Showing papers on "Groundwater flow published in 2006"

Representative hydraulic conductivities in saturated groundwater flow

Abstract: [1] Heterogeneity is the single most salient feature of hydrogeology. An enormous amount of work has been devoted during the last 30 years to addressing this issue. Our objective is to synthesize and to offer a critical appraisal of results related to the problem of finding representative hydraulic conductivities. By representative hydraulic conductivity we mean a parameter controlling the average behavior of groundwater flow within an aquifer at a given scale. Three related concepts are defined: effective hydraulic conductivity, which relates the ensemble averages of flux and head gradient; equivalent conductivity, which relates the spatial averages of flux and head gradient within a given volume of an aquifer; and interpreted conductivity, which is the one derived from interpretation of field data. Most theoretical results are related to effective conductivity, and their application to real world scenarios relies on ergodic assumptions. Fortunately, a number of results are available suggesting that conventional hydraulic test interpretations yield (interpreted) hydraulic conductivity values that can be closely linked to equivalent and/or effective hydraulic conductivities. Complex spatial distributions of geologic hydrofacies and flow conditions have a strong impact upon the existence and the actual values of representative parameters. Therefore it is not surprising that a large body of literature provides particular solutions for simplified boundary conditions and geological settings, which are, nevertheless, useful for many practical applications. Still, frequent observations of scale effects imply that efforts should be directed at characterizing well-connected stochastic random fields and at evaluating the corresponding representative hydraulic conductivities.

Modeled impacts of predicted climate change on recharge and groundwater levels

Abstract: [1] A methodology is developed for linking climate models and groundwater models to investigate future impacts of climate change on groundwater resources. An unconfined aquifer, situated near Grand Forks in south central British Columbia, Canada, is used to test the methodology. Climate change scenarios from the Canadian Global Coupled Model 1 (CGCM1) model runs are downscaled to local conditions using Statistical Downscaling Model (SDSM), and the change factors are extracted and applied in LARS-WG stochastic weather generator and then input to the recharge model. The recharge model simulated the direct recharge to the aquifer from infiltration of precipitation and consisted of spatially distributed recharge zones, represented in the Hydrologic Evaluation of Landfill Performance (HELP) hydrologic model linked to a geographic information system (GIS). A three-dimensional transient groundwater flow model, implemented in MODFLOW, is then used to simulate four climate scenarios in 1-year runs (1961–1999 present, 2010–2039, 2040–2069, and 2070–2099) and compare groundwater levels to present. The effect of spatial distribution of recharge on groundwater levels, compared to that of a single uniform recharge zone, is much larger than that of temporal variation in recharge, compared to a mean annual recharge representation. The predicted future climate for the Grand Forks area from the downscaled CGCM1 model will result in more recharge to the unconfined aquifer from spring to the summer season. However, the overall effect of recharge on the water balance is small because of dominant river-aquifer interactions and river water recharge.

Driving mechanisms for groundwater flow and salt transport in a subterranean estuary

Abstract: This paper presents field measurements and numerical simulations of groundwater dynamics in the intertidal zone of a sandy meso-tidal beach. The study, focusing on vertical hydraulic gradients and pore water salinities, reveals that tides and waves provide important forcing mechanisms for flow and salt transport in the nearshore aquifer. Such forcing, interacting with the beach morphology, enhances the exchange between the aquifer and ocean. The spatial and temporal variations of vertical hydraulic gradients demonstrate the complexity and dynamic nature of the processes and the extent of mixing between fresh groundwater and seawater in a subterranean estuary''. These results provide evidence of a potentially important reaction zone in the nearshore aquifer driven by oceanic oscillations. Land-derived contaminants may undergo important biogeochemical transformations in this zone prior to discharge.

Faults as conduit-barrier systems to fluid flow in siliciclastic sedimentary aquifers

Abstract: [1] We argue that the observed conduit-barrier behavior of fault zones in siliciclastic sedimentary aquifer systems can be understood by considering a strongly anisotropic hydraulic structure in the fault. Hydraulic anisotropy in the fault is expected from a variety of mechanisms including clay-smearing, drag of sand, grain re-orientation and vertical segmentation of the fault plane. In this paper, we present an algorithm to predict fault zone width, lithological heterogeneity and hydraulic anisotropy. Estimation of these parameters is based upon the amount of fault throw and the clay-content of the lithologies flanking the fault zone. A suite of steady-state flow models are presented using an idealized stratigraphy consisting of alternating clay and sand-rich layers that are offset by a fault zone. These conceptual simulations show the impact of a fault zone on shallow (<500 m) fluid flow patterns and solute transport for different scenarios of fault throw. Fault width varies along the fault zone and increases from an average width of ~2 m for a throw of 50 m to ~8 m for a throw of 200 m. Hydraulic anisotropy in the fault zone in these scenarios is predicted to range between two to three orders of magnitude. Our results show that faults can form a preferential path way between aquifers at different depths over vertical distances of several hundreds of meters (that are otherwise separated by confining units) when fault permeability is strongly anisotropic. However, in the same scenario anomalously high hydraulic head gradients across the fault would still suggest that they act as an effective barrier to lateral groundwater flow. This has important implications for the assessment of the risk of a spread of contaminated groundwater or the reconstruction of hydrocarbon migration within sedimentary basins.

A modelling study of hyporheic exchange pattern and the sequence, size, and spacing of stream bedforms in mountain stream networks, Oregon, USA

Abstract: Studies of hyporheic exchange flows have identified physical features of channels that control exchange flow at the channel unit scale, namely slope breaks in the longitudinal profile of streams that generate subsurface head distributions. We recently completed a field study that suggested channel unit spacing in stream longitudinal profiles can be used to predict the spacing between zones of upwelling (flux of hyporheic water into the stream) and downwelling (flux of stream water into the hyporheic zone) in the beds of mountain streams. Here, we use two-dimensional groundwater flow and particle tracking models to simulate vertical and longitudinal hyporheic exchange along the longitudinal axis of stream flow in second-, third-, and fourth-order mountain stream reaches. Modelling allowed us to (1) represent visually the effect that the shape of the longitudinal profile has on the flow net beneath streambeds; (2) isolate channel unit sequence and spacing as individual factors controlling the depth that stream water penetrates the hyporheic zone and the length of upwelling and downwelling zones; (3) evaluate the degree to which the effects of regular patterns in bedform size and sequence are masked by irregularities in real streams. We simulated hyporheic exchange in two sets of idealized stream reaches and one set of observed stream reaches. Idealized profiles were constructed using regression equations relating channel form to basin area. The size and length of channel units (step size, pool length, etc.) increased with increasing stream order. Simulations of hyporheic exchange flows in these reaches suggested that upwelling lengths increased (from 2Ð 7mt o 7 Ð6 m), and downwelling lengths increased (from 2Ð 9mt o 6 Ð0 m) with increase in stream order from second to fourth order. Step spacing in the idealized reaches increased from 5Ð 3mt o 13Ð7 m as stream size increased from second to fourth order. Simulated downwelling lengths increased from 4Ð 3m in second-order streams to 9Ð7 m in fourth-order streams with a POOL–RIFFLE–STEP channel unit sequence, and increased from 2Ð 5m to 6 Ð1 m from second- to fourth-order streams with a POOL–STEP–RIFFLE channel unit sequence. Upwelling lengths also increased with stream order in these idealized channels. Our results suggest that channel unit spacing, size, and sequence are all important in determining hyporheic exchange patterns of upwelling and downwelling. Though irregularities in the size and spacing of bedforms caused flow nets to be much more complex in surveyed stream reaches than in idealized stream reaches, similar trends emerged relating the average geomorphic wavelength to the average hyporheic wavelength in both surveyed and idealized reaches. This article replaces a previously published version (Hydrological Processes, 19(17), 2915–2929 (2005) [DOI:10.1002/hyp.5790]. Copyright  2006 John Wiley & Sons, Ltd.

Tidally driven groundwater flow and solute exchange in a marsh: Numerical simulations

Abstract: [1] Tidal fluctuations drive groundwater flow in salt marsh sediments. This flow could cause significant chemical exchange across the sediment-water interface and could affect marsh ecology. Numerical models of a generalized tidal creek and marsh were constructed to calculate flow patterns and solute exchange between the marsh and creek. The governing equation for saturated/unsaturated flow was modified to account for tide-related changes in total stress. Groundwater flow occurred primarily in the creek bank, even when the marsh platform was inundated at high tide. For marsh sediments with a hydraulic conductivity of 10−4 m s−1, groundwater ages in simulations lasting 60 days were on the order of days near the creek bank and increased to 50–60 days with distance into the marsh. The volume of water that discharged between high and low tide was 0.22–0.31 m3 per meter length of channel, which, for a creek drainage density of 0.012 m−1, corresponds to 10–14 L m−2 d−1. Sediment permeability and capillarity were important controls on flow and groundwater age in the marsh. Sediment compressibility affected groundwater age for compressible sediments representative of mud but not for sediments with lower compressibilities representative of sand. Simulations were relatively insensitive to dispersivity. A comparison of simulation results with other estimates of groundwater exchange from the North Inlet, South Carolina, suggest that tides could drive observed exchange there only if the Pleistocene sands underlying muddy marsh sediments outcrop within the tidal range.

Groundwater dynamics and arsenic mobilization in Bangladesh assessed using noble gases and tritium.

Abstract: The contamination of groundwater by geogenic arsenic is the cause of major health problems in south and southeast Asia. Various hypotheses proposing that As is mobilized by the reduction of iron (oxy)hydroxides are now under discussion. One important and controversial question concerns the possibility that As contamination might be related to the extraction of groundwater for irrigation purposes. If As were mobilized by the inflow of re-infiltrating irrigation water rich in labile organic carbon, As-contaminated groundwater would have been recharged after the introduction of groundwater irrigation 20-40 years ago. We used environmental tracer data and conceptual groundwater flow and transport modeling to study the effects of groundwater pumping and to assess the role of reinfiltrated irrigation water in the mobilization of As. Both the tracer data and the model results suggest that pumping induces convergent groundwater flow to the depth of extraction and causes shallow, young groundwater to mix with deep, old groundwater. The As concentrations are greatest at a depth of 30 m where these two groundwater bodies come into contact and mix. There, within the mixing zone, groundwater age significantly exceeds 30 years, indicating that recharge of most of the contaminated water occurred before groundwater irrigation became established in Bangladesh. Hence, at least at our study site, the results call into question the validity of the hypothesis that re-infiltrated irrigation water is the direct cause of As mobilization; however, the tracer data suggest that, at our site, hydraulic changes due to groundwater extraction for irrigation might be related to the mobilization of As.

Multiscale geomorphic drivers of groundwater flow paths: subsurface hydrologic dynamics and hyporheic habitat diversity

Abstract: Application of a hydrogeologic computer model underscored the importance of geomorphic controls on groundwater and surface-water flow dynamics in the Nyack Floodplain, a montane alluvial floodplain in Montana, USA. The model represented the floodplain as a hierarchy of geomorphic patches, which facilitated analysis of model results using independent (predictor) variables at multiple scales. The analyses revealed that geomorphic structures at various spatial scales interact with the flow regime to influence the direction, magnitude, and stability of hyporheic flow within individual floodplain patches. Specifically: 1) the hydrologic flow network within the hyporheic zone is more responsive to seasonal changes in river discharge if floodplain topography is complex and aquifer properties are heterogeneous, 2) simplification of internal patch structure across the floodplain eliminates the influence of fine-scale geomorphic structures on the stability of groundwater flow paths, although the influence o...

An assessment of the tracer‐based approach to quantifying groundwater contributions to streamflow

Abstract: [1] The use of conservative geochemical and isotopic tracers along with mass balance equations to determine the pre-event groundwater contributions to streamflow during a rainfall event is widely used for hydrograph separation; however, aspects related to the influence of surface and subsurface mixing processes on the estimates of the pre-event contribution remain poorly understood Moreover, the lack of a precise definition of “pre-event” versus “event” contributions on the one hand and “old” versus “new” water components on the other hand has seemingly led to confusion within the hydrologic community about the role of Darcian-based groundwater flow during a storm event In this work, a fully integrated surface and subsurface flow and solute transport model is used to analyze flow system dynamics during a storm event, concomitantly with advective-dispersive tracer transport, and to investigate the role of hydrodynamic mixing processes on the estimates of the pre-event component A number of numerical experiments are presented, including an analysis of a controlled rainfall-runoff experiment, that compare the computed Darcian-based groundwater fluxes contributing to streamflow during a rainfall event with estimates of these contributions based on a tracer-based separation It is shown that hydrodynamic mixing processes can dramatically influence estimates of the pre-event water contribution estimated by a tracer-based separation Specifically, it is demonstrated that the actual amount of bulk flowing groundwater contributing to streamflow may be much smaller than the quantity indirectly estimated from a separation based on tracer mass balances, even if the mixing processes are weak

Exact three-dimensional spectral solution to surface-groundwater interactions with arbitrary surface topography

Abstract: It has been long known that land surface topography governs both groundwater flow patterns at the regional-to-continental scale and on smaller scales such as in the hyporheic zone of streams. Her ...

Three-dimensional model for multi-component reactive transport with variable density groundwater flow

Abstract: PHWAT is a new model that couples a geochemical reaction model (PHREEQC-2) with a density-dependent groundwater flow and solute transport model (SEAWAT) using the split-operator approach. PHWAT was developed to simulate multi-component reactive transport in variable density groundwater flow. Fluid density in PHWAT depends not on only the concentration of a single species as in SEAWAT, but also the concentrations of other dissolved chemicals that can be subject to reactive processes. Simulation results of PHWAT and PHREEQC-2 were compared in their predictions of effluent concentration from a column experiment. Both models produced identical results, showing that PHWAT has correctly coupled the sub-packages. PHWAT was then applied to the simulation of a tank experiment in which seawater intrusion was accompanied by cation exchange. The density dependence of the intrusion and the snow-plough effect in the breakthrough curves were reflected in the model simulations, which were in good agreement with the measured breakthrough data. Comparison simulations that, in turn, excluded density effects and reactions allowed us to quantify the marked effect of ignoring these processes. Next, we explored numerical issues involved in the practical application of PHWAT using the example of a dense plume flowing into a tank containing fresh water. It was shown that PHWAT could model physically unstable flow and that numerical instabilities were suppressed. Physical instability developed in the model in accordance with the increase of the modified Rayleigh number for density-dependent flow, in agreement with previous research.

Streamline-based simulation of carbon dioxide storage in a North Sea aquifer

Abstract: [1] We model carbon dioxide (CO2) storage into a deep North Sea aquifer using streamline-based simulation. We assume incompressible flow of liquid-like CO2 and aqueous phases. We simulate dissolution of CO2 and a rate-limited precipitation reaction. Advective transport and reactions are solved along streamlines, while dispersion and flow due to gravity segregation of the phases are solved on the underlying grid. Geological storage is modeled on a one million cell model. The distribution of CO2 after injection is dominated by advective transport due to multiphase flow, and CO2 moves preferentially through high-permeability channels. Without reaction the regional groundwater flow causes the CO2 to continue to migrate until it reaches residual saturation, where it continues, slowly, to dissolve. Precipitation leads to a decrease in porosity and permeability, while CO2 is stored in the solid phase. The storage efficiency is low, around 2%, because of aquifer heterogeneity.

Occurrence of salt water above fresh water in dynamic equilibrium in a coastal groundwater flow system near De Panne, Belgium

Abstract: A salt water lens is found above fresh water under the shore between Dunkerque (France) and Nieuwpoort (Belgium). This inverse density distribution is in a dynamic equilibrium. It develops due to the infiltration of salt water on the back shore during high tide. Under this salt water lens, water infiltrated in the adjacent dune area flows towards the sea and discharges at the seabed. This water quality distribution differs from the classic salt water wedge under fresh water described in the literature. Here, the evolution to this water quality distribution is simulated with a density dependent numerical model. A large tidal range, shore morphology and a permeable groundwater reservoir are the main conditions for the observed water quality distribution. By altering these conditions, intermediate water quality distributions between the classic salt water wedge and the one discussed here develop. Based on these simulations, it is expected that similar kinds of inverse density distribution could be present in a number of coastal areas, which have tides, a gently sloping shore and a permeable substratum.

Evaluation and numerical modeling of seawater intrusion in the Gaza aquifer (Palestine)

Abstract: A numerical assessment of seawater intrusion in Gaza, Palestine, has been achieved applying a 3-D variable density groundwater flow model. A two-stage finite difference simulation algorithm was used in steady state and transient models. SEAWAT computer code was used for simulating the spatial and temporal evolution of hydraulic heads and solute concentrations of groundwater. A regular finite difference grid with a 400 m2 cell in the horizontal plane, in addition to a 12-layer model were chosen. The model has been calibrated under steady state and transient conditions. Simulation results indicate that the proposed schemes successfully simulate the intrusion mechanism. Two pumpage schemes were designed to use the calibrated model for prediction of future changes in water levels and solute concentrations in the groundwater for a planning period of 17 years. The results show that seawater intrusion would worsen in the aquifer if the current rates of groundwater pumpage continue. The alternative, to eliminate pumpage in the intruded area, to moderate pumpage rates from water supply wells far from the seashore and to increase the aquifer replenishment by encouraging the implementation of suitable solutions like artificial recharge, may limit significantly seawater intrusion and reduce the current rate of decline of the water levels.

Importance of groundwater in the water balance of an alpine headwater lake

Abstract: [1] The response of snow- and glacier-fed mountain streams and lakes to climate warming is of growing concern. A full understanding of these systems must include the role of groundwater, but this is poorly understood, especially for high-elevation lakes. This study addresses the role of groundwater in an alpine watershed, located at the continental divide of North America, with a focus on quantifying the groundwater exchange with Lake O’Hara in British Columbia, Canada. This is facilitated using a water balance approach and measurements of electrical conductivity in inflowing streams. The water balance indicates that groundwater inflow is substantial, as it was equivalent to at least 30–67% and 35–74% of the total outflow for the 2004 and 2005 field seasons, respectively. Hydrological and chemical data also suggest contributions from both deep and shallow groundwater flow paths. Citation: Hood, J. L., J. W. Roy, and M. Hayashi (2006), Importance of groundwater in the water balance of an alpine headwater lake, Geophys. Res. Lett., 33, L13405, doi:10.1029/ 2006GL026611.

Chemical characterization of the Neogene Aquifer, Belgium

Abstract: The evolution of groundwater chemistry along the direction of groundwater flow was studied using hydrochemical data from samples collected along a flow line in the Neogene Aquifer, Belgium. Infiltrating water was found to have a very low mineral content and low pH because the sediments are strongly decalcified. Increasing SiO2 and cation concentrations along the groundwater flow line indicate silicate-weathering processes, confirmed with the aid of saturation indices, calculated with PHREEQC, and stability diagrams. A classification system based on redox sensitive species was developed and shows that an extensive redox sequence is present in the aquifer. At a shallow depth, pyrite oxidation has caused an increase in sulphate, while iron is precipitated as hydroxides. Elevated arsenic concentrations are related to the reduction of these iron hydroxides at a relatively shallow depth and to the dissolution of siderite at greater depth. Dissolution of carbonate in the aquifer material, present in deep layers and to the north, has lead to increased Ca2+ and HCO3 − concentrations. The Ca2+ from the groundwater is exchanged for Na+, Mg2+ and K+ adsorbed to the clay surfaces at the bottom of the groundwater reservoir. Although the Neogene Aquifer is well flushed, there are still some marine influences present in the deepest parts.

A regional coupled surface water/groundwater model of the Okavango Delta, Botswana

Abstract: [1] In the endorheic Okavango River system in southern Africa a balance between human and environmental water demands has to be achieved. The runoff generated in the humid tropical highlands of Angola flows through arid Namibia and Botswana before forming a large inland delta and eventually being consumed by evapotranspiration. With an approximate size of about 30,000 km2, the Okavango Delta is the world's largest site protected under the convention on wetlands of international importance, signed in 1971 in Ramsar, Iran. The extended wetlands of the Okavango Delta, which sustain a rich ecology, spectacular wildlife, and a first-class tourism infrastructure, depend on the combined effect of the highly seasonal runoff in the Okavango River and variable local climate. The annual fluctuations in the inflow are transformed into vast areas of seasonally inundated floodplains. Water abstraction and reservoir building in the upstream countries are expected to reduce and/or redistribute the available flows for the Okavango Delta ecosystem. To study the impacts of upstream and local interventions, a large-scale (1 km2 grid), coupled surface water/groundwater model has been developed. It is composed of a surface water flow component based on the diffusive wave approximation of the Saint-Venant equations, a groundwater component, and a relatively simple vadose zone component for calculating the net water exchange between land and atmosphere. The numerical scheme is based on the groundwater simulation software MODFLOW-96. Since the primary model output is the spatiotemporal distribution of flooded areas and since hydrologic data on the large and inaccessible floodplains and tributaries are sparse and unreliable, the model was not calibrated with point hydrographs but with a time series of flooding patterns derived from satellite imagery (NOAA advanced very high resolution radiometer). Scenarios were designed to study major upstream and local interventions and their expected impacts in the Delta. The scenarios' results can help decision makers strike a balance between environmental and human water demands in the basin.