Showing papers on "Groundwater flow published in 1997"

Hydrochemical Evolution of Sodium-Sulfate and Sodium-Chloride Groundwater Beneath the Northern Chihuahuan Desert, Trans-Pecos, Texas, USA

Abstract: Groundwater beneath the northern Chihuahuan Desert, Trans-Pecos, Texas, USA, occurs in both carbonate and siliciclastic aquifers beneath a thick unsaturated zone in shallow Rio Grande alluvium. Groundwater hydrochemical evolution was investigated by analyzing soils, soils leachates, bolson-fill sediments, water from the unsaturated zone, and groundwater from three major aquifers. Ionic relations, mineral saturation states, and geochemical modeling show that groundwater compositions are controlled by reactions in the unsaturated zone, mineralogy of unsaturated sediments and aquifers, position in the groundwater flow system, and extensive irrigation. Recharge to aquifers unaffected by irrigation is initially a Ca-HCO3 type as a result of dissolving carbonate surficial salts. With continued flow and mineral-water interaction, saturation with calcite and dolomite is maintained, gypsum is dissolved, and aqueous Ca and Mg are exchanged for adsorbed Na to produce a Na-SO4 water. Groundwater in Rio Grande alluvium is a Na-Cl type, reflecting river-water composition and the effects of irrigation, evapotranspiration, and surficial salt recycling. These results document two hydrochemical evolution paths for groundwater in arid lands. If recharge is dilute precipitation, significant changes in water chemistry can occur in unsaturated media, ion exchange can be as important as dissolution-precipitation reactions in determining groundwater composition, and mineral-water reactions ultimately control groundwater composition. If recharge is return flow of irrigation water that already contains appreciable solutes, mineral-water reactions are less important than irrigation-water composition in determining groundwater chemistry.

Multiphase groundwater flow near cooling plutons

Abstract: We investigate groundwater flow near cooling plutons with a computer program that can model multiphase flow, temperatures up to 1200°C, thermal pressurization, and temperature-dependent rock properties. A series of experiments examines the effects of host-rock permeability, size and depth of pluton emplacement, single versus multiple intrusions, the influence of a caprock, and the impact of topographically driven groundwater flow. We also reproduce and evaluate some of the pioneering numerical experiments on flow around plutons. Host-rock permeability is the principal factor influencing fluid circulation and heat transfer in hydrothermal systems. The hottest and most steam-rich systems develop where permeability is of the order of 10−15 m2. Temperatures and life spans of systems decrease with increasing permeability. Conduction-dominated systems, in which permeabilities are ≤10−16 m2, persist longer but exhibit relatively modest increases in near-surface temperatures relative to ambient conditions. Pluton size, emplacement depth, and initial thermal conditions have less influence on hydrothermal circulation patterns but affect the extent of boiling and duration of hydrothermal systems. Topographically driven groundwater flow can significantly alter hydrothermal circulation; however, a low-permeability caprock effectively decouples the topographically and density-driven systems and stabilizes the mixing interface between them thereby defining a likely ore-forming environment.

Quantitative solutions in hydrogeology and groundwater modeling.

Abstract: CONTENTS: Porosity and Related Parameters. Laboratory Permeameters. Grain Size Analysis. One-Dimensional Steady State Flow. One-Dimensional Transient Flow. Flow Nets. Water Wells, Steady State Flow. Water Wells, Transient Flow. Spring Flow. Time Series Modeling. Numeric Groundwater Flow Modeling. References and Recommended Readings. Appendices. Index. Back Cover Copy

Microorganisms as tracers in groundwater injection and recovery experiments: a review

Abstract: Modern day injection and recovery techniques designed to examine the transport behavior of microorganisms in groundwater have evolved from experiments conducted in the late 1800s, in which bacteria that form red or yellow pigments were used to trace flow paths through karst and fractured-rock aquifers. A number of subsequent groundwater hydrology studies employed bacteriophage that can be injected into aquifers at very high concentrations (e.g., 1013 phage ml−1) and monitored through many log units of dilution to follow groundwater flow paths for great distances, particularly in karst terrain. Starting in the 1930s, microbial indicators of fecal contamination (particularly coliform bacteria and their coliphages) were employed as tracers to determine potential migration of pathogens in groundwater. Several injection and recovery experiments performed in the 1990s employed indigenous groundwater microorganisms (both cultured and uncultured) that are better able to survive under in situ conditions. Better methods for labeling native bacteria (e.g. by stable isotope labeling or inserting genetic markers, such as the ability to cause ice nucleation) are being developed that will not compromise the organisms' viability during the experimental time course.

Flow reversals in peatlands influenced by local groundwater systems

Abstract: Piezometric head data from various depths were examined at two peatlands in Ontario, Canada and one peatland in Sweden influenced by small-scale, shallow groundwater systems. Data from different hydrogeological settings show reversals in groundwater flow leading to discharge in topographically high regions of peatlands in isolation from large-scale groundwater flow. It is suggested that subsurface flow within peat can reverse in direction in response to water deficit and water-table drawdown. The data presented here refute the assumption that local groundwater flow in peatlands is unidirectional and further illustrate the fact that measurable subsurface water flow can occur at depth in peat isolated from large-scale groundwater flow systems. In the light of implicit assumptions made by many workers on water movement in peatlands, especially when connected to small-scale groundwater systems, the consequences of such reversals are paramount in understanding the hydrology and biogeochemistry of peatlands. © 1997 by John Wiley & Sons, Ltd.

Sulphate dynamics in relation to groundwater–surface water interactions in headwater wetlands of the southern canadian shield

Abstract: The spatial and temporal distribution of sulphate (SO4) concentrations in peat pore water and the outlet streams of two forested swamps was related to variations in the magnitude of upland runoff, wetland water levels and flow path. The swamps were located in headwater catchments with contrasting till depths typical of the southern Canadian Shield. Inputs of SO4 from shallow hillslope tills and streams showed little seasonal variation in either source or concentration in both swamps. Sulphate dynamics at the outlet stream reflected hydrological and biogeochemical processes within the valley wetlands, which in turn were partly controlled by catchment hydrogeology. During high runoff, maximum water table elevations and peak surface flow in the swamps resulted in upland inputs largely bypassing anoxic peat. Consequently, SO4 concentrations of 8–10 mg/l at the swamp outlets were similar to stream and groundwater inputs. During periods of low flow, concentrations of SO4 at the swamp outlets declined to less than 3 mg/l. At this time lower water table elevations resulted in increased interaction of input water with anoxic peats, and therefore, SO4 reduction. Contrasts in till depth and the nature of groundwater flow between catchments resulted in differences in SO4 dynamics between years and swamps. In dry summers the absence of groundwater inputs to the swamp in the catchment with thin till resulted in a large water table drawdown and re-oxidation of accumulated S, which contributed to maximum SO4 concentrations (up to 35 mg/l) during storm runoff. Continuous groundwater input to the swamp in the catchment with deeper till was critical to maintaining saturated surfaces and efficient SO4 retention during both dry and wet summers. A conceptual model of wetland SO4 retention and export, based on catchment hydrogeology, is developed to generalize the SO4 dynamics of valley bottom wetlands at the landscape scale. © 1997 by John Wiley & Sons, Ltd.

A method for the optimal location of monitoring wells for detection of groundwater contamination in three‐dimensional heterogenous aquifers

Abstract: We present an optimization method for the design of monitoring well networks to detect initial groundwater contamination in three-dimensional heterogenous aquifers. A Monte Carlo–based approach generates a large number of equally likely realizations of a random hydraulic conductivity field and a contaminant leak location. A finite difference groundwater flow model and a particle-tracking model generate a contaminant plume for each realization. Information from the flow and transport simulations is passed to an optimization model based upon a facility location analogy. The optimization model is a large integer programming problem which is solved approximately by the method of simulated annealing to determine optimal trade-off curves among the following three conflicting objectives: (1) maximum detection probability, (2) minimum cost (i.e., number of monitoring wells), and (3) minimum volume of contaminated groundwater at the time of detection. The method is applied to a unit-scaled hypothetical three-dimensional site to determine the sensitivity of the trade-off curves to various model parameters. Application to an existing landfill site reveals that the existing well network is suboptimal with respect to the considered objectives.

A view of water quality from the Plynlimon watershed

Abstract: . A summary of hydrochemical research in the Plynlimon catchments from 1983 to the present is related to identifying water flow pathways within catchments and the modelling of soil and stream water acidification. The study reveals a highly heterogeneous system that barely conforms with current understanding of hydrology and acidification theory. The role of hydrochemical mixing processes and groundwater flow routing is emphasised as is the need for maintaining long term monitoring studies and enhancing process based studies of water and chemical fluxes through catchments. The applicability of current environmental impact models for predictive and environmental management purposes is questioned and it is proposed that new hydrochemical modelling structures are needed to examine the highly heterogeneous systems being modelled.

Acid neutralization capacity measurements in surface and ground waters in the Upper River Severn, Plynlimon: from hydrograph splitting to water flow pathways

Abstract: . Acid Neutralization Capacity (ANC) data for ephemeral stream and shallow groundwater for the catchments of the upper River Severn show a highly heterogeneous system of within-catchment water flow pathways and chemical weathering on scales of less than 100m. Ephemeral streams draining permeable soils seem to be supplied mainly from shallow groundwater sources. For these streams, large systematic differences in pH and alkalinity occur due to the variability of the groundwater sources and variability in water residence times. However, the variability cannot be gauged on the basis of broad based physical information collected in the field as geology, catchment gradients and forest structure are very similar. In contrast, ephemeral streams draining impermeable soils are of more uniform chemistry as surface runoff is mainly supplied from the soil zone. Groundwater ANC varies considerably over space and time. In general, the groundwaters have higher ANCs than the ephemeral streams. This is due to increased chemical weathering from the inorganic materials in the lower soils and groundwater areas and possibly longer residence times. However, during the winter months the groundwater ANCs tend to be at their lowest due to additional event driven acidic soil water contributions and intermediate groundwater residence times. The results indicate the inappropriateness of a blanket approach to classifying stream vulnerability to acidification simply on the basis of soil sensitivity. However, the results may well indicate good news for the environmental management of acidic and acid sensitive systems. For example, they clearly indicate a large potential supply of weathering components within the groundwater zone to reduce or mitigate the acidifying effects of land use change and acidic deposition without the environmental needs for Aiming. Furthermore, the high variability of ephemeral stream runoff means that certain areas of catchments where there are specific problems associated with acidification can be identified for focused remediation work for the situation where liming is required. The case for focused field campaigns and caution against over reliance on blanket modelling approaches is suggested. The results negate the conventional generalizations within hydrology of how water moves through catchments to generate streamflow events (from Hortonian overland flow to catchment contributing areas).

WEKU – a GIS-Supported stochastic model of groundwater residence times in upper aquifers for the supraregional groundwater management

Abstract: The supraregional GIS-supported stochastical model, WEKU, for the determination of groundwater residence times in the upper aquifers of large groundwater provinces is presented. Using a two-dimensional analytical model of groundwater flow, groundwater residence times are determined within two extreme cases. In the first case, maximal groundwater residence times are calculated, representing the part of groundwater, that is drained by the main surface water of a groundwater catchment area. In the second case, minimal groundwater residence times for drainage into the nearest surface water are determined. Using explicit distribution functions of the input parameters, mean values as well as potential ranges of variations of the groundwater residence times are derived. The WEKU model has been used for the determination of groundwater residence times throughout Germany. The model results – mean values and deviations of the groundwater velocity and the maximal and minimal groundwater residence times in the upper aquifers – are presented by general maps and discussed in detail. It is shown that the groundwater residence times in the upper aquifer vary regionally, differentiated between less than 1 year and more than 2000 years. Using this information, the time scales can be specified, until measures to remediate polluted groundwater resources may lead to a substantial groundwater quality improvement in the different groundwater provinces of Germany. With respect to its supraregional scale of application, the WEKU model may serve as a useful tool for the supraregional groundwater management on a state, federal or international level.

An application of factor analysis for the study of the hydrogeological conditions in Plio-Pleistocene aquifers of NW Achaia (NW peloponnesus, Greece)

Abstract: R- andQ-mode factor analysis is applied to 51 groundwater samples collected from wells drilled in the Plio-Pleistocene aquifers of NW Achaia, Greece. The purpose ofR- andQ-mode factor analysis application is to identify (i) the regional groundwater flow pattern, and (ii) the deterioration of groundwater quality. Sixteen hydrogeological parameters are used in order to examine their importance and to provide significant insight into their correlations. In theR-mode factor analysis, a six-factor model is suggested which can explain more than 77.5% of the total variance. The contribution of each factor at every site (factor scores) also is computed. Maps are constructed showing the geographical distribution of the factor scores. From these maps, the high salinity areas are delineated (seawater intrusion, possible appearance of halite layers) and the areas with elevated contribution of karastic-water are defined. Using theQ-mode correspondence analysis the meaning of the electrical conductivity as the most important variable in groundwater quality characterization is demonstrated.

A tracer study of the floridan aquifer in southeastern georgia : implications for groundwater flow and paleoclimate

Abstract: Distributions of stable isotopes of water, radiocarbon, noble gases, and chloride (C1) in groundwater from the Upper Floridan aquifer in southeastern Georgia suggest that down gradient of the Gulf Trough this aquifer contains waters representative of both regional and local groundwater flow systems. In this area, locally recharged waters tend to remain near the top of the aquifer and do not mix substantially with the regional groundwater flow system. Noble gas temperatures suggest that this region of Georgia was 4.0 ± 0.6°C cooler during the last glacial period (LGP). Similar temperature changes have been reported for southern Texas and northern New Mexico, suggesting that the southern United States cooled uniformly during the LGP. Stable isotopes of water became enriched down gradient from the recharge area. These enrichments which have been observed before appear to result from local influx of shallow groundwater into the regional aquifer system rather than representing a climate change signal. An inland gradient of the stable isotope composition (0.60 ± 0.14‰/100 km in δ18O) was found in young (Holocene) water. After correcting for the change in the stable isotope composition of the ocean during the LGP, water that was recharged during the LGP was found to be slightly depleted in stable isotopes relative to modern recharge (Δδ18O = 0.6 ± 0.4‰). Assuming the modern inland gradient, the change in δ18O is consistent with a shift in the coastline, which was caused by the lower sea level during the LGP.

The occurrence of groundwater in the Lower Palaeozoic rocks of upland Central Wales

Abstract: A series of boreholes of up to 50 m depth, drilled into Lower Palaeozoic mudstone, shale and greywacke bedrock in the headwater catchment areas of the River Severn at Plynlimon in Central Wales, shows an extensive chemically- and hydrologically-active shallow groundwater fracture flow system. Groundwater chemistry varies in space and time with lowest water levels and highest alkalinities occurring during the drier summer months. The groundwaters are enriched in base cations, silica, sulphate and alkalinity relative to surface waters indicating significant silicate weathering sources and sulphide oxidation. These sources provide important contributions to both stream water quality and flow. At one site, the introduction of a borehole near to the main river opened bedrock fractures which increased the amount of groundwater entering the river. This had a profound effect on the river water quality by increasing the pH, alkalinity and calcium concentrations. As well as pointing to the possibility of the wider availability of groundwater resources in upland areas, the results highlight (a) the potential value of groundwater as a acid neutralizing resource, (b) the importance of weathering processes and flow routing within the groundwater environment for stream water chemistry, (c) the potential for altering stream water quality by manipulation of groundwater routing and (d) the need to include groundwater characteristics in hydrochemical management models of surface water acidification.

Hydrogeological modeling of the saline hot springs at the Sea of Galilee, Israel

Abstract: Meteoric fresh groundwater from shallow aquifers and hot brines from deep aquifers mix while emerging from several springs along the western coast of the Sea of Galilee, a freshwater lake located within the Dead Sea Rift Valley, Israel. After the rainy season, when elevations of the groundwater table rise in the regional aquifers and discharge rates of springs increase, solute concentrations decrease at Tabha springs but, surprisingly, increase at Fulya springs, apparently suggesting two different salinization mechanisms. Two detailed geologic cross sections were constructed, one across the rift valley at Tabha and a second at Fulya, each about 6 km deep and 70 km long. The hydrodynamics in these cross sections were analyzed using a two-dimensionalfinite element code that solves the coupled variable-density groundwaterflow and heat transfer equations. Numerical simulations indicate that a topography-drivenflow model explains both spring systems, and the opposite salinity behavior results from the different hydrogeological configurations of the two subsurface drainage basins. At Fulya, both aquifers, the shallow one and the deeper one, are partially phreatic, whereas at Tabha, the deeper aquifer is totally confined. The response of springs to changes in elevation of groundwater table were simulated, reproducingfield observations. This analysis has implications for the management scheme for the lake and its surrounding aquifers.

Method for directing groundwater flow and treating groundwater in situ

Abstract: A method for treating groundwater in situ in rock or soil. An elongate permeable upgradient zone (2) and an elongate permeable downgradient zone, each in hydraulic communication with a permeable subsurface treatment zone (14) and having a major axis parallel to a non-zero component of the general flow direction (8), are provided in the subsurface by any of a number of construction methods. The upgradient zone, downgradient zone, and treatment zone are situated within the subsurface medium (4) and have permeabilities substantially greater than the adjacent subsurface medium's permeability.