摘要:

A generalized soft water acidification model has been developed. The enhanced trickle‐down (ETD) model is driven by precipitation, evaporation, acidity, sulfate, and chloride loading time series daily input data. The hydrologic component simulates snowmelt, interflow, overland flow, groundwater flow, frozen ground processes, seepage, and evapotranspiration. Physicochemical and biological processes that affect the alkalinity or sulfate balance and are included in the formulation are cation exchange, chemical weathering, sulfate sorption, and sulfate reduction. The system of 20 ordinary differential equations is solved by using a variable time step fourth‐order predictor‐corrector numerical scheme. Shown here is calibration of the ETD model for two lakes in the Adirondack Park of New York. ETD is relatively simple and requires limited input data, and yet it accounts for the predominant hydrologic and biochemical processes of the ecos

ISSN:0043-1397    

DOI:10.1029/WR024i012p01983

年代:1988

数据来源: WILEY

摘要:

This paper extends the work of Gilliom and Helsel (1986) on procedures for estimating descriptive statistics of water quality data that contain “less than” observations. Previously, procedures were evaluated when only one detection limit was present. Here we investigate the performance of estimators for data that have multiple detection limits. Probability plotting and maximum likelihood methods perform substantially better than simple substitution procedures now commonly in use. Therefore simple substitution procedures (e.g., substitution of the detection limit) should be avoided. Probability plotting methods are more robust than maximum likelihood methods to misspecification of the parent distribution and their use should be encouraged in the typical situation where the parent distribution is unknown. When utilized correctly, less than values frequently contain nearly as much information for estimating population moments and quantiles as would the same observations had the detection limit been below t

ISSN:0043-1397    

DOI:10.1029/WR024i012p01997

年代:1988

数据来源: WILEY

摘要:

The stochastic nature of soil water pressure head and vertical and horizontal flux density under two‐dimensional unsaturated steady state flow conditions was examined by using a Monte Carlo technique. Random autocorrelated scale factors were generated by a first‐order nearest neighbor model and were used to describe a stationary random field of hydraulic conductivity functions. Scale factor values were assumed to vary only in the horizontal direction, perpendicular to the mean flow. The water flow equation was numerically solved for steady state infiltration and transpiration in a two‐dimensional domain, of which the lower boundary was described by a groundwater level. Differences in results between a saturated and unsaturated soil water system were most likely caused by the direction in which the hydraulic properties are randomly autocorrelated and differences in the applied boundary conditions. In contrast to earlier findings with respect to the saturated system, the variability in fluxes for the unsaturated system decreased with an increase in the autocorrelation length, and the variation in soil water pressure head for one‐ and two‐dimensional systems tended to be equal with increasing autocorrelation lengths. When considering a two‐layer system it was found that the variability in the lateral flow component at the layer boundary was severely increased as compared to the single‐laye

ISSN:0043-1397    

DOI:10.1029/WR024i012p02005

年代:1988

数据来源: WILEY

摘要:

The results and interpretation of five induced‐gradient tracer tests performed at five different average interborehole fluid velocities in a single fracture in monzonitic gneiss are described. The experiments were conducted using radioactive82Br and a fluorescent dye as conservative tracers where the tracers were pulse injected into radial convergent and injection‐withdrawal flow fields. The flow fields were established between straddle packers isolating the fracture in three boreholes over distances of 12.7–29.8 m. The tracer breakthrough curves were determined from samples of the withdrawn groundwater and were interpreted using residence time distribution (RTD) theory and two deterministic simulation models. The RTD curves of the tracer experiments were interpreted by fitting to the field data a simple advection‐dispersion model and an advection‐dispersion model with transient solute storage in immobile fluid zones. Both models consider the different flow field geometries associated with injection‐withdrawal and radial convergent tests. Comparison of the fits obtained by the simulation models suggest that the initial period of solute transport in single fractures is advection dominated and with increasing tracer residence time or decreasing fluid velocity, transport progresses toward more Fickian‐like behavior. During the advective‐dominated period, the transient solute storage model is shown to adequately describe the asymmetries and long tails characteristic of the fracture RTDs. Interpretation of the tracer experiments using both simulation models further suggests that induced‐gradient tracer experiments are likely to underestimate the dispersive characteristics of single fractures under natura

ISSN:0043-1397    

DOI:10.1029/WR024i012p02019

年代:1988

数据来源: WILEY

摘要:

Calculations for the flow and solute transport through a single rough‐surfaced fracture were carried out. The fracture plane was discretized into a square mesh to which variable apertures were assigned. The spatially varying apertures of each single fracture were generated using geostatistical methods, based on a given aperture probability density distribution and a specified spatial correlation length. Constant head boundary conditions were assumed for the flow in thexdirection of a single fracture with no flow boundaries in theydirection. The fluid potential at each node of the discretization mesh was computed and the steady state flow rates between all the nodes were obtained. Our calculations showed that fluid flow occurs predominantly in a few preferred paths. Hence, the large range of apertures in the single fracture gives rise to flow channeling. The solute transport was calculated using a particle tracking method. Both the spatial and time variations of tracer breakthrough results are presented. The spatial variation of tracer transport between a line of injection points and a line of observation points are displayed in contour plots which we labeled “transfer matrix.” Our results indicate that such plots can give information on the spatial correlation length of the heterogeneity in the fracture. The tracer breakthrough curve obtained from a line of point measurements is shown to be controlled by the aperture density distribution and is insensitive to statistical realization and spatial correlation length. These results suggest the importance of making line measurements in the laboratory and the field. Sensitivity of our results on parameter variations was also investi

ISSN:0043-1397    

DOI:10.1029/WR024i012p02033

年代:1988

数据来源: WILEY

摘要:

Field evidences indicate that the bulk of water flow in fractured crystalline rock often occurs in preferred flow paths, or channels. A theoretical approach was proposed by Tsang and Tsang (1987) to interpret flow and transport through a two‐ or three‐dimensional fractured medium in terms of a system of statistically equivalent one‐dimensional channels. The apertures along the flow channels are characterized by an aperture density distribution and a spatial correlation length. In this paper, we present detailed studies on the properties of these channels: channel volume, channel residence time, and channel volumetric flow rate. We also calculated the dispersion in tracer transport through groups of statistically equivalent channels. The one‐dimensional channel model is then applied to breakthrough data from transport in a two‐dimensional single fracture (Moreno et al., this issue) in both a forward and an inverse calculation. We show from the inverse calculation that the aperture density distribution parameters of the one‐dimensional flow channels may be estimated from the dispersion and mean residence time of the tracer data. The tracer breakthrough curve should be obtained from line measurements with tracer sampled over several spatial correlation lengths of the variable apertures. This is in contrast to conventional point tracer measurements which is expected to fluctuate with statistical realization and may not yield pertinent information on the flow system. Based on the insight gained in such calculations, design, and analysis of field measurements are discussed. Both tracer breakthrough measurements and flow rate measurements are needed to obtain the aperture parameters of the flow systems. The permeability measurements alone are controlled by the small constrictions along the flow paths and therefore do not yield a good measure of the mean aperture

ISSN:0043-1397    

DOI:10.1029/WR024i012p02049

年代:1988

数据来源: WILEY

摘要:

We present a second‐order kinetic approach for the description of solute retention during transport in soils. The basis for this approach is that it accounts for the sites on the soil matrix which are accessible for retention of the reactive solutes in solution. This approach was incorporated with the fully kinetic two‐site model where the difference between the characteristics of the two types of sites is based on the rate of kinetic retention reactions. We also assume that the retention mechanisms are site‐specific, e.g., the sorbed phase on type 1 sites may be characteristically different in their energy of reaction and/or the solute species from that on type 2 sites. The second‐order two‐site (SOTS) model was capable of describing the kinetic retention behavior of Cr(VI) batch data for Olivier, Windsor, and Cecil soils. Using independently measured parameters, the SOTS model was successful in predicting experimental Cr breakthrough curves (BTC's). The proposed second‐order approach was also extended to the diffusion controlled mobile‐immobile or two‐region (SOMIM) model. The use of estimated parameters (e.g., the mobile water fraction and mass transfer coefficients) for the SOMIM model did not provide improved predictions of Cr BTC's in comparison to the SOTS model. The failure of the mobile‐immobile model was attributed to the lack of nonequilibrium conditions for the two regio

ISSN:0043-1397    

DOI:10.1029/WR024i012p02061

年代:1988

数据来源: WILEY

摘要:

Models of depth‐averaged hydraulic head are used with data from a water balance study in a New England salt marsh to describe horizontal pore water water fluxes near a creek bank. Three hydrologically distinct regions are identified in the sediment. In the marsh studied, semidiurnal tides may drive an oscillating horizontal flux in the narrow region within about 2.5 m of the creek bank. Farther than 15 m from the creek there is essentially no horizontal water movement. In the region between 2.5 and 15 m, drainage to the creek is driven by alternating periods of surface flooding and nonflooding due to the spring neap variation in tidal amplitudes. There is little or no input of fresh groundwater to the marsh sediment at this site. The spatial extent of the drained region depends on the duration of the nonflooded period, the morphology of the sediment surface, and the ratio of hydraulic conductivity to specific storage of the sedimen

ISSN:0043-1397    

DOI:10.1029/WR024i012p02077

年代:1988

数据来源: WILEY

摘要:

As concern over the effects of trace amounts of pollutants has increased, so has the need for statistical methods that deal appropriately with data that include values reported as “less than” the detection limit. It has become increasingly common for water quality data to include censored values that reflect more than one detection limit for a single analyte. For such multiply censored data sets, standard statistical methods (for example, to compare analyte concentration in two areas) are not valid. In such cases, methods from the biostatistical field of survival analysis are applicable. Several common two‐sample censored data rank tests are explained, and their behaviors are studied via a Monte Carlo simulation in which sample sizes and censoring mechanisms are varied under an assumed lognormal distribution. These tests are applied to shallow groundwater chemistry data from two sites in the San Joaquin Valley, California. The best overall test, in terms of maintained α level, is the normal scores test based on a permutation variance. In cases where the α level is maintained, however, the Peto‐Prentice statistic based on an asymptotic variance performs as well

ISSN:0043-1397    

DOI:10.1029/WR024i012p02087

年代:1988

数据来源: WILEY

摘要:

The variance in particle position, a measure of dispersion, is reviewed in the context of certain models of flow in random porous media. Asymptotic results for a highly stratified medium and an isotropic medium are particularly highlighted. Results of the natural gradient tracer test at the Borden site are reviewed in light of these models. This review suggests that the moments obtained for the conservative tracers at the Borden site could as well be explained by a model that more explicitly represents the three‐dimensional nature of the flow fiel

ISSN:0043-1397    

DOI:10.1029/WR024i012p02099

年代:1988

数据来源: WILEY