential repository as a containment system for high-level radioactive waste. However, the geologic complexities of this site, its low precipitation and net infiltration, with numerous mechanisms operating simultaneously to move water through the system, provide many challenges for the estimation of the spatial distribution of recharge. A variety of methods appropriate for arid environments has been applied, including water-balance techniques, calculations using Darcy's law in the unsaturated zone, a soil-physics method applied to neutron-hole water-content data, inverse modeling of thermal profiles in boreholes extending through the thick unsaturated zone, chloride mass balance, atmospheric radionuclides, and empirical approaches. These methods indicate that near-surface infiltration rates at Yucca Mountain are highly variable in time and space, with local (point) values ranging from zero to several hundred millimeters per year. Spatially distributed net-infiltration values average 5 mm/year, with the highest values approaching 20 mm/year near Yucca Crest. Site-scale recharge estimates range from less than 1 to about 12 mm/year. These results have been incorporated into a site-scale model that has been calibrated using these data sets that reflect infiltration processes acting on highly variable temporal and spatial scales. The modeling study predicts highly non-uniform recharge at the water table, distributed significantly differently from the non-uniform infiltration pattern at the surface.

Hydrogeochemistry and cation-exchange processes in the coastal aquifer of Mar Del Plata, Argentina

Abstract: The aquifer of Mar del Plata is unconfined and composed of silt and fine sand. The sand fraction is mainly quartz, potassium feldspars, chalcedony, and gypsum. Volcanic-glass shards (40–60%) dominate the silt fraction, and the clays are of the smectite and illite groups. Calcium carbonate, in caliche form, constitutes about 10–20% of the sediment.

Geochemical and stable isotopic evolution of the Guarani Aquifer System in the state of São Paulo, Brazil

Abstract: The purpose of this report is to explain geochemical and stable isotopes trends in the Brazilian unit of the Guarani Aquifer System (Botucatu and Piramboia aquifers) in Sao Paulo State, Brazil. Trends of dissolved species concentrations and geochemical modeling indicated a significant role of cation exchange and dissolution of carbonates in downgradient evolution of groundwater chemistry. Loss of calcium by the exchange for sodium drives dissolution of carbonates and results in Na–HCO3 type of groundwater. The cation-exchange front moves downgradient at probably much slower rate compared to the velocity of groundwater flow and at present is located near to the cities of Sertaozinho and Aguas de Santa Barbara (wells PZ-34 and PZ-148, respectively) in a shallow confined area, 50–70 km from the recharge zone. Part of the sodium probably enters the Guarani Aquifer System. together with chloride and sulfate from the underlying Piramboia Formation by diffusion related to the dissolution of evaporates like halite and gypsum. High concentrations of fluorine (up to 13.3 mg/L) can be explained by dissolution of mineral fluoride also driven by cation exchange. However, it is unclear if the dissolution takes place directly in the Guarani Aquifer System or in the overlying basaltic Serra Geral Formation. There is depletion in δ2H and δ18O values in groundwater downgradient. Values of δ13C(DIC) are enriched downgradient, indicating dissolution of calcite under closed system conditions. Values of δ13C(DIC) in deep geothermal wells are very high (>–6.0‰) and probably indicate isotopic exchange with carbonates with δ13C about –3.0‰. Future work should be based on evaluation of vertical fluxes and potential for penetration of contamination to the Guarani Aquifer System.

Spatial and temporal distribution of groundwater recharge in northern Nigeria

Abstract: Moisture samples obtained from unsaturated-zone profiles in sands from northern Nigeria were used to obtain recharge estimates using the chloride (Cl) mass-balance method and to produce records of past recharge and climatic events. Recharge rates range from 14–49 mm/year, on the basis of unsaturated-zone Cl values and rainfall chemistry measured over eight years at three local stations. The unsaturated-zone results also provide a record of the changing recharge and climatic events of the past 80 years; this record compares quite well with modelling results using precipitation data from Maiduguri, especially for the late 20th-century period of drought. The best fit for the model is made, however, by using a lower mean rainfall Cl (0.65 mg/l) than that obtained from the mean of the field results (1.77 mg/l Cl). This result implies that the measured rainfall Cl probably overestimates the depositional flux of Cl, although the lower value is comparable to the minimum of the measured rainfall Cl values (0.6 mg/l Cl). Recharge estimates made using these lower Cl values range from 16–30 mm/year. The spatial variability was then determined using results from 360 regional shallow wells over 18,000 km2.

The influence of faults in basin-fill deposits on land subsidence, Las Vegas Valley, Nevada, USA

Abstract: The role of horizontal deformation caused by pumping of confined-aquifer systems is recognized as contributing to the development of earth fissures in semiarid regions, including Las Vegas Valley, Nevada. In spite of stabilizing water levels, new earth fissures continue to develop while existing ones continue to lengthen and widen near basin-fill faults. A three-dimensional granular displacement model based on Biot's consolidation theory (Biot, MA, 1941, General theory of three-dimensional consolidation. Jour. Applied Physics 12:155–164) has been used to evaluate the nature of displacement in the vicinity of two vertical faults. The fault was simulated as (1) a low-permeability barrier to horizontal flow, (2) a gap or structural break in the medium, but where groundwater flow is not obstructed, and (3) a combination of conditions (1) and (2). Results indicate that the low-permeability barrier greatly enhances horizontal displacement. The fault plane also represents a location of significant differential vertical subsidence. Large computed strains in the vicinity of the fault may suggest high potential for failure and the development of earth fissures when the fault is assumed to have low permeability. Results using a combination of the two boundaries suggest that potential fissure development may be great at or near the fault plane and that horizontal deformation is likely to play a key role in this development.

Remote sensing of soil moisture: implications for groundwater recharge

Abstract: Remote sensing provides information on the land surface. Therefore, linkages must be established if these data are to be used in groundwater and recharge analyses. Keys to this process are the use of remote sensing techniques that provide information on soil moisture and water-balance models that tie these observations to the recharge. Microwave remote sensing techniques are used to map the spatial domain of surface soil moisture and to monitor its temporal dynamics, information that cannot be measured using other techniques. The physical basis of this approach is presented with examples of how microwave remote sensing is utilized in groundwater recharge and related studies.

Estimating impacts of changed land use on recharge: review of modelling and other approaches appropriate for management of dryland salinity

Abstract: To manage dryland salinity, one needs to know how changed land use affects groundwater recharge. Few techniques are available for comparing 'deep drainage' under different land uses. Soil-tracer methods, although good for replication and remote field sites, are subject to spatial variability. Lysimeters are good for comparisons but are difficult for drier areas and sloping land. Agronomic water-balance studies, where appropriate soil-water measurements exist, may be used with a soil-vegetation model to estimate long-term deep drainage. Complex models are required to analyze specific land-use differences, such as perenniality and root and leaf area dynamics, but models require intensive and extensive data for calibration. This approach is time-consuming, labour-intensive, and difficult in remote locations. Because of the one-dimensionality of most soil-vegetation models and the small fraction of the total water balance that is deep drainage, little success has occurred in extrapolating beyond the research plot, or to spatially heterogeneous systems such as alley farming. Some 'top-down' modelling and landscape disaggregation approaches have been partially successful in making catchment or regional-scale predictions. The direction for further work depends on the level of recharge reduction that is required for most groundwater systems and difficulties that it imposes.

Unsaturated zones as archives of past climates: toward a new proxy for continental regions

Abstract: Natural chemical and isotopic tracers contained in unsaturated-zone moisture profiles are being developed as potential new archives for reconstructing recharge history, as well as palaeoclimatic or palaeobotanical conditions over time scales ranging from 20–120,000 years. Results worldwide to date are reviewed, and examples from northern Africa and the western USA are discussed in detail. Encouraging results are obtained from relatively homogeneous deposits such as Quaternary dune sands, where Cl profiles are compared both with the instrumental record, such as rainfall and river-gauging records, and 3H profiles. Model studies have helped to define the persistence time of unsaturated-zone signals, where evidence of a 20-year event such as the Sahel drought may persist for 1,000 years.

Groundwater-flow modeling in the Yucatan karstic aquifer, Mexico

Abstract: The current conceptual model of the unconfined karstic aquifer in the Yucatan Peninsula, Mexico, is that a fresh-water lens floats above denser saline water that penetrates more than 40 km inland The transmissivity of the aquifer is very high so the hydraulic gradient is very low, ranging from 7–10 mm/km through most of the northern part of the peninsula The computer modeling program AQUIFER was used to investigate the regional groundwater flow in the aquifer The karstified zone was modeled using the assumption that it acts hydraulically similar to a granular, porous medium As part of the calibration, the following hypotheses were tested: (1) karstic features play an important role in the groundwater-flow system; (2) a ring or belt of sinkholes in the area is a manifestation of a zone of high transmissivity that facilitates the channeling of groundwater toward the Gulf of Mexico; and (3) the geologic features in the southern part of Yucatan influence the groundwater-flow system The model shows that the Sierrita de Ticul fault, in the southwestern part of the study area, acts as a flow barrier and head values decline toward the northeast The modeling also shows that the regional flow-system dynamics have not been altered despite the large number of pumping wells because the volume of water pumped is small compared with the volume of recharge, and the well-developed karst system of the region has a very high hydraulic conductivity

Combining stochastic facies and fractal models for representing natural heterogeneity

Abstract: Sedimentary deposits are often characterized by various distinct facies, with facies structure relating to the depositional and post-depositional environments. Permeability (k) varies within each facies, and mean values in one facies may be several orders of magnitude larger or smaller than those in another facies. Empirical probability density functions (PDFs) of log(k) increments from multi-facies structures often exhibit properties well modeled by the Levy PDF, which appears unrealistic physically. It is probable that the statistical properties of log(k) variations within a facies are very different from those between facies. Thus, it may not make sense to perform a single statistical analysis on permeability values taken from a mix of distinct facies. As an alternative, we employed an indicator simulation approach to generate large-scale facies distributions, and a mono-fractal model, fractional Brownian motion (fBm), to generate the log(k) increments within facies. Analyses show that the simulated log(k) distributions for the entire multi-facies domain produce apparent non-Gaussian log(k) increment distributions similar to those observed in field measurements. An important implication is that Levy-like behavior is not real in a statistical sense and that rigorous statistical measures of the log(k) increments will have to be extracted from within each individual facies.

Integrating geophysical and hydrochemical borehole-log measurements to characterize the Chalk aquifer, Berkshire, United Kingdom

Abstract: Geophysical and hydrochemical borehole-logging techniques were integrated to characterize hydraulic and hydrogeochemical properties of the Chalk aquifer at boreholes in Berkshire, UK. The down-hole measurements were made to locate fissures in the chalk, their spatial extent between boreholes, and to determine the groundwater chemical quality of the water-bearing layers. The geophysical borehole logging methods used were caliper, focused resistivity, induction resistivity, gamma ray, fluid temperature, fluid electrical conductivity, impeller and heat-pulse flowmeter, together with borehole wall optical-imaging. A multiparameter data transmitter was used to measure groundwater temperature, electrical conductivity, dissolved oxygen, pH, and redox potential of the borehole fluid down-hole. High permeability developed at the Chalk Rock by groundwater circulation provides the major flow horizon at the Banterwick Barn study site and represents a conduit system that serves as an effective local hydraulic connection between the boreholes. The Chalk Rock includes several lithified solution-ridden layers, hardgrounds, which imply a gap in sedimentation possibly representing an unconformity. Lower groundwater temperature, high dissolved-oxygen content, and flowmeter evidence of preferential groundwater flow in the Chalk Rock indicated rapid groundwater circulation along this horizon. By repeating the logging at different times of the year under changing hydraulic conditions, other water-inflow horizons within the Chalk aquifer were recognized.

Movement of water through the thick unsaturated zone underlying Oro Grande and Sheep Creek Washes in the western Mojave Desert, USA

Abstract: Previous studies indicate that a small quantity of recharge occurs from infiltration of streamflow in intermittent streams in the upper Mojave River basin, in the western Mojave Desert, near Victorville, California. Chloride, tritium, and stable isotope data collected in the unsaturated zone between 1994 and 1998 from boreholes drilled in Oro Grande and Sheep Creek Washes indicate that infiltration of streamflow occurs to depths below the root zone, and presumably to the water table, along much of Oro Grande Wash and near the mountain front along Sheep Creek Wash. Differences in infiltration at sites along each wash are the result of hydrologic variables such as proximity to the mountain front, quantity of streamflow, and texture of the subsurface deposits. Differences in infiltration between the washes are the result of large-scale geomorphic processes. For example, Oro Grande wash is incised into the Victorville fan and infiltration has occurred at approximately the same location over recent geologic time. In contrast, Sheep Creek Wash overlies an active alluvial fan and the stream channel can move across the fan surface through time. Infiltration does not occur to depths below the root zone at control sites outside of the washes.

Towards a policy for sustainable use of groundwater by non-governmental organisations in Afghanistan

Abstract: A 'first pass' groundwater management policy has been developed for use by non-governmental organisations (NGOs) in Afghanistan, designed to prevent derogation of existing traditional water sources, aquifer over-abstraction and chemical deterioration of soil and groundwater quality. Key elements include (1) continuing promotion of groundwater as a drinking water source, (2) a presumption against use of motorised pumps to abstract groundwater for irrigation unless other options (surface water, qanats) are not available, (3) the use of groundwater for irrigation as a temporary alternative to surface water (i.e. a strategy for drought survival) rather than as a long-term development policy, (4) limiting groundwater abstraction to a long-term average of 1 l s–1 km–2, (5) siting irrigation wells at least 500 m from other groundwater sources and (6) analysing irrigation groundwater for electrical conductivity, sodium absorption ratio, boron and residual sodium carbonate alkalinity. Analyses of these parameters indicate that groundwater from some areas is of dubious suitability for irrigation. In some villages and towns, groundwater contains elevated nitrate and faecal bacteria concentrations, probably derived from latrines, sewage or animal wastes.

Assessing the potential for significant and episodic recharge in southwestern Australia using rainfall data

Abstract: Agricultural practices in semi-arid parts of southwestern Australia have increased recharge and raised groundwater levels. As a result, land salinization has occurred. Managers aim to address the problem by reducing recharge, but it is not known whether all recharge is regular and seasonal or whether a substantial component is episodic (i.e. occurs in irregular pulses). Approaches that reduce regular recharge may not be effective at reducing recharge that is episodic.

The gravity method in groundwater exploration in crystalline rocks: a study in the peninsular granitic region of Hyderabad, India

Abstract: The application of variations in the earth's gravity in groundwater exploration on a regional scale, especially in sedimentary basins, metamorphic terrains, valley fills, and for buried alluvial channels, is well established. However, its use in hard crystalline rocks is little known. In granite, for example, the upper weathered layer is a potential primary aquifer, and the underlying fractured rock can form a secondary aquifer. Fracturing and weathering increases the porosity of a rock, thereby reducing the bulk density. Changes in gravity anomalies of 0.1–0.7 mGal for granites, due to weathering or variations in lithology, can be detected.

Groundwater flow and solute transport at the Mourquong saline-water disposal basin, Murray Basin, southeastern Australia

Abstract: Saline groundwater and drainage effluent from irrigation are commonly stored in some 200 natural and artificial saline-water disposal basins throughout the Murray-Darling Basin of Australia. Their impact on underlying aquifers and the River Murray, one of Australia's major water supplies, is of serious concern. In one such scheme, saline groundwater is pumped into Lake Mourquong, a natural groundwater discharge complex. The disposal basin is hydrodynamically restricted by low-permeability lacustrine clays, but there are vulnerable areas in the southeast where the clay is apparently missing. The extent of vertical and lateral leakage of basin brines and the processes controlling their migration are examined using (1) analyses of chloride and stable isotopes of water (2H/1H and 18O/16O) to infer mixing between regional groundwater and lake water, and (2) the variable-density groundwater flow and solute-transport code SUTRA. Hydrochemical results indicate that evaporated disposal water has moved at least 100 m in an easterly direction and that there is negligible movement of brines in a southerly direction towards the River Murray. The model is used to consider various management scenarios. Salt-load movement to the River Murray was highest in a "worst-case" scenario with irrigation employed between the basin and the River Murray. Present-day operating conditions lead to little, if any, direct movement of brine from the basin into the river.

Monitoring artificially stimulated fluid movement in the Cretaceous Dakota aquifer, western Kansas

Abstract: Aquifer properties can be evaluated by monitoring artificially stimulated fluid movements between wells, if the fluid is heated. Changes in the temperature profile recorded in observation wells indicate the flow path of the heated fluid, which in effect acts as a tracer. A fluid-flow experiment in the Cretaceous Dakota Formation at the Hodgeman County site, west-central Kansas, demonstrated the advantage of using the distributed optical-fiber temperature sensing method for monitoring transient temperature conditions in this hydrological application. The fluid flow in the aquifer was increased by producing water from a pumping well and injecting heated water in an injection well 13 m (43 ft) distant from the pumping well. The time-temperature series data obtained and compared with results from previous pumping tests point to interwell heterogeneity of the aquifer and to a zone in the sandstone aquifer of high hydraulic conductivity. However, the experiment would have allowed further clarification of aquifer heterogeneity and thermal properties if at least one observation well had been present between the injection and production wells.

The use of aquifer-media characteristics to model vulnerability to contamination, Manawatu region, New Zealand

Abstract: The Manawatu region of New Zealand consists largely of alluvial deposits which form extensive unconfined aquifers. Intensification of land use, with consequent increases in contamination potential, places the groundwater at serious risk. Aquifer-vulnerability assessment is the first step in protecting this vital resource. Characteristics of the aquifer medium affect vulnerability because the concentration of contaminants is controlled partly by the ability of the aquifer to dilute them. Static water levels in over 1,100 bores were used as point estimates to generate the phreatic surface for the study area. Well-drillers' descriptions of the aquifer medium below this surface were then assigned to grain-size classes. These descriptions were treated as point estimates from which variations in aquifer-media characteristics were interpolated. The point estimates were used to build a network of Theissen polygons which, in turn, were used to estimate the spatial distribution of the aquifer-media characteristics. Analysis of bores drilled subsequently has shown that the regional trends in characteristics predicted from the point data were realistic. Aquifer vulnerability was then assessed on the basis of grain size. Larger grain sizes indicate higher hydraulic conductivity, thus promoting greater dilution. This results in lower vulnerability to contamination. Nitrate concentrations, the major contaminant in the area, show a high correlation with the assigned vulnerability classes. Regional assessment of aquifer-medium vulnerability is therefore an effective tool in groundwater resource management and protection.