摘要:

Seasonal wastewater discharge programs employ different effluent standards during different times of the year to take advantage of the variation in a receiving water's susceptibility to adverse impacts. These programs should try to achieve the maximum economic benefits possible without increasing the risk of water quality impairment. A method is developed for designing seasonal programs for individual dischargers that limits the risk of one or more water quality standard violations in any year. The key elements involve (1) treating seasonal receiving water susceptibility to water quality impairment as a random variable, (2) assuming Markov‐like behavior of these random variables between seasons, and (3) using a nonlinear programming model to find seasonal discharge limits that minimize waste treatment efforts while maintaining an acceptable annual risk of water quality violation. A case study of controlling ammonia toxicity is presented as well as a comparison of the potential savings available from seasonalization for several pollutants on two rivers with differing seasonal regime

ISSN:0043-1397    

DOI:10.1029/WR025i010p02083

年代:1989

数据来源: WILEY

摘要:

Rates of atmospheric deposition of sulfur are as high in areas of the southeastern United States as they are in the northeastern United States and southeastern Canada. Sulfate concentrations and acidity of surface waters in the southeastern United States, however, remain low because of sulfate adsorption by soils. Because the capacity of soils to adsorb sulfate is finite, the questions of when and how fast the surface waters in the southeast might respond to the high rates of acidic deposition are of some importance. Modeling studies have suggested that time constants for sulfate breakthrough may be of the order of decades. Given that acidic deposition levels have been high in the southeastern United States since the 1940s, it might be expected that trends of increasing sulfate concentration and acidity would be apparent in data from monitored catchments. An analysis of 8 years of discharge chemistry data for two catchments in Shenandoah National Park, Virginia, indicates that sulfate concentrations and acidity are increasing in these catchments. The discharge‐weighted mean annual sulfate concentrations increased at a rate of 2 μeq L−1yr−1for both streams over the 8 years of the study. Both catchments are currently retaining approximately 65% of deposited sulfate. The ratios of the rate of decrease of alkalinity to the rate of increase of sulfate concentration are 34% and 25% for the two streams. If deposition rates remain constant, the observed rates of increase indicate that steady state conditions will be approached in 100 years, at which time the stream alkalinities may be as low as −65

ISSN:0043-1397    

DOI:10.1029/WR025i010p02091

年代:1989

数据来源: WILEY

摘要:

A quantitative conceptual model of soil sealing is proposed, based on the assumption that the compaction of the upper soil layer by raindrops and the “washed in” material result in a nonuniform change in bulk density, which can be approximated by an exponential function. The change in density causes in turn variations in soil properties, such as porosity, water retention, and hydraulic conductivity. Empirical functions are suggested to simulate the pattern by which these properties vary as dependent upon the density change. The model was calibrated using data of two experimental studies related to crust properties of three different soils: sandy loam, loam (loess), and clay loam. The model reproduced quite well the observed features of the experimental sealing characteristics. Seals developed in two different flow systems subjected to simulated rain were analyzed: (1) seal developed under saturated conditions over a shallow soil bed and (2) seal developed under unsaturated flow conditions. The proposed model highlights some soil parameters that are quite important and should be observed in soil sealing investigations. It also presents a basis for quantitative analysis of the soil sealing phenomenon in terms of measurable physical paramet

ISSN:0043-1397    

DOI:10.1029/WR025i010p02101

年代:1989

数据来源: WILEY

摘要:

Most of the existing data on vadose zone field scale solute transport have been obtained from experiments conducted under transient, nonmonotonic water flow. However, the majority of the theoretical analyses of these experiments have used models which assume monotonic steady state water flow and uniform water content for the entire profile. In this study, transport of nonreactive solutes under nonmonotonic, transient water flow is analyzed numerically. The effect of hysteresis on solute transport is evaluated by making the soil hydraulic properties hysteretic using the procedure of Kool and Parker (1987). The effect of profile heterogeneity on solute transport is analyzed by assuming that the medium is scale heterogeneous in a vertical direction, with a random scale factor. Results of the simulations show that under transient water flow, analysis of solute transport data with a steady state water flow model may considerably overestimate the effective vertical pore water velocity. Under nonmonotonic water flow, when the hysteretic characteristics of the soil are important, transient flow models which neglect hysteresis can also seriously overestimate the solute velocity. In addition, failure to account for profile heterogeneity will also overestimate the solute velocity, because both hysteresis and profile heterogeneity change the water content profile and concurrently retard solute transport relative to the movement predicted if the soil water system is considered as homogeneous and nonhysteretic. Analyses of the computed breakthrough curves suggest that direct stimates of the amount of water drained below a given depth may improve the goodness of fit of the solution of the convection dispersion equation with constant effective parameters to the breakthrough curves obtained under transient conditions. The fitted parameters, however, are depth dependent and the resulting effective solute velocity is smaller than the steady state pore water velocity.

ISSN:0043-1397    

DOI:10.1029/WR025i010p02109

年代:1989

数据来源: WILEY

摘要:

In this study, the two‐component mobile‐immobile water model is modified to describe solute transport under transient water flow induced by periodic cycles of irrigation, evaporation, and redistribution. To characterize transient flow, the soil water content is divided into a time‐invariant immobile water content Θi, and a time‐dependent mobile water content Θmwhich obeys the Richards equation. Solute transport is described by the two‐component mobile‐immobile water model, modified to take into account the time dependence of Θm. Results of the calculations show important differences between the behavior of the flux and residence concentrations under steady state water flow, particularly when the mass transfer coefficient α is relatively small. In this case, both resident and flux concentrations show a dispersion scale effect in their rate of spreading about the center of mass position in the soil. There were significant differences in solute transport predicted by the transient two‐component model compared to the steady state model. The apparent velocity derived from the time variations of the position of the center of mass of the resident concentration decreased with time in the transient model, but was relatively constant in the steady state one. Apparent dispersion about the center of mass increased with time in both models, but was much higher in the transient model, caused principally by a buildup of solute at the surface during e

ISSN:0043-1397    

DOI:10.1029/WR025i010p02119

年代:1989

数据来源: WILEY

摘要:

Oscillatory water levels in observation wells have commonly been recorded at the beginning of aquifer tests in highly transmissive fractured formations. In this paper, oscillatory water levels are predicted by the equations coupling the fluid movement in the observation well and the fluid movement in the surrounding formation. The equivalent‐porous medium and dual‐porosity models of fractured rock are two models considered in this analysis; however, other conceptual models of fractured media can also be coupled with the model presented here for fluid movement in the observation well. Type curves for the response of water levels in observation wells due to pumping in another well are generated by numerical inversion of the Laplace transform solution to the governing equations. Overdamped conditions, where inertial effects are insignificant, and underdamped conditions, where oscillations arise, are predicted by the solution to the governing equations. By matching water level measurements with the appropriate type curve, a conceptual model of the formation can be identified, and aquifer properties can be estimated. This analysis is applied in the interpretation of an aquifer test conducted in a fractured dolomite in northeastern Illinois. If the early time oscillations are ignored, the measured water levels can be explained by an equivalent‐porous medium model. By analyzing the early time oscillations, however, the formation is shown to respond as a dual‐porosity medium with a storativity of the fracture porosity that is an order of magnitude smaller than the storativity estimated from the equivalent‐porous med

ISSN:0043-1397    

DOI:10.1029/WR025i010p02129

年代:1989

数据来源: WILEY

摘要:

A study was conducted to quantify the source of increased dissolved aluminum concentrations during acidic storm flows on a small Pennsylvania stream. Data for six episodes during fall 1986 and spring 1987 showed depressions in streampH and increases in sulfates, conductivity, dissolved organic carbon, and dissolved aluminum. Flow separation analyses were conducted using18O as a tracer in a three‐component mass balance tracer model. Results showed that soil water and groundwater are the dominant flow sources, accounting for approximately 20 and 75% of total flow during storms, respectively. Channel precipitation generally provided less than 5% of total flows. Hydrograph separation using aluminum agreed with18O results, while other chemical parameters produced unsatisfactory results. The data support Hewlett's (1982) variable source area concept of storm flow generation with inputs of older, deep circulating groundwater from low‐elevation source areas early in an event and later inputs of younger soil water and possibly shallow groundwater from expanding source areas at higher elevations. The results suggest that the most toxic runoff events for aquatic life occur during large storms when the greatest inputs of soil water cause elevated stream dissolved aluminum concentrations. Reductions in storm flow acidity and dissolved aluminum concentrations on this catchment will be most dependent upon changes in soil water and/or groundwater chemis

ISSN:0043-1397    

DOI:10.1029/WR025i010p02139

年代:1989

数据来源: WILEY

摘要:

The effects of heterogeneity on one‐dimensional, steady state infiltration are studied using numerical simulations where the soil hydrologic properties are assumed to be spatial stochastic processes. Analytical solutions to one‐dimensional, steady state infiltration in heterogeneous soils are developed and applied to the stochastic random fields. The effects of spatial variability of parameters of an exponential unsaturated hydraulic conductivity model on the soil‐water pressure profiles are examined. The amount of variation in pressure heads is found to vary with infiltration rates and mean pressure heads, while the cross‐correlation between parameters is shown to have important influences on the value of the head variance. An inverse procedure is developed to determine the effective hydraulic conductivity parameters. The effective parameter is found to vary with mean pressures. Effective hydraulic conductivities and pressure head variances estimated from the numerical simulations were compared with those obtained from a spectral method by Yeh et al. (1985a, b, c). A unit mean gradient approach was used to estimate the effective unsaturated hydraulic conductivity, and the result shows that this approach is adequate for heterogeneou

ISSN:0043-1397    

DOI:10.1029/WR025i010p02149

年代:1989

数据来源: WILEY

摘要:

A theoretical investigation of factors affecting the gas phase transport of evaporating organic liquids in the unsaturated zone is presented. Estimates of density‐driven advective gas flow using a simple analytic expression indicate that significant advective gas flow will result from the evaporation of volatile liquids in soils having a high permeability. Numerical simulations using a two‐dimensional cylindrical geometry and including the effects of phase partitioning between the solid, gas, water, and organic liquid phases show that mass transfer due to density‐driven flow may dominate the gas phase transport of some organic chemical vapors in the unsaturated

ISSN:0043-1397    

DOI:10.1029/WR025i010p02159

年代:1989

数据来源: WILEY

摘要:

A dynamic model is derived for the transport of a conservative solute along a stream connected to an aquifer contaminated by the solute. It relates downstream concentration to upstream concentration, stream discharge, and piezometric levels in the aquifer. Submodels for solute travel time in the stream and aquifer inflow into the stream are presented. In paper 2, (Jakeman et al., this issue), the model is applied to a reach of the River Murray in Australia and the model structure and probability distribution of parameter values are determined using linear system identification methods.

ISSN:0043-1397    

DOI:10.1029/WR025i010p02171

年代:1989

数据来源: WILEY