ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00859.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractLocalized ground‐water mounding beneath larger on‐site sewage disposal fields (septic drain fields) can reduce and even eliminate the waste‐water treatment that occurs in the unsaturated soil zone. Such mounding was previously predicted for longer times by a procedure of uncertain accuracy and having a number of limitations. In this paper, a new procedure is developed on a stronger theoretical basis. The governing equation may be solved by four different methods. The new procedure considerably extends the time range of applicability, and no longer restricts the proportions of the rectangular disposal field. Errors in the previous methods approached 20% in some cases. Comparisons of different methods to reduce mound heights where they are critical indicate that the most effective method is to subdivide the disposal field into separated sub

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00860.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThis paper presents a subset of the family of diagonally implicit Runge‐Kutta (DIRK) time‐stepping methods for finite‐difference models of parabolic (diffusion‐like) equations. It includes the first‐order‐accurate Euler implicit (backward in time, DIRK1) and the second‐order Crank‐Nicolson (DIRK2) methods as special cases. It combines a series of DIRK1 partial time steps so as to eliminate additional power terms of the time step δt, in the local error, by the number of partial steps used. This offers a large increase in computational efficiency, going from a DIRK1 to a DIRK2 method, and improves on the Crank‐Nicolson method with a better choice of Runge‐Kutta parameters. For a linear diffusion example, an optimal‐parameter DIRK2 method offers the same accuracy as the Euler implicit method at two orders of magnitude larger time step, with an order of magnitude better accuracy than the Crank‐Nicholson method. In a highly nonlinear horizontal unsaturated water flow example, using eight simulated medium to coarse soils, a DIRK2 method produces either an average maximum accuracy improvement of 4.7 times over the Euler implicit method, without Newton or Picard iteration, or from 3.8 to 48 times faster computer run tim

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00861.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe low conductivity of glacial till is usually thought to preclude pumping tests, which has led to the predominate use of laboratory permeameter and single well response (slug) tests for estimating hydraulic conductivity. Slug tests only sample a small region around the well and may be more representative of the disturbance of well installation or development than in situ conductivity. To provide a larger scale comparison to slug test estimates of hydraulic conductivity for unweathered Wisconsin age till in Iowa, a pumping test was performed, with both the pumping and monitoring wells placed in unweathered till. Maintaining a constant drawdown of 439 cm in the pumping well for over five days yielded an average flowrate of only 26.3 ml/min, but produced a maximum drawdown of 104 cm at a radial distance of 178 cm. Assuming radial and vertical flow, least‐squares parameter estimation yielded a hydraulic conductivity estimate of 7.5 · 10‐7cm/s and a specific storage estimate of 6.6 · 10‐6cm‐1. Slug test results compared favorably, with geometric mean hydraulic conductivity estimates of 4.9 · 10‐7cm/s from a Cooper et al. (1967) analysis and 7.1 · 10‐7cm/s from a Hvorslev (1951) analysis. For this relatively low clay till (20% clay fraction), borehole smearing does not appear to have a significant impact on slug tests. The results indicate pumping tests can be performed, on an appropriate scale, in some un

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00862.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractCokriging and ordinary kriging of geoelectric and well data are used to determine the thickness, lateral extent, and spatial variability of a naturally occurring clay layer near Ashland, Nebraska. The clay layer that protects the underlying ground water from contaminant sources is defined by mapping the estimated clay thickness; variance maps provide an assessment of the uncertainty of the thickness estimates. Two types of information are combined in the cokriging procedure: clay thickness data estimated from drillers'logs of 128 irrigation and municipal supply wells, and thicknesses estimated from interpretation of 44 vertical electrical soundings (VES); for the ordinary kriging, well data are used alone. Both types of maps demonstrate the existence of a continuous near‐surface clay layer extending over much of the Ashland wellfield; however, the cokriged map shows much more detail in the central part of the study area, where the well data are undersampled. The relatively dense VES network in the central area provides an important information bridge between the northern and southern areas making the cokriged map more continuous and accurate than the map prepared using the well data alone and ordinary krigin

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00863.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractLaboratory batch and column studies with two subsurface media demonstrated that structural isomers of rhodamine WT (RWT) are responsible for the observed two‐step sigmoidal breakthrough curve. A linear isotherm was observed at RWT concentrations100 μ/1. In column studies, the breakthrough curve for RWT was observed to exhibit a plateau at values of C/ Coaround 0.43 to 0.49. Titration curves for RWT indicated a single pKavalue of 5.1. The pH effects on the breakthrough curve for RWT were evaluated and showed no evidences related to the observed two‐step breakthrough curve. Sorption of RWT was observed to be much more significant at low values of pH (e.g., 3.0), and to decrease at elevated values of pH (e.g., 11.3); both observations are consistent with the ionogenic nature of RWT and the surface charge of the media. Chromatographic separation of RWT resulted in two UV spectrophotoscopically distinct peaks indicating two structural isomers. Combination of the individual breakthrough curves for each of the two RWT isomers resulted in a two‐step breakthrough curve similar to that observed for the original RWT solution. The distinct sorptive characteristic of the different isomers is explained by the relative positions of two carboxyl groups (COO‐) attached to the phenyl ri

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00864.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe yield characteristics in clusters of closely spaced and sparsely spaced wells in a typical granitic environment in South India are analyzed. Contrary to the belief of earlier workers, the analysis shows that the yield of the wells is not directly influenced by depth, topography, number of fractures, or the thickness of weathered zones. In a few cases, reduction of discharges were observed at certain depths, and in general, ground‐water yield was contributed by shallow fractured zones. The data were statistically processed further to quantify the observed relationships among these parameters. The analysis of the results indicated similar trends regardless of whether the wells are closely spaced or otherwise throughout the basin are

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00865.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractMost numerical models of ground‐water flow incorporate lakes as constant head nodes. We developed a computer package for the widely used MODFLOW code to simulate three‐dimensional steady‐state and transient ground‐water flow with fluctuating lake levels. This package considers precipitation to and evaporation from the lake surface, stream flow, and ground‐water inflow and outflow for multiple lakes. We demonstrate the use of the package through the results of steady‐state and transient simulations for a hypothetical ground‐water flow problem involving a lake an

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00866.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractNitrate is one of the most common ground‐water contaminants, and denitrification, the bacterially mediated conversion of nitrate into molecular nitrogen, is the only geochemical process that contributes significantly to the renovation of nitrate‐contaminated aquifers. Previous workers have shown that denitrification occurs in some, but not all, nitratecontaminated unconfined aquifers in southern Ontario, Canada. Denitrification tends to occur in aquifers that have a very shallow water table, but not in aquifers whose water table is deeper than about two to three meters.This paper describes an investigation of the importance of organic carbon in controlling the occurrence of denitrification in two shallow sandy aquifers in southern Ontario, Canada. In situ and laboratory measurements of denitrification rates using the acetylene block technique show that: (1) organic carbon availability decreases with depth below the ground surface; (2) denitrification occurs in an aquifer with a very shallow water table, but organic carbon availability limits the denitrification rate; and (3) denitrification does not occur in an aquifer with a deeper water table because there is insufficient labile organic carbon pres

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00867.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractIn this paper we apply the Bayesian decision analysis to the engineering design of a ground‐water interception well whose purpose is to capture a contaminant plume. Two decision variables are considered: (1) pumping rate of an interception well when the desired width of interception zone is known, and (2) optimal number of slug tests needed to estimate the statistics of mean log‐conductivity. The optimal number of slug tests is calculated for the case when no prior information regarding the mean of log‐conductivity is available. The analysis is performed for steady‐state ground‐water flow in a linear‐type aquifer. Two utility functions are considered. The utility functions account for the risk associated with not capturing the contaminant plume, cost of pumping, and cost of aqui

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00868.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY