摘要:

Stable isotopes of oxygen, and natural and fallout radionuclides, have been used, respectively, to identify water sources and to quantify watershed fluxes of precipitation‐borne solutes and particles that come in contact with tundra vegetation at Imnavait Creek, Alaska (68° 37′N, 149° 17′W). Oxygen 18/oxygen 16 ratios of water in the snowpack, stream, and unfrozen soil during the peak of snowmelt showed that less than ∼14% of streamwater was derived from sources other than snow. Within a month of snowmelt,18O/16O ratios in the stream indicated that only negligible amounts of water derived from snow remained in the watershed, in contrast to the low degree of mixing between snow and underlying soil water, greater than 90% of the atmospherically derived7Be (53‐day half‐life) that was deposited in the snowpack was adsorbed onto the surface 2–3 cm of frozen organic soils and vegetation. A slightly lower degree of absorption (∼2/3) on tundra was observed for atmospherically derived35S (87‐day half‐life) released during snowmelt. These data indicate that atmospherically derived substances in the snowpack are often retained strongly by sorption or chemical interaction with vegetation and soil during snowmelt. This conclusion is also supported by the vegetation and soil inventories of137Cs (30.2‐year half‐life), which are similar to values expected as a result of atmospheric depositi

ISSN:0043-1397    

DOI:10.1029/91WR01243

年代:1991

数据来源: WILEY

摘要:

Richards' equation of the unsaturated flow has been scaled in an invariant form with regard to the variation of boundary conditions within a defined class of flow problems. Two classes of problems were examined: (1) vertical infiltration into a homogeneous soil with a constant surface boundary flux, and (2) vertical infiltration into a soil topped by a seal layer of negligible thickness and with a given positive pressure head above the seal layer. The solution of the problem when plotted as function of scaled variables is unchanged for any variation at the boundary, i.e., variation of the flux in the first class and variation of the seal in the second class.

ISSN:0043-1397    

DOI:10.1029/91WR01550

年代:1991

数据来源: WILEY

摘要:

A network of piezometers was installed in a surficial lacustrine clay aquitard overlying a thin saline water aquifer of volcanoclastic origin at a study site near Mexico City in the Basin of Mexico. The aquifer is underlain by additional lacustrine sediments which in turn overlie a thick regional freshwater aquifer. The regional aquifer provides approximately 70% of the water supply for 20 million people in the Basin of Mexico. In the study area, major ions, oxygen 18, and deuterium in the pore water of the surficial aquitard exhibit large variations with depth. The nature of these variations suggests that the saline pore water is being displaced downward by infiltrating meteoric water. The infiltration has been induced by strong downward hydraulic gradients imposed two to three decades ago when heavy aquifer pumping of the thin saline water aquifer began. One‐dimensional analytical models representing solute transport in both fractured and unfractured porous media were used to simulate the geochemical profiles in the surficial aquitard. The fractured porous medium model, using a realistic mean hydraulic gradient and fracture spacing (1.5 m) and small but significant fracture aperture (30 μm) provide nearly an exact match to the field data. From this we infer that, because of vertical fractures, there is a much greater potential for downward leakage of water and contaminants through the Mexico City clay into underlying aquifers than has been previously thoug

ISSN:0043-1397    

DOI:10.1029/91WR01306

年代:1991

数据来源: WILEY

摘要:

This work presents new results on a method for optimal aquifer remediation when available information is limited. The methodology combines computer simulation models of solute transport and fate, descriptions of spatial variability, probabilistic analysis of uncertainty, and optimization. The objective is to find the most cost‐effective management policy for aquifer decontamination. Advantages of the method include the following: (1) it utilizes measurements in real time, (2) it simultaneously estimates aquifer parameters and makes decisions for remediation, and (3) it devises a more cost‐effective and reliable aquifer remediation strategy than deterministic optimization, specifically, the method known as deterministic feedback control. Subject to constraints and for a given reliability of meeting water quality standards, this method minimizes the expected value of the cost in the remaining periods. That is, because of incomplete information about the site the cost of a decontamination strategy is not known a priori. The objective is to minimize the cost weighted by the probability that it will be incurred. The optimal aquifer management policy is expressed as the sum of a deterministic and a stochastic control term. The former is obtained by solving a deterministic optimization problem through constrained differential dynamic programming, and the latter is obtained by a perturbation approximation to the stochastic optimal control problem. Extended Kalman filtering is incorporated into the optimization method to improve the accuracy of the estimated state and parametric variables using available measurements. A hypothetical contamination case with two‐dimensional unsteady flow and transport for a persistent solute is studied to illustrate the applicability of the methodology. The effectiveness in terms of cost and reliability of the proposed method is studied under various conditions and then compared with the cost and reliability of the deterministic feedback control method through Monte Carlo simulations. The proposed methodology is shown to be superior to deterministic feedback co

ISSN:0043-1397    

DOI:10.1029/91WR01307

年代:1991

数据来源: WILEY

摘要:

Four different methods of parameterizing spatially varying log transmissivities in an inverse approach are compared with respect to prediction accuracy of simulated flow and transport. Transport parameter estimation is included by two‐stage feedback optimization. In stage one the log transmissivities are estimated by fitting both head and concentration data, given initial values of the source concentration and the dispersivities. In stage two, the source concentration and the dispersivities are estimated by fitting the concentration data. With the updated transport parameters, final estimates of the log transmissivities are obtained by repeating the optimization of stage one. The formulated objective functions are minimized using Levenberg‐Marquardt's algorithm. The models are applied to synthetic two‐dimensional transport problems in steady state flow regimes. The “true” log transmissivity fields are generated by the turning bands method, thereby incorporating spatial variability. The test cases differ in the input variances of the generated fields and with respect to the amount and accuracy of “observed” transm

ISSN:0043-1397    

DOI:10.1029/91WR00990

年代:1991

数据来源: WILEY

摘要:

A study was undertaken to quantitatively evaluate the hydrologic processes affecting the chemical and isotopic composition of drain lateral water in a drained agricultural field in the western San Joaquin Valley, California. The results elucidate the process of mixing of deep and shallow groundwater (below and within 6 m from land surface) entering the drain laterals. The deep groundwater was subject to evapoconcentration prior to drainage system installation and has been displaced downward (to depths greater than 6 m) in the groundwater system. The proportions of deep and shallow groundwater entering the drain laterals was calculated from the end‐member oxygen 18 compositions determined in groundwater samples. The percentage of total drain lateral flow which is deep groundwater flow is about 30% for the shallow drain lateral (1.8 m below land surface) (drain lateral 1)) and 60% for the deep drain lateral (2.7 m below land surface (drain lateral 2)). During irrigation, the percentages of deep groundwater flow decrease to 0 and 30% for the shallow and deep drain laterals, respectively. Selenium concentrations in drain lateral waters decrease during irrigation but selenium loads increase. Total estimated annual loads were 1.1 and 5.4 kg of selenium for drain laterals 1 and 2, respectively. Substantial percentages of the annual load occurred during 8 days of irrigation, 23 and 9% for drain laterals 1 and 2, respectivel

ISSN:0043-1397    

DOI:10.1029/91WR01367

年代:1991

数据来源: WILEY

摘要:

Groundwater flow modeling was used to quantitatively assess the hydrologic processes affecting ground water and solute movement to drain laterals. Modeling results were used to calculate the depth distribution of groundwater flowing into drain laterals at 1.8 m (drain lateral 1) and 2.7 m (drain lateral 2) below land surface. The simulations indicated that under nonirrigated conditions about 89% of the flow in drain lateral 2 was from groundwater originating from depths greater than 6 m below land surface. The deep groundwater has higher selenium concentrations than shallow groundwater. Simulation of irrigated conditions indicates that as recharge (deep percolation) increases, the proportional contribution of deep groundwater to drain lateral flow decreases. Groundwater flow paths and travel times estimated from the simulation results indicate that groundwater containing high concentrations of selenium (greater than 780 μg L−1) probably will continue to enter drain lateral 2 for decad

ISSN:0043-1397    

DOI:10.1029/91WR01368

年代:1991

数据来源: WILEY

摘要:

A flexible finite element transport model, which includes the impact of vapor sorption, is developed to simulate the movement of volatile organic compounds (VOCs) in variably saturated porous media. The two‐dimensional numerical model predicts contaminant transport by aqueous advection, aqueous dispersion, aqueous and vapor diffusion, and surface volatilization. For a soil with increasing water contents with depth, one‐dimensional simulations demonstrate that strong vapor sorption may reduce the total amount of VOC that volatilizes in 100 days from 84.6% of the initial mass volatilized without vapor sorption to 73.2%. Yet, for a soil with low water contents at depth, such as in an area with limited recharge, vapor sorption enhanced the rate of volatilization in 100 days from 72.4% without vapor sorption to a peak of 90.3%. When low soil moisture was combined with a soil type that has strong vapor sorption characteristics, VOC transport was significantly retar

ISSN:0043-1397    

DOI:10.1029/91WR00223

年代:1991

数据来源: WILEY

摘要:

Apparent, or effective, infiltration rates on grassland hillslopes vary with rainfall intensity and flow depth because of the interaction between rainfall, runoff, and vegetated microtopography. The higher parts of the microtopography are occupied by greater densities of macropores and therefore have much greater hydraulic conductivities than the intervening microdepressions. On short hillslopes and plots the apparent infiltration rate is simply the spatial average of the saturated and unsaturated conductivities of this surface. The proportion of the surface which is saturated and the value to which the unsaturated conductivity is raised depends on the rainfall intensity. On longer hillslopes the downslope increase in flow depth in microtopographic depressions progressively inundates more permeable, vegetated mounds so that the hydraulic conductivity of a greater proportion of the surface is raised to its saturated value. For this reason the apparent infiltration rate increases downslope, even in the absence of spatial trends in any of the surface characteristics that affect infiltration. Apparent, or effective, infiltration rate depends on hillslope length. Consequently, steady state discharge does not increase linearly with distance downslope. These two fundamental relationships between infiltration, rainfall intensity, and runoff are analyzed on the basis of sprinkling‐infiltrometer measurements and a mathematical mode

ISSN:0043-1397    

DOI:10.1029/91WR01585

年代:1991

数据来源: WILEY

摘要:

The transport of linearly interacting solutes in porous media is investigated with the help of residence time distributions, transfer functions, methods of system dynamics, and time‐moment analyses. The classical one‐dimensional convection‐dispersion equation is extended to two‐region (mobile‐immobile water) transport by including diffusional mass transfer limitations characteristic of aggregated soils. The two‐region model is further revised by incorporating the effects of multiple retention sites (in parallel or in series), multiple porosity levels, and arbitrary but steady flow fields. It is shown that different physical situations can be represented by a relatively small number of transfer functions containing only two types of parameters: distribution coefficients to account for equilibrium properties and characteristic times reflecting kinetic processes. Relevant kinetic processes include convective transport, hydrodynamic dispersion, adsorption‐desorption, and physical or chemical mass transfer limitations. In most situations, theoretical breakthrough curves are found to be relatively insensitive to the mathematical structure of the transfer function, irrespective of the physical interpretation of the distribution coefficients and the characteristic times in the model. This means that alternative physical and chemical interpretations of model parameters can lead to nearly identical breakthrough curves. Certain transfer time distributions can lead to quite unusual shapes in the breakthrough curves; these curves strongly depend on the characteristic times and a few operational variables. Results of this study show that the transfer time distribution is an extremely useful tool for explaining some unexpected experimental results in the solute transpo

ISSN:0043-1397    

DOI:10.1029/91WR01034

年代:1991

数据来源: WILEY