摘要:

This study focuses on water flow and solute migration through unsaturated fractured chalk in an arid area. The chalk underlies a major industrial complex in the northern Negev desert, where groundwater contamination has been observed. Four dry‐drilling holes were bored through the vadose zone. Core and auger samples, collected at 30‐ to 50‐cm intervals, were used for chemical and isotopic analyses, enabling the construction of the following profiles: (1) a tritium profile, to estimate the rate of water flow through the unsaturated zone; (2) oxygen 18 and deuterium profiles, to assess the evaporation of water at land surface before percolation, and in the upper part of the vadose zone after infiltration; and (3) chloride and bromide profiles, as tracers for inert solutes and pollutants. The tritium and bromide profiles showed the rate of infiltration through the unsaturated matrix to be very slow (1.6–11 cm/yr). The chemical and isotopic data from the core holes suggested that the pore water changes characteristics with depth. Close to land surface, the pore water is strongly evaporated (δ18O = +5.94‰) and highly concentrated (∼29 meq Cl/100 g rock), but changes gradually with depth to amore dilute concentration (∼4 meq Cl/100 g rock) and isotopically depleted composition (δ18O = −4.4‰), closer to the isotopic composition of precipitation and groundwater. Nearby monitoring wells have shown anthropogenic contribution of heavy metals, organic compounds, and tritium (Nativ and Nissim, 1992). A conceptual model is proposed in which a small portion of the rainwater percolates downward through the matrix, while a larger percentage of the percolating water moves through preferential pathways in fractures. The water flowing through the fractures penetrates the matrix across the fracture walls, where it increases the tritium concentrations, depletes the stable isotopic composition, and dilutes the salt concentrations. The observed rapid downward migration of tritium and heavy metals through the profuse fractures makes the chalk inefficient as a

ISSN:0043-1397    

DOI:10.1029/94WR02536

年代:1995

数据来源: WILEY

摘要:

Chlorofluorocarbons and krypton 85 are trace gases whose atmospheric concentrations have been increasing over the past few decades. As they are soluble in water, they have been used as groundwater age indicators over timescales ranging from a few years to a few decades. In this paper we show that the time lag for transport of these atmospheric trace gases through the unsaturated zone is an important consideration when dating groundwaters that are recharged through thick unsaturated zones. The apparent time lag is largely dependent on the gas solubility, the gas diffusion coefficient, and the soil water content. In coarse‐grained soils the lag time will typically range between 1 and 2 years for a water table depth of 10 m to between 8 and 15 years for a water table depth of 30 m. For the shallower water tables (10 m), if this effect is not considered, the use of these gaseous tracers will overestimate the age of the groundwater. In very fine‐grained soils where the soil water content in the unsaturated zone may be close to saturation, the effect will be much more pronoun

ISSN:0043-1397    

DOI:10.1029/94WR02232

年代:1995

数据来源: WILEY

摘要:

Continuous flow column experiments were used at different temperatures to study the importance of motility on advective transport of bacteria through repacked, but otherwise unaltered, natural aquifer sediment. The bacterium used was A0500, a flagellated, spore‐forming rod isolated from the deep subsurface (180 m). At 4°C, A0500 was nonmotile because here was no flagellar metabolism. Bacteria removal was greater at 4°C than at 18°C. Similar experiments with microspheres showed an opposite effect, ith greater removal at 18° than 4°C, which was consistent with colloid filtration theory. The sticking efficiency (α) for nonmotile A0500 (4°C), estimated using a steady state colloid filtration model, was over 3 times that of the motile A0500 (18°C), 0.073 versus 0.022. Analysis of complete breakthrough curves using a nonsteady, kinetically limited, transport model suggested that motile A0500 bacteria traveled twice as far as nonmotile A0500 bacteria before becoming attached to the sediment grains. Once attached, nonmotile bacteria detached on a timescale of 9–17 days versus 4–5 days for the motile bacteria. Bacterial motility facilitates advective transport through sediments by changing the attachment‐detachment kinetics to effectively reduce retardation and increase the fraction of time bacteria spend in a detached versus an attached state. Consequently, travel times to deep aquifers from recharge waters could be significantly affected by bac

ISSN:0043-1397    

DOI:10.1029/94WR02569

年代:1995

数据来源: WILEY

摘要:

Numerical models of flow in the vadose zone require capillary pressure‐saturation relationships for the grid blocks discretizing the medium. However, the scale of these grid blocks is generally much larger than the sample scale at which capillary pressure‐saturation relationships are measured. This study investigates the upscaling of capillary curves under static conditions of capillary‐gravity equilibrium. The averaging regions are horizontal layers, parallel to the phreatic surface, as in one‐dimensional models of infiltration. At the point or sample scale it is assumed that all capillary curves can be normalized using the Leverett (1941∥ approach to a common dimensionless form described by the Corey‐Brooks model. Static water saturation at any point in the medium is then expressed as a parametric function of three regionalized variables, permeability, porosity, and irreducible water saturation. Water saturations at the layer scale are obtained by spatial averaging of point‐scale values. With certain assumptions it is shown that the nondimensional relationship between capillary pressure and saturation at the layer scale is the same as at the sample scale. In general, the nondimensional scaling of layer‐averaged capillary pressure requires exhaustive knowledge of the spatial distribution of the permeability‐porosity ratio within the averaging region. However, when permeability and porosity are jointly lognormal, this spatial distribution may be approximated by a lognormal model requiring only two parameters for each averaging region. The developments are illustrated using published field data from the Apache Leap tuff site. Exact and estimated upscaled capillary curves are compared in simulated coregionalized fields of permeability, porosity, and irreducible water saturations. Results show that the upscaled capillary pressure‐saturation curves accurately reproduce the vertical saturation profile obtained by exhaustive spatial averaging of point‐sc

ISSN:0043-1397    

DOI:10.1029/94WR02677

年代:1995

数据来源: WILEY

摘要:

Many problems in subsurface hydrology involve the flow and transport of solutes that affect liquid density. When density variations are large (>5%), the flow and transport are strongly coupled. Density variations in excess of 20% occur in salt dome and bedded‐salt formations which are currently being considered for radioactive waste repositories. The widely varying results of prior numerical simulation efforts of salt dome groundwater‐brine flow problems have underscored the difficulty of solving strongly coupled flow and transport equations. We have implemented a standard model for hydrodynamic dispersion in our general purpose integral finite difference simulator, TOUGH2. The residual formulation used in TOUGH2 is efficient for the strongly coupled flow problem and allows the simulation to reach a verifiable steady state. We use the model to solve two classic coupled flow problems as verification. We then apply the model to a salt dome flow problem patterned after the conditions present at the Gorleben salt dome, Germany, a potential site for high‐level nuclear waste disposal. Our transient simulations reveal the presence of two flow regimes: (1) recirculating and (2) swept forward. The flow dynamics are highly sensitive to the strength of molecular diffusion, with recirculating flows arising for large values of molecular diffusivity. For pure hydrodynamic dispersion with parameters approximating those at Gorleben, we find a swept‐forward flow field at steady state rather than the recirculating flows found in previous investigations. The time to steady state is very sensitive to the initial conditions, with long time periods required to sweep out an initial brine pool in the lower region of the domain. Dimensional analysis is used to demonstrate the tendency toward brine recirculation. An analysis based on a dispersion timescale explains the observed long time to steady state when the initial condition has a brine pool in the lower part of the system. The nonlinearity of the equations and the competing effects of dispersion and gravity make this variable‐density problem a challenge for any numerical simulati

ISSN:0043-1397    

DOI:10.1029/94WR02272

年代:1995

数据来源: WILEY

摘要:

Transport in single fractures has recently been intensely examined. Potential applications of these studies include nuclear waste storage and infiltration of rainwater into soil desiccation cracks. We modeled hydrodynamic dispersion in single fractures, using a variable‐aperture model and particle‐tracking techniques. We examined issues of scale of heterogeneity, particle‐tracking method, and grid topology. Hydrodynamic dispersion tends to zero as the scale of the transport path increases in relation to the scale of heterogeneity. Since this is not observed in nature, it implies either that fractures have fractal structure or that hydrodynamic dispersion alone does not account for all the dispersion that occurs in fractures. Dispersion and retardation as simulated using a node‐to‐node or mixing type algorithm are greater than when they are simulated using an interpolation algorithm, and the difference cannot be attributed to molecular diffusion. Differences in conductivity and dispersion between different grid types (serial, parallel, square, and random field) are related to the coordination number (degree of connectedness) of the grid, with lower coordination number grids having higher d

ISSN:0043-1397    

DOI:10.1029/94WR02674

年代:1995

数据来源: WILEY

摘要:

A control volume model of solute transport through a single fracture in a porous matrix is developed. Application to problems of contaminant transport through fractured clay demonstrates several strong features of the method. The control volume approach inherently conserves mass and treats dispersivity at interfaces in a physically correct manner. By employing an upstream weighting scheme, based on the exact solution to the one‐dimensional steady state advection‐dispersion equation, the model proves to be more efficient than previous single‐fracture models. The significance of matrix diffusion in the direction parallel to the fracture axis is investigated. For the transport of a nonreactive tracer through a 20‐micrometer‐wide fracture in clay material, analytical solutions based on one‐dimensional matrix diffusion are erroneous for flow velocities of less than 1 m/day. The influence of boundary conditions on two‐dimensional matrix diffusion is considered, and the clean‐up of a contaminated fractu

ISSN:0043-1397    

DOI:10.1029/94WR01967

年代:1995

数据来源: WILEY

摘要:

A stochastic model for solute transport in streams with transient storage zones is presented. This model has an analytic solution in terms of a rapidly converging infinite series. Besides being computationally efficient, this solution is of interest because the terms of the series can be interpreted physically. Simple methods for roughly estimating storage zone parameters directly from tracer injection data are introduced. These methods give quick estimates of these parameters without requiring solutions to the model equations.

ISSN:0043-1397    

DOI:10.1029/94WR02739

年代:1995

数据来源: WILEY

摘要:

The temporal dynamics in the lake water mixture derived from river flooding, local rainfall and runoff, exchange with an adjacent lake, groundwater exchange, and evaporation was computed from detailed measurements over an annual cycle for a permanent lake on the central Amazon floodplain. River water invaded the lake at the start of rising water, but by mid‐rising water, lake water steadily flowed out from the lake and into the river, while river levels continued to rise. Despite a relatively low ratio of catchment area to water surface area, the lake exported to the river three times the amount of water originally received from the river. The lake water mixture was dominated by river water early during the water year, increasing to 70% of the mixture. When lake water began flowing out from the lake, the river water fraction was steadily diluted as runoff became dominant. By the end of the water year, runoff contributed 57% of the total water input, river inflow 21%, rainfall 11%, inflow from an adjacent lake 6%, and seepage inflow 4%. However, local runoff, river inflow, and lake water carried over between water years have the potential for considerable interannual variation. The finding that runoff from relatively small local catchments can be sufficiently large to prevent flood waters from entering the lakes during periods of rising water may critically limit our ability to characterize the long‐term frequency and duration of flooding in lakes on the floodplains of large rivers in the wet trop

ISSN:0043-1397    

DOI:10.1029/94WR02271

年代:1995

数据来源: WILEY

摘要:

Laboratory experiments were used to identify and quantify processes having a significant effect on molybdate (MoO42−) adsorption in a shallow alluvial aquifer on Cape Cod, assachusetts. Aqueous chemistry in the aquifer changes as a result of treated sewage effluent mixing with groundwater. Molybdate adsorption decreased aspH, ionic strength, and the concentration of competing anions increased. A diffuse‐layer surface complexation model was used to simulate adsorption of MoO42−, phosphate (PO43−), and sulfate (SO42−) on aquifer sediment. Equilibrium constants for the model were calculated by calibration to data from batch experiments. The model was then used in a one‐dimensional solute transport program to successfully simulate initial breakthrough of MoO42−from column experiments. A shortcoming of the solute transport program was the inability to account for kinetics of physical and chemical processes. This resulted in a failure of the model to predict the slow rate of desorption of MoO42−from the columns. The mobility of MoO42−ncreased with ionic strength and with the formation of aqueous complexes with calcium, magnesium, and sodium. Failure to account for MoO42−speciation and ionic strength in the model resulted in overpredicting MoO42−adsorption. Qualitatively, the laboratory data predicted the observed behavior of MoO42−in the aquifer, where retardation of MoO42−was greatest in uncontaminated roundwater having lowpH, low ionic strength, and low concent

ISSN:0043-1397    

DOI:10.1029/94WR02675

年代:1995

数据来源: WILEY