Spatial Distribution Of Hydraulic Conductivity Research Articles

Modelling the effect of physical and chemical characteristics of shallow aquifers on water and nitrate transport in small agricultural catchments

The aim of this study is threefold: (1) to evaluate the effect of the physical properties of weathered granitic material (geometry and spatial distribution of hydraulic conductivity) on water fluxes at the outlets of two small and nearby agricultural catchments, Kerbernez and Kerrien (0.12 and 0.095 km 2), underlain by granite bedrock; (2) to explain the variations of nitrate concentration in streamwater in relation to the spatial distribution of dissolved nitrate in the groundwater; (3) to investigate the origin of the groundwater nitrate by analysing the reaction times of groundwater to variations of nitrate concentration in water recharge. These objectives were attained by developing a flow and nitrate transport model for the two catchments from geophysical measurements on the geometry and hydraulic conductivity of the weathered granite layer, and using data collected from soil surveys. The models were calibrated and validated from spatial and temporal observations of hydraulic heads and nitrate concentrations in groundwater and stream water. The flow models appeared to be less sensitive to the geometry of the weathered granite layer than the spatial distribution of hydraulic conductivity. Model results show that seasonal patterns of nitrate concentrations in streamwater result partly from the spatial distribution of nitrate concentration in the groundwater. The horizontal distribution of nitrate concentrations in groundwater derives from denitrification in the downslope domain. An analysis of the groundwater reaction times for both catchments shows that, following a variation in the recharge nitrate concentration; the time to reach equilibrium in the whole groundwater body is more than 14 years. In the Kerbernez catchment, the vertical distribution observed in groundwater nitrate concentrations appears to be caused by a temporal decrease of the nitrate concentration in the water recharge over the last 15 years.

RELATIONSHIP BETWEEN SPATIAL DISTRIBUTIONS OF HYDRAULIC CONDUCTIVITY AND DISPERSIVITY IN ADVECTION DISPERSION PHENOMENA

It is known that observed macroscopic dispresivity depends on observation scale, so that a single value cannot represent the field property for dispersion. In this study, numerical simulations of the advection dispersion phenomena in various random-permeability fields generated by using stochastic fractal model were carried out to investigate the relationship between spatial distributions of hydraulic conductivity and dispersivity. The results show that various concentration distributions occur even when the hydraulic conductivities of flow fields are statistically the same. It is also shown that a dispesivity distribution model expressed by function of scale can well reproduce the ensemble mean of concentration distributions.

Reliable groundwater management in hydroecologically sensitive areas

A stochastic groundwater management model is formulated to account for prediction uncertainty when maximizing regionally distributed groundwater production yet obeying regulations to maintain the hydroecological balance in wetland areas. The water table elevation in sensitive wetland areas is lowered by the withdrawal of groundwater at supply wells. Substantial uncertainty exists because drawdowns depend on both the unknown spatial distribution of hydraulic conductivity and regional boundary conditions. Planning in the face of uncertain predictions of water table changes means that optimal production must be prudently reduced. A stochastic simulation‐optimization formulation is developed that provides a robust water production plan. Prediction uncertainty is dealt with through stochastic simulation‐optimization using a multiple‐realization approach. On the basis of analyses involving solution of over 8 million aquifer models and 36,000 stochastic‐optimization solutions, the nature and reliability of the optimal groundwater production scheme are inspected to determine the effects of uncertainty in spatially variable hydraulic conductivity, conditioning on local measurements, and the type of boundary conditions imposed in the nonlinear aquifer model. We propose a new measure to predict the expected reliability of meeting water level constraints in wetland areas. Monte Carlo simulations based on numerous optimal groundwater production schemes confirm that the expected reliability is a quantifiable function of the number of hydraulic conductivity realizations included in the stochastic‐optimization formulation and the variance of log hydraulic conductivity.

Improving groundwater-flow modeling using optimal zoning methods

Hydraulic conductivity sometimes exhibits complicated spatial variation over a site. A thorough understanding of the spatial distributions of hydraulic conductivity helps to make deterministic models of groundwater more accurate. This study presents a novel procedure that combines simulated annealing algorithms (SA) and the shortest distance method (SD) with the modular three-dimensional groundwater flow model (MODFLOW). The procedure is applied to a hypothetical site with groundwater-monitoring wells to minimize the difference between simulated and observed hydraulic head for optimal zoning of the spatial distribution of hydraulic conductivity. The results of this optimal zoning method indicate that this new procedure not only improves the efficiency of optimization, but also increases the probability of finding the global optimum, minimizing the errors of the hydraulic head simulated by MODFLOW in two scenarios, one with known and the other with unknown hydraulic conductivity. The results also illustrated that the procedure can effectively determine and delineate hydrogeological zones.

Deformation mechanisms and hydraulic properties of fault zones in unconsolidated sediments; the Roer Valley Rift System, The Netherlands

In general, faults cutting through the unconsolidated sediments of the Roer Valley Rift System (RVRS), The Netherlands, form strong barriers to horizontal groundwater flow. The relationships between deformation mechanisms along fault zones and their impact on the hydrogeological structure of the fault zone are analyzed in a shallow (0–5 m below land surface) trench over one of the faults in the study area. Recently developed digital-image-analysis techniques are used to estimate the spatial distribution of hydraulic conductivity at the millimeter-scale and to describe the micromorphologic characteristics of the fault zone. In addition, laboratory measurements of hydraulic conductivity on core-plug samples show the larger-scale distribution of hydraulic conductivity in the damage zone flanking the main fault plane. Particulate flow is the deformation mechanism at shallow depths, which causes the damage zone around the fault, in the sand-rich parts, to have a relatively enhanced hydraulic conductivity. The fault core is characterized by reduced hydraulic conductivity due to clay smearing, grain-scale mixing, and iron-oxide precipitation.

Using direct current resistivity sounding and geostatistics to aid in hydrogeological studies in the Choshuichi alluvial fan, Taiwan.

Ground water reservoirs in the Choshuichi alluvial fan, central western Taiwan, were investigated using direct-current (DC) resistivity soundings at 190 locations, combined with hydrogeological measurements from 37 wells. In addition, attempts were made to calculate aquifer transmissivity from both surface DC resistivity measurements and geostatistically derived predictions of aquifer properties. DC resistivity sounding data are highly correlated to the hydraulic parameters in the Choshuichi alluvial fan. By estimating the spatial distribution of hydraulic conductivity from the kriged well data and the cokriged thickness of the correlative aquifer from both resistivity sounding data and well information, the transmissivity of the aquifer at each location can be obtained from the product of kriged hydraulic conductivity and computed thickness of the geoelectric layer. Thus, the spatial variation of the transmissivities in the study area is obtained. Our work is more comparable to Ahmed et al. (1988) than to the work of Niwas and Singhal (1981). The first "constraint" from Niwas and Singhal's work is a result of their use of linear regression. The geostatistical approach taken here (and by Ahmed et al. [1988]) is a natural improvement on the linear regression approach.

Application of kriging with external drift to estimate hydraulic conductivity from electrical-resistivity data in unconsolidated deposits near Montalto Uffugo, Italy

In the framework of various studies to characterize the aquifer at the groundwater experimental field near Montalto Uffugo, Italy, the present work estimates the spatial distribution of hydraulic conductivity of the unconsolidated deposits that underlie the area, by applying the geostatistical technique of kriging with external drift to electrical-resistivity and hydraulic-conductivity data.

SPATIAL VARIABILITY OF CONDUCTWITY AS APPLIED TO DISTRIBUTED PARAMETER INFILTRATION MODELS1

ABSTRACT: Two infiltration models, called VVSIM (variable variance stochastic infiltration model) and EVVSIM (enhanced variable variance stochastic infiltration model), are developed in this study. A distributed parameter infiltration model can estimate the amount of infiltration over a field area by computing the infiltration over zones of the field area. Hydraulic conductivity is the most important parameter determining infiltration in simulations by infiltration models. The performance of an infiltration model depends on how well the model accommodates the complicated spatial distribution of hydraulic conductivity. The two proposed models include the effects of spatial correlation of the conductivity distribution. Virtual conductivity fields are generated using the turning bands method. Monte Carlo simulations show that the proposed models give infiltration estimates more accurate than those obtained by the other models employed.

Numerical experiments on the residence time distributions of heterogeneous groundwatersheds

Groundwater residence time distributions are important when studying the impacts of non-point source pollution in shallow groundwatersheds (aquifer-stream systems) on stream water quality. The theoretical residence time distribution F(T) for unstratified groundwatersheds that are homogeneous with respect to porosity, recharge and aquifer thickness is a simple exponential function. This function F(T) is insensitive to watershed size, stream network and spatial distribution of hydraulic conductivity. It has been found that certain of the conditions previously presented as necessary for the validity of F(T) can be somewhat relaxed. In this paper, we explore the behavior of residence time distributions when these assumptions used in the development of F(T) are no longer met, and test the robustness of the theoretical distribution F(T) in such cases. Numerical experiments suggest that F(T) provides a suitable approximation to many groundwatersheds' residence time distributions, except when aquifer heterogeneities are few and distinct. Additionally, a new residence time distribution F̂(T) is derived for a class of semi-confined stratified aquifers.

Evaluation of Hydraulic Conductivity of Carson County Well Field, Amarillo, Texas

AbstractEnvironmental assessment, ground‐water management, and aquifer remediation designs rely on comprehension of the hydraulic parameters of aquifers. Because of the heterogeneity of most aquifers, a number of pumping tests are commonly required to provide a reasonable hydraulic parameter distribution. Data for 11 pumping tests, conducted at the Carson County well field, Amarillo, Texas in the 1950s to 1970s were analyzed by the Theis solution and the Cooper and Jacob solution to provide information for optimal ground‐water management. The unconfined aquifer at the well field consists of sands, sandstone, gravels, and clay. A delayed gravity response was observed in the drawdown curves from pumping tests conducted in the unconfined aquifer. Because the Theis solution and the Cooper and Jacob solution do not take this delayed response, or other unsaturated effect into account, these two solutions may overestimate hydraulic conductivity of an unconfined aquifer. Therefore, the Neuman solution which considers the delayed gravity response was used to estimate the hydraulic parameters using the software AQTESOLV (by Geraghty & Miller, Inc., 1990). This paper presents the results of a study of the unconfined aquifer at Carson County well field, Amarillo, Texas. The analysis shows that the results obtained by the Neuman method are more reasonable than those obtained by the Theis solution and the Cooper and Jacob solution. According to results from the Neuman solution, the hydraulic conductivity of the unconfined aquifer of Carson County well field varies from 2.55 to 5.97 × 10−5 m/s (weighted average hydraulic conductivity is 4.13 × 10−5 m/s). The small difference of the estimated hydraulic conductivity between wells shows that the unconfined aquifer under Carson County well field is relatively uniform. The effects of infinitesimal borehole assumption, accuracy of individual parameters, skin effects, and spatial distribution of hydraulic conductivity are discussed as well. It is also shown that benefits from obtaining accurate results should be weighted against budget requirements.

Simulations of Advective Transport and Importance of High Permeability Facies in an Outwash Deposit: ABSTRACT

Predictions of clean-up times for groundwater pump-and-treat systems are difficult to make for heterogeneous aquifers due to uncertainty in our knowledge of this hydraulic conductivity distribution. Our approach has been to specify the spatial distribution of hydraulic conductivity in terms of different facies and to examine the importance of highly permeable facies on transport. A 50 by 60 meter section of proximal glacial outwash deposits were mapped by a previous investigator in a quarry in Wisconsin and separated into seven lithofacies. Hydraulic conductivity values were estimated for facies based on grain size characteristics of sediment samples. These values range over 4 orders of magnitude, with the highest and lowest hydraulic conductivity facies each representing less than 5 percent of the deposit. We simulated contaminant transport using a particle tracking code coupled to a three-dimensional model for advective flow to trace the path of particles in a steady-state flow field. The model grid is composed of 7,500 nodes in 10 layers. Simulations assuming homogeneous facies and interfaces heterogeneity were conducted. Breakthrough curves were generated for particle arrival at the model exit face and for particle capture by a pumping well, and exhibit such features as early particle arrival and tailing thatmore » are characteristic of heterogeneous aquifers. Slow and fast particle pathways were examined and related to the distribution of hydraulic conductivity. Simulations were performed to examine the sensitivity of particle transport to the orientation of the flow field, pumping well location and absence of high conductivity facies. Particle recovery is sensitive to both well location and high conductivity facies. Less converging of particle pathways and later arrival occurred without the high conductivity facies.« less

Similarity in Catchment Response: 1. Stationary Rainstorms

The variability in Hortonian surface runoff discharge and volume produced by stationary rainstorms on watersheds with spatially distributed soil saturated hydraulic conductivity is examined using a two‐dimensional runoff model and a Monte Carlo methodology. Results indicate that rainfall duration tr, rainfall intensity i, representative time to equilibrium tre, mean saturated hydraulic conductivityKm, and coefficient of variation Cυ play major roles in the variability of surface runoff. Similarity in surface runoff generated on heterogeneous soils is governed by the following dimensionless parameters: T* = tr/tre, K* = Km/i, and Cυ. The variability in both discharge and runoff volume for randomly distributed systems increases with K* and Cυ, compared to the runoff generated from uniformly distributed systems. Runoff variability decreases whenT* increases unless the mean value of hydraulic conductivity approaches the rainfall intensity (K* → 1). In highly pervious watersheds the steady state discharge depends on the spatial distribution of hydraulic conductivity. Lumped values of saturated hydraulic conductivity are found to typically underestimate the peak discharge and runoff volume.

Three‐Dimensional Modeling of a Two‐Well Tracer Test

AbstractA three‐dimensional (3‐D), advection‐based numerical model has been developed for simulating two‐well injection withdrawal tracer tests in heterogeneous granular aquifers, and the model has been applied to perform various simulations of an actual two‐well tracer test conducted previously in a confined aquifer at a field site near Mobile, Alabama. Information obtained from a series of independent field tests is used as input in the model to account for the 3‐D spatial distribution of the hydraulic conductivity (K) at the test site.The numerical model is based on a simplified, Lagrangian approach in which the transport of the tracer between the injection and withdrawal wells is modeled taking into account advection only. Processes such as dispersion, sorption, and chemical or biological reactions are neglected. Despite field data limitations and various simplifying assumptions, the model predictions of the withdrawal‐well concentration breakthrough and of the mean tracer arrival times at various levels of two multilevel observation wells are in good overall agreement with the corresponding field observations.The results of this study indicate that it will be possible to construct realistic, predictive models of contaminant transport in heterogeneous granular aquifers if the necessary effort is made to obtain field measurements of the 3‐D spatial distribution of hydraulic conductivity. The extent and feasibility of such an effort are expected to depend on the particular contamination problem at hand. Future research should be directed toward the problems of field measurement scale selection and the development of K distributions for models from sparse data sets.

Calibration of a Texture‐Based Model of a Ground‐Water Flow System, Western San Joaquin Valley, Californiaa

AbstractThe occurrence of selenium in agricultural drain water from the western San Joaquin Valley, California, has focused concern on the semiconfined ground‐water flow system, which is underlain by the Corcoran Clay Member of the Tulare Formation. A two‐step procedure is used to calibrate a preliminary model of the system for the purpose of determining the steady‐state hydraulic properties. Horizontal and vertical hydraulic conductivities are modeled as functions of the percentage of coarse sediment, hydraulic conductivities of coarse‐textured (Kcoarse) and fine‐textured (Kfine) end members, and averaging methods used to calculate equivalent hydraulic conductivities. The vertical conductivity of the Corcoran (Kcorc) is an additional parameter to be evaluated.In the first step of the calibration procedure, the model is run by systematically varying the following variables: (1) Kcoarse/Kfine, (2) Kcoarse/Kcorc, and (3) choice of averaging methods in the horizontal and vertical directions. Root mean square error and bias values calculated from the model results are functions of these variables. These measures of error provide a means for evaluating model sensitivity and for selecting values of Kcoarse, Kfine, and Kcorc for use in the second step of the calibration procedure. In the second step, recharge rates are evaluated as functions of Kcoarse, Kcorc, and a combination of averaging methods. The associated Kfine values are selected so that the root mean square error is minimized on the basis of the results from the first step.The results of the two‐step procedure indicate that the spatial distribution of hydraulic conductivity that best produces the measured hydraulic head distribution is created through the use of arithmetic averaging in the horizontal direction and either geometric or harmonic averaging in the vertical direction. The equivalent hydraulic conductivities resulting from either combination of averaging methods compare favorably to field‐ and laboratory‐based values.

Distributed of downward flux in unsaturated heterogeneous hydrogeology environments

We used the finite-element computer program UNSAT2 to quantify the horizontal distribution of unsaturated downward flux in porous tuff as a function of hydrogeologic heterogeneities under two different recharge rates. The distribution of downward flux is important because it influences ground-water travel time at a potential geologic repository for high-level radioactive wastes. Hydraulic properties of the Topopah Spring Member of the Paintbrush Tuff Formation at Yucca Mountain, Nevada, were selected for use in the eight simulations that were conducted. All simulations were run essentially to steady state (referred to herein as quasi-steady state). The simulations show that (1) heterogeneities in an isotropic porous rock matrix will cause downward flux to be distributed nonuniformly, (2) zones in which saturated matrix hydraulic conductivity is less than the downward flux tend to develop positive pressures that may divert flow into fractures if they exist, (3) one-dimensional analysis of vertical flow in the unsaturated zone is insufficient to ensure that fracture flow does not occur even if the true magnitude of vertical flux is known, and (4) the true spatial distribution of hydraulic conductivity above the regional water table must be determined in order to obtain the true spatial distribution of downward flux. This analysis assumes constant spatial and temporal distribution of recharge for each simulation, but the differences among simulations verify the fact that changing recharge rates also play a significant role in the distribution of downward flux owing to the influence of recharge rate on the development of positive pore pressures in the otherwise unsaturated rock. The primary significance of this research is as follows. (1) We have introduced a new method of UNSAT2 time step control that facilitates convergence even in complex, large-scale partially saturated hydrogeologic environments. Previous studies have experienced convergence problems with UNSAT2 under complex hydrogeologic conditions. (2) Nonuniform downward flow should be anticipated in heterogeneous hydrogeologic environments at a heterogeneity scale of 50 m and a model scale of hundreds of meters, even if recharge is uniform spatially. (3) The impact of heterogeneities in the unsaturated zone on downward flux can now be quantified. (4) Hydrogeologic heterogeneities can create zones of positive pressure and consequent possible fracture flow in the unsaturated zone even when the recharge rate is less than the average saturated hydraulic conductivity, but most certainly when the recharge rate exceeds saturated hydraulic conductivity. (5) This type of study provides a basis for designing hydrogeologic property testing programs and recharge distribution testing programs that are compatible with the design of models that can characterize downward flux. (6) This work shows that field documentation of the absence of positive pressures above the water table constitutes the best evidence of exclusive matrix flow (absence of fracture flow) within Yucca Mountain. If positive pressures are encountered in the field, then fracture flow probably exists within Yucca Mountain.

Estimation of Spatial Distribution of Hydraulic Conductivity by Statistical Linearization Technique

Estimation of Spatial Distribution of Hydraulic Conductivity by Statistical Linearization Technique

空間分布構造を有する透水係数の統計的推定法

The complex spatial distributions of hydraulic conductivity within a natural aquifer represented as a statistical homogeneous random process which is characterized by a new function, called the variogram. This variogram is related to the stationary covariance of the hydraulic conductivity difference between two points separated by the distance d. Using variogram and actual field data at the sample points, a statistical linearization technique is developed to estimate the spatial distributions of hydraulic conductivity.A case study is presented in order to illustrate the statistical methodology. Once the variogram is determined throughout an aquifer, this estimation method has proven to be adapted solving groundwater system's behavior.