Showing papers on "Groundwater flow published in 1998"

Detection of aquifer system compaction and land subsidence using interferometric synthetic aperture radar, Antelope Valley, Mojave Desert, California

Abstract: Interferometric synthetic aperture radar (InSAR) has great potential to detect and quantify land subsidence caused by aquifer system compaction. InSAR maps with high spatial detail and resolution of range displacement (±10 mm in change of land surface elevation) were developed for a groundwater basin (∼103 km2) in Antelope Valley, California, using radar data collected from the ERS-1 satellite. These data allow comprehensive comparison between recent (1993–1995) subsidence patterns and those detected historically (1926–1992) by more traditional methods. The changed subsidence patterns are generally compatible with recent shifts in land and water use. The InSAR-detected patterns are generally consistent with predictions based on a coupled model of groundwater flow and aquifer system compaction. The minor inconsistencies may reflect our imperfect knowledge of the distribution and properties of compressible sediments. When used in conjunction with coincident measurements of groundwater levels and other geologic information, InSAR data may be useful for constraining parameter estimates in simulations of aquifer system compaction.

Seasonal variation in surface‐subsurface water exchange and lateral hyporheic area of two stream‐aquifer systems

Abstract: . We used two-dimensional unconfined transient groundwater flow models to investigate the interface between stream and groundwater flow systems, or hyporheic zone, of two first-order streams that drain catchments with distinctly different alluvial sediments and bedrock lithology. Particle tracking showed that lateral hyporheic area (planimetric area of flow paths lateral to the stream that are recharged by and return to the stream with travel times of 10 days or less) differed between the two study streams and varied with discharge within each system. At the Rio Calaveras (welded tuff), lateral hyporheic area ranged from 1.7 to 4 m 2 over the annual cycle. In the Aspen Creek system (sandstone), lateral hyporheic area (1-1.5 m 2) was restricted to roughly half of that observed at Rio Calaveras. The size of the hyporheic zone lateral to the streams at both sites decreased by approximately 50% during high flows. Sensitivity analyses indicated that changes in the hydraulic conductivity of alluvial and streambed sediments and variation in recharge rates have greatest impact on the magnitude, direction, and spatial distribution of stream-groundwater exchange.

Groundwater in Geologic Processes

Abstract: Preface List of symbols 1. Groundwater flow 2. Solute transport 3. Heat transport 4. Regional-scale flow and transport 5. Ore deposits 6. Hydrocarbons 7. Geothermal processes 8. Earthquakes 9. Evaporites 10. Diagenesis and metamorphism References.

The handbook of groundwater engineering

Abstract: From an engineering perspective, this book provides a practical treatment of groundwater flow; substance transport, well construction, groundwater production, site characterization, and remediation of groundwater pollution.

Effect of groundwater springs on NO3− concentrations during summer in Catskill Mountain streams

Abstract: Groundwater and stream water data collected at three headwater catchments in the Neversink River watershed indicate that base flow is sustained by groundwater from two sources: a shallow flow system within the till and soil and a deep flow system within bedrock fractures and bedding planes that discharges as perennial springs. Data from eight wells finished near the till/bedrock interface indicate that saturated conditions are not maintained in the shallow flow system during most summers. In contrast, the discharge of a perennial spring remained constant during two summer rainstorms, providing evidence that the deep flow system is disconnected from the shallow flow system in summer. Discharge from perennial springs was the principal source of streamflow in a headwater reach during low flow. Mean NO3− concentrations were 20–25 μmol L−1 in five perennial springs during the summer but only 5–10 μmol L−1 in shallow groundwater. Thus the deep flow system does not reflect typical NO3− concentrations in the soil during summer. A hydrologic budget at a headwater drainage reveals that March and late fall are the principal groundwater recharge periods. Residence time modeling based on analyses of 18O and 35S indicates that groundwater in the deep flow system is 6–22 months old. These data indicate that summer base flow largely originates from previous dormant seasons when available soil NO3− is greater. In these Catskill watersheds, high base flow concentrations of NO3− during summer do not provide sufficient evidence that the atmospheric N deposition rate exceeds the demand of terrestrial vegetation.

Basin-scale geohydrologic drought flow features of riparian aquifers in the Southern Great Plains

Abstract: Low-streamflow hydrographs from 22 subbasins in the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) Washita River Experimental Watershed complex in central Oklahoma were subjected to recession slope analysis; this method, after that of Brutsaert and Nieber [1977], was derived from a Dupuit-Boussinesq formulation for the groundwater outflows from the adjoining phreatic aquifers. The longtime aquifer response characteristics were generally found to be close to linear, and the short-time response characteristics were consistent with Boltzmann similarity. Representative values of the resulting basin-scale effective groundwater parameters were (35 days)−1 for the low-flow extinction coefficient (i.e., a storage half-life of 25 days); 0.021 m2 s−1 for the hydraulic diffusivity, Dh; 0.0035 m2s−½ for the hydraulic desorptivity, Deh; 8 × 10−4 ms−1 for the hydraulic conductivity k; and 0.018 for the drainable porosity (or specific yield), ƒ. The variabilities of Dh, Deh, and k from basin to basin could be better represented by the log-normal than by the normal distribution; ƒ could be described nearly equally well by both. The storage half-life is moderately and positively correlated with basin size; in the case of k the correlation is negative but weaker. Any scale dependence of Dh, Deh, and ƒ appears to be negligible.

Noble gases as natural tracers of water circulation in the Paris Basin: 2. Calibration of a groundwater flow model using noble gas isotope data

Abstract: Using the rare gas concentrations in the aquifers of the Paris Basin (see part 1 of this series), a numerical model of a two-dimensional cross section of the entire Paris Basin was built to simulate groundwater flow and the transport of 3He, 4He, and 40Ar isotopes. The model included seven aquifers separated by seven aquitards in a steady state flow regime. Transport of the gases is by advection, diffusion, and dispersion in steady or transient states. The 4He transport was simulated first and made it possible to calibrate both the crustal flux of this isotope and the average permeability of each aquifer, which were then favorably compared with measured values. These values present a high variability from aquifer to aquifer, between 8.5 × 10−7 and 3.5 × 10−4 m s−1. The water velocities and average residence times were also estimated. Average turnover times for the different aquifers are highly variable, ranging from 8700 years for the shallowest one (Ypresian) to 30 Myr for the deepest one (Trias). The calibrated model was also able to correctly represent the distribution of 3He and 40Ar in the basin. Diffusion proved to be an important mechanism for vertical transfer through the aquitards of the helium isotopes, as opposed to 40Ar, which is transported mainly by advection. On the basis of the (4He/40Ar) radiogenic ratio a constant value of 10−l1 m s−1 was attributed to the permeability of all the aquitards. A sensitivity study showed that the permeability of the aquitards situated in the lower part of the basin (Lias and aquitards in the Triassic and Dogger) could not be higher than 10−11 m s−1 given the observed distribution of the radiogenic 4He/40Ar ratio, but a lower limit could not be defined. The crustal fluxes of 3He, 4He, and 40Ar in the basin were estimated at 4.33 × 10−13 mol m−2 yr−1, 4. × 10−6 mol m−2 yr−1 and 2.52 × 10−7 mol m−2 yr−1, respectively. The simulation of the 3He and 4He transport showed that theR/Ra ratio (value of the measured R = 3He/4He ratio normalized to the atmospheric ratio Ra) entering at the base of the Trias from the bedrock remained constant while crossing the basin except in the zones close to the recharge areas where it is influenced by the atmospheric component. This constancy is due to the low radiogenic/nucleogenic production rate of these isotopes inside the basin, as compared to the crustal flux.

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

Abstract: 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.

Groundwater flow influences the biomass and nutrient ratios of epibenthic algae in a north temperate seepage lake

Abstract: Groundwater flow influenced epibenthic algal biomass and N : P ratios at a seepage lake (Sparkling Lake, Wisconsin). During seasonal studies, biomass and seepage flux were positively associated (r = 0.453; P < 0.001). Pore-water soluble reactive phosphorus (SRP) concentrations (29.2-110.7 μg PO 4 liter -1 ), SRP fluxes, and algal biomass were significantly higher at high groundwater discharge sites than at low flow sites (<10.0 to 27.7 μg PO 4 liter -1 ). Pore-water ammonia (NH 3 ) concentrations were significantly lower at high groundwater discharge sites (<10.0 to 566.0 μg NH 3 liter 1 ) than at low groundwater discharge and recharge sites (61.4-1464.9 μg NH 3 liter -1 ). The coupling between pore-water nutrient concentrations and local groundwater flow dynamics suggests a mechanism for the observed spatial patterns in biomass. In situ experimental chambers evaluated coupling between epibenthic algal biomass, N: P ratios, and groundwater flow patterns. Biomass responded rapidly in chambers, reaching ambient levels within 1.5 months of initiation. Free-flow chambers in discharge regions had consistently higher soluble reactive phosphorus (SRP), NO 3 -NO 2 , and O 2 concentrations, higher phosphate and nitrate-nitrite fluxes, higher algal biomass, and lower N:P ratios in the developing mat. Free-flow chambers in recharge regions had high ammonia (NH 3 ) concentrations, lower algal biomass, and higher N: P ratios. These results confirm that groundwater-related nutrient fluxes influence the local physicochemical environment and affect epibenthic algal biomass.

Simulation of seawater intrusion into the Khan Yunis area of the Gaza Strip coastal aquifer

Abstract: The Gaza Strip coastal aquifer is under severe hydrological stress due to over-exploitation. Excessive pumping during the past decades in the Gaza region has caused a significant lowering of groundwater levels, altering in some regions the normal transport of salts into the sea and reversing the gradient of groundwater flow. The sharp increase in chloride concentrations in groundwater indicates intrusion of seawater and/or brines from the western part of the aquifer near the sea.

The baseflow and storm flow hydrology of a precambrian shield headwater peatland

Abstract: A hydrological investigation was conducted in a small headwater peatland located in the Experimental Lakes Area, north-western Ontario, Canada, to determine the subsurface and surface flow paths within the peatland, and between the peatland and an adjacent forested upland during baseflow and storm flow conditions. Distinct zones of groundwater recharge and discharge were observed within the peatland. These zones are similar to those found in much larger flow systems even though the peatland was only influenced by local groundwater flow. Groundwater emerging in seeps and flowing beneath the peatland sustained the surface wetness of the peatland and maintained a constant baseflow. The response of the peatland stream to summer rain events was controlled by peatland water table position when the basin was dry and antecedent moisture storage on the uplands when the basin was wet. The magnitude and timing of peak runoA during wet conditions were controlled by the degree of hydrological connectivity between the surrounding upland terrain and the peatland. #1998 John Wiley Sons, Ltd. Hydrol. Process., Vol. 12, 57‐72 (1998)

Change in Groundwater Nitrate Concentration in a Large River Floodplain: Denitrification, Uptake, or Mixing?

Abstract: We examined the buffering capacities of different riparian vegetation (natural riparian forest, 3- and 15-y-old poplar plantations, and a wet meadow) on non-point-source nitrogen pollution along a 7th-order reach of the Garonne River in southwest France. Groundwater nitrate concentration was measured monthly for 1.5 y in 51 wells installed within the aquifer of the river floodplain and in River Garonne water. The mixing of river water and groundwater was estimated using a 2-end-member model based on measured concentrations of chloride. Nitrate concentrations in groundwater decreased significantly along groundwater flow paths crossing the riparian zones. Mixing of nitrate-rich groundwater with nitrate-poor river water accounted for most of the change in nitrate concentration along groundwater flow paths. The fraction of river water in wells increased from the margin of the floodplain with an alluvial terrace (31% river water) to the river; an average of 80% river water occured in the natural riparian fores...

Preferential transport of nitrate to a tile drain in an intermittent-flood-irrigated field: Model development and experimental evaluation

Abstract: A comprehensive field experiment was conducted near Las Nutrias, New Mexico, to study field-scale flow and transport in the vadose zone. The field data were analyzed in terms of a two-dimensional numerical model based on the Richards equation for variably saturated water flow, convection-dispersion equations with first-order chemical decay chains for solute transport, and bimodal piecewise-continuous unsaturated hydraulic functions to account for preferential flow of water and nitrate-nitrogen (NO3-N; loosely used as NO3 ) following flood irrigation events at the experimental site. The model was tested against measured NO3 flux concentrations in a subsurface tile drain, several monitoring wells and nested piezometers, and against resident NO3 concentrations in the soil profile (obtained at 52 spatial locations and four depths along a transect). NO3 transport at the field site could be described better with the bimodal hydraulic functions than using the conventional approach with unimodal van Genuchten-Mualem type hydraulic functions. Average resident nitrate concentrations measured across the soil profile were predicted reasonably well. However, NO3 flux concentrations in the subsurface tile drain and piezometers at the field site were occasionally underestimated or overestimated depending upon the irrigation sequence in three field benches, probably reflecting unrepresented three-dimensional regional flow/transport processes. Limiting the capture zone to a region closer to the tile drain did lead to a better match with observed sharp increases and decreases in predicted NO3 flux concentrations during the irrigation events. On the basis of this result we inferred that the preferential flow intercepted by the tile drain was generated in close proximity of the drain and essentially oriented vertically. In summary, our study suggests that irrigation scheduling in adjacent field plots, drainage design (e.g., spacing between tiles, drain depth, drain diameter) and effectiveness (e.g., drain blockage), preferential flow in (horizontal) surface-opened shallow cracks and (vertical) macropores, and transient regional groundwater flow can add significant uncertainty to the predictions of (local-scale) flow and transport to a tile drain.

Gas transport in a confined unsaturated zone during atmospheric pressure cycles

Abstract: Gas transport induced by temporal pressure fluctuations in the atmosphere can be an important mechanism for transport of atmospheric oxygen within the unsaturated zone. Moreover, the presence of oxygen in the unsaturated zone may be a factor controlling oxidation of sulphide minerals and other redox processes. A field study was carried out in a glacial aquifer with a 10–12 m thick sandy unsaturated zone to explore gas exchange between the atmosphere and the unsaturated zone. The exchange occurs through a “geological window” in a till layer which covers the sandy unsaturated zone. Observed pressure distribution and oxygen concentrations within the unsaturated zone were compared to numerical simulations with SUTRA, a finite element and fluid density dependent groundwater flow model. The simulations were carried out by modeling the gas pressure distribution within the unsaturated zone based on atmospheric pressure time series. The spatial variation in permeability observed from borehole logging was implemented in the model. The analysis demonstrated a good match between the field observations and the numerical simulations. During an atmospheric pressure cycle, atmospheric oxygen migrated more than 10 md−1 horizontally in the capped unsaturated zone. The analysis shows that both the amplitude and the length of the period of pressure variations are important for the transport of oxygen, and it shows that the combined effects determine the extent of a subsurface zone where atmospheric oxygen can reach.

Deciphering large landslides: linking hydrological, groundwater and slope stability models through GIS

Abstract: Large landslides can deliver substantial volumes of sediment to river channels, with potentially adverse consequences for water quality and fish habitat. When planning land use activities, it is important both to consider the risks posed by landslides and to account for the effects of land use on rates of landslide movement. Of particular interest in the Pacific Northwest are the effects of timber harvest in groundwater recharge areas of landslides. Because of variability between sites, and variability over time in precipitation and other natural environmental factors affecting landslide behaviour, empirical data are usually insufficient for making such determinations. We describe here the use of simple numerical models of site hydrology, groundwater flow and slope stability for estimating the effects of timber harvest on the stability of the Hazel Landslide in northwestern Washington State. These effects are examined relative to those of river bank erosion at the landslide toe. The data used are distributed in time and space, as are the model results. A geographical information system (GIS) provides an efficient framework for data storage, transfer and display. Coupled with process-based numerical models, a GIS provides an effective tool for site-specific analysis of landslide behaviour. © 1998 John Wiley & Sons, Ltd.

Selecting remediation goals by assessing the natural attenuation capacity of groundwater systems

Abstract: Remediation goals for the source areas of a chlorinated ethene‐contaminated groundwater plume were identified by assessing the natural attenuation capacity of the aquifer system. The redox chemistry of the site indicates that sulfate‐reducing (H2 ∼ 2 nanomoles [nM]) per liter conditions near the contaminant source grade to Fe(III)‐reducing conditions (H2 ∼ 0.5 nM) downgradient of the source. Sulfate‐reducing conditions facilitate the initial reduction of perchloroethene (PCE) to trichloroethene (TCE), cis‐dichloroethene (cis‐DCE), and vinyl chloride (VC). Subsequently, the Fe(III)‐reducing conditions drive the oxidation of cis‐DCE and VC to carbon dioxide and chloride. This sequence gives the aquifer a substantial capacity for biodegrading chlorinated ethenes. Natural attenuation capacity (the slope of the steady‐state contaminant concentration profile along a groundwater flowpath) is a function of biodegradation rates, aquifer dispersive characteristics, and groundwater flow velocity. The natura...

Watershed Delineation and Ground Water Discharge to a Coastal Embayment

Abstract: Nonpoint source management strategies for watersheds to eutrophic coastal waters require an adequate characterization of subsurface hydrogeology to understand the sources and fate of ground water and contaminant flow. An investigation was conducted to define the watershed to Waquoit Bay, a shallow eutrophic coastal embayment at the margin of the sand and gravel aquifer of Cape Cod, Massachusetts. This investigation included: (1) the development of a subregional water table map over 56 square kilometers; (2) delineation of a watershed and subwatersheds by plotting reverse ground water flowpaths; (3) a water budget based on a mass balance between recharge rates over riverine watershed segments and measured stream flows; and (4) a series of well points driven beneath the bed of the embayment and a transect of deep geologic borings perpendicular to the shore to characterize submarine ground water and hydrogeologic conditions at depth. The principal aquifer thins from the apex of the watershed, where it is approximately 46 m thick, to approximately 11 m thick as it nears Waquoit Bay. The detailed water table map shows extremely high hydraulic gradients to the rivers and comparably smaller gradients near the bay, indicating the thinning aquifer forces ground water to discharge into the rivers and causes them to function as significant surface drains for the aquifer. A classic sharp salt water interface was found in the upper 11.3 m thick High K unit, while the interface in the lower 34 m thick Low K unit was estimated to extend 700 m from the shoreline beneath the saline water of the bay. Eighty-nine percent (89%) of the bay water budget is from ground water flow (55% channeled into rivers and 34% direct ground water discharge), while 11% is from atmospheric deposition. The Low K unit is an insignificant source of fresh water to the embayment. The conceptual hydro-geologic model of the watershed is consistent with the water table configuration and the mass balance water budget and was confirmed by subsurface characterization of the near shore hydrogeology. This type of watershed characterization provides a firm basis for the development of nonpoint source management options for coastal embayments.

MOC3D adapted to simulate 3D density-dependent groundwater flow

Abstract: The three-dimensional computer code MOC3D (Konikow et al., 1996) is adapted for density differences: MOCDENS3D. As a result, it is possible to model transient three-dimensional groundwater flow in large-scale hydrogeologic systems where non-uniform density distributions occur. A special field of application is the simulation of salt water intrusion in coastal aquifers. The groundwater flow equation is solved by the MODFLOW module of MOCDENS3D. Density differences are taken into account through adding buoyancy terms to the RHS term of the basic groundwater flow equation of MODFLOW. The advection-dispersion equation is solved by the MOC module, using the method of characteristics. Advective transport of solutes is modeled by means of particle tracking and dispersive transport by means of the finite difference method. An advantage of applying the method of characteristics is that the condition of spatial discretisation is not strict. As a consequence, the displacement of fresh, brackish and saline groundwater in large-scale hydrogeologic systems can easily be modeled. Finally, the evolution of a freshwater lens at a circular sandy island is shortly discussed.

The role of groundwater flow in controlling the spatial distribution of soil salinity and rooted macrophytes in a southeastern salt marsh, USA

Abstract: Groundwater flow is an important factor in governing botanical zonation in the salt marsh at North Inlet, SC Areas of the marsh adjacent to upland forest are characterized by upward flow of fresh groundwater This inhibits the infiltration and evapoconcentration of saline tidal water and the development of a habitat for hypersaline-tolerant fugitive species such as Salicornia europaea Areas of high marsh that are not adjacent to extensive upland forest are characterized by downward gradients in hydraulic head This allows the infiltration and evapoconcentration of tidal water and the development of hypersaline conditions that are suitable for salt-tolerant fugitives

Mixing and stretching efficiency in steady and unsteady groundwater flows

Abstract: Mixing is the physical process through which solute is spread into a fluid by the stretching and folding of material lines and surfaces. Mixing, as compared to dilution, is important to solute spreading by groundwater because it operates on much shorter timescales than does dilution, and it provides the increased plume boundary area and high local concentration gradients that promote effective solute dilution. In this paper, the mixing process is investigated theoretically for subsurface tracer plume movement, using as heuristic examples both steady and unsteady groundwater flows in a perfectly stratified aquifer whose properties mimic those of the sand aquifer at the Borden site. It is shown that the stretching efficiency, a parameter that characterizes the effectiveness of mixing, is largest at transitions between regions of highly contrasting hydraulic conductivity and, more broadly, that pronounced spatial variability in the hydraulic conductivity is conducive to good mixing because of the periodic resurgences in material line stretching that it causes. Unsteady groundwater flow resulting from a decrease in vertical groundwater flux with time leads to greater rates of material line stretching than do steady flows, whereas little difference from the steady flow case occurs for unsteady groundwater flow under a time-varying horizontal hydraulic head gradient. Overall, pronounced spatial variability in the hydraulic conductivity is the most important contributor to good mixing of a tracer solute plume, but highly effective mixing requires additional physical conditions that induce chaotic solute pathlines.