ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00390.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe USGS two‐dimensional mass‐transport model (MOC) is widely used in the analysis of ground‐water contamination problems. A need exists to examine the accuracy of the code in situations dominated by radially convergent and divergent flow around wells. The model is applied here to situations that commonly exist in remedial actions involving recovery wells. The cases simulated are a recharge/recovery single well, a recharge/recovery doublet, and plume capture by one or two production wells. The results were tested against analytical and semianalytical solutions. Inaccuracies in model results occurred especially for the doublet case under continued long‐time simulation. Inaccuracies are caused not only by the mainly radial‐flow situation, or by the curvature nature of streamlines, but also by the arrival of contamination at the sink nodes. Better agreement of numerical and analytical solutions was obtained for the single‐well and plume‐capture situations. However, a large mass‐balance error exists for the single‐well case. Inaccuracies can be reduced by modifying the code and reducing the finite‐difference mesh (e.g., Erickson, 1985). However, the use of a very fine mesh (i.e., on the order of a few feet) may prevent the use of the code in large‐scale problems. Care must be taken in applying the model to situations where production or injection wells ar

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00391.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTAn optimization model for computer‐assisted estimation of aquifer parameters has been developed. It is based upon the principle of minimization of the sum of squares of the differences between observed and computed drawdowns. The objective strategy eliminates the subjectivity associated with the conventional curve‐matching procedures. Apart from the parameter estimation, the model can assist in identifying certain hydrogeological features like leakage, discontinuities, vertical anisotropy, etc. It also can provide a check on consistency among different data sets monitored during a test pumping (e.g., drawdown and recovery data). The application of the proposed model has been illustrated by analysis of field data. The model‐computed parameters honored the observed data more closely than the graphical estimates. The model could also identify leakage as the cause of departure of the time‐drawdown curve from the Theis type curve. In another study, the model results lead to an inference that the recovery data of the pumped well are inconsistent with the rest of t

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00392.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTA finite‐element model is developed to solve the two‐dimensional, transient, ground‐water flow equation on a regional scale over geologic time for the purpose of assessing the rate of hydraulic head decay at depth in response to erosional modifications of the land surface. The model includes a time‐stepping procedure that allows consideration of sedimentary basins with large hydraulic conductivity contrasts. Results confirm that hydraulic head patterns that are not in equilibrium with existing topography can persist at depth over geologically significant periods of time. A sensitivity analysis on a hypothetical layered system shows that the rate of hydraulic head adjustment, due to erosional modifications of the topography, is particularly sensitive to the thickness, continuity, and vertical hydraulic conductivity of the low‐permeability layers. Application of the two‐dimensional model to the Red earth region of northern Alberta, Canada predicts response times that are reduced by two‐thirds over those predicted with one‐dimensional models. The concept of delayed hydraulic response over geologic time has important implications in the siting and design of nuclear waste repositories, the analysis of contaminant plume migration, and the use of the hydrodynamic approach to the exploration for petroleum resources and stratabou

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00393.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTAs a result of the recent drought in central Africa, an accelerated well drilling program was commissioned in Victoria Province, Zimbabwe, to help alleviate the shortage of water.An analysis of the data collected during the siting of these boreholes showed that geophysical techniques can be used reliably to determine regolith thickness and provide an indication of regolith lithology. Valleys and the zone of deep weathering around bornhardts were found to have the greatest potential for water‐supply boreholes. The relationship between regolith resistivity and specific capacity is masked by the clay mineralogy of the regolith which must be taken into account when siting boreholes. Despite this effect, optimum values of regolith resistivity for production boreholes in Victoria Province were established which may be of value in similar Basement terrain elsewher

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00394.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTHydrochemical investigations of aquifer contamination by brine are usually approached using conventional graphical methods (i.e. Stiff, Piper, and Schoeller diagrams). While these methods may be used successfully to identify brine in distinction from other contaminants (e.g., road deicing agents) in ground water, they are unsuccessful in differentiating between brine contaminants from different stratigraphic origins. We present a methodology to be employed in hydrochemical investigations of suspected brine contamination, which incorporates the use of select ionic ratios [Ca:Mg, Na:Ca, Na;Mg, Na:Cl, K:Cl, K:Na, Mg:K, Ca:K, Cl:Mg, Cl:Ca, and (Ca + Mg):(Na + K)] in discriminant analysis and modified graphical methods. The method is tested by applying it to an area of known brine contamination in northeastern Ohio. The results of the test indicate that the method is successful in identifying brine as a contaminant in ground water and in identifying the stratigraphic origin of the brine. This method, when combined with information obtained from drilling reports, can be used to suggest a point source for the brine contamination.

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00395.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTMost measures of ground‐water quality are positively skewed with a few observations occurring with significantly large values. Using a set of mathematical expressions and readily available software, the effect of deleting one or more large‐valued observation(s) on selected estimates of parameters of underlying distributions are evaluated. The effect of deleting a single large‐valued observation on the mean is linear, and on the standard deviation it is nonlinear (approximately quadratic). In the case of multiple deletions, the effect of sum of the large‐valued observations on the mean is linear and on the standard deviation it is nonlinear. The effect on the standard deviation is a function of the sum of squares of deleted observations. The standard deviation is much more sensitive to the magnitude of deleted observations than the mean. Analysis of a large data set consisting of 17 primary and secondary drinking‐water constituents showed that (with a few exceptions) the effect of deleting large‐valued observations on the standard deviation is considerably higher than that on the mean, the 95th and the 99th percentiles. Seldom are percentiles below the 75th affected by the deletion of large‐valued observations. Analyses of upper quantiles and the maximum can be of much value in the study of maximum contaminant level (MCL) violations and population exposures to toxic chemicals above and beyond certain threshold levels. Since large‐valued observations affect the estimates of different parameters differently, it is extremely important to choose the relevant parameter first and then study the change in its estimate based on the deletion of large‐valued observations.Note: For the sake of brevity, the terms mean, standard deviation, and quantiles (or percentiles) are often used in this paper, instead of the more precise estimated mean, estimated standard deviation, and estimated quantiles

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00396.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTNatural occurrences of ground water with moderate (10 to 50 micrograms per liter) to high (greater than 50 micrograms per liter) concentrations of arsenic are common throughout much of the Western United States. High concentrations of arsenic are generally associated with one of four geochemical environments: (1) basin‐fill deposits of alluvial‐lacustrine origin, particularly in semiarid areas, (2) volcanic deposits, (3) geothermal systems, and (4) uranium and gold‐mining areas. These findings are based on an extensive literature review, compilation of unpublished reports and data, and the review of data bases containing more than 7,000 analyses of ground‐water samples for arsenic. In the first two environments, arsenic appears to be associated with sediments derived, in part, from volcanic rocks of intermediate to acidic composition. Dissolved arsenic concentrations in water from volcanic aquifers in the same regions, however, may be low (less than 10 micrograms per liter). Solid phases (minerals, amorphous solids, and sedimentary organic matter) that supply the dissolved arsenic have not been identified in most areas. Alluvial and lacustrine sedimentary deposits appear to be an important source of arsenic in volcanic areas (such as Lane County, Oregon) and in areas underlain by basin‐fill deposits (such as Carson Desert in Nevada and the Tulare Lake basin in California). Mobilization of arsenic in sedimentary aquifers may be, in part, a result of changes in the geochemical environment due to agricultural irrigation. In the deeper subsurface, elevated arsenic concentrations are associated with compaction caused by groundwater wi

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00397.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.A sampling manifold has been developed that allows the rapid collection of ground water from multilevel samplers during tracer tests. Its reliability was proven in an experiment lasting 51 days.

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00398.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Abstract.For a classroom exercise, nine groups of graduate students calibrated a numerical ground‐water flow model to a set of perfectly observed hydraulic head data for a hypothetical phreatic aquifer. All groups used exactly the same numerical model and identical sets of observed data. After calibration, the students predicted the hydraulic head distribution in the aquifer resulting from a modification in one boundary condition. A quantitative analysis of the results of this calibration‐prediction exercise vividly demonstrates some of the difficulties in parameter identification for ground‐water flow models. Group predictions differed significantly. Successful prediction was strongly correlated with successful estimation of conductivity values, and was essentially unrelated to successful estimation of aquifer bottom elevations or with the number of trial‐and‐error simulations required for calibration. Most importantly, success in prediction was unrelated to success in matching observed heads under premodification conditions. In this sense, good calibration did not lead to good p

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1988.tb00399.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY