ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00522.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractDecomposition of organic‐rich bottom sediment in a tidal creek in Maryland results in production of gas bubbles in the bottom sediment during summer and fall. In areas where volatile organic contaminants discharge from ground water, through the bottom sediment, and into the creek, part of the volatile contamination diffuses into the gas bubbles and is released to the atmosphere by ebullition. Collection and analysis of gas bubbles for their volatile organic contaminant content indicate that relative concentrations of the volatile organic contaminants in the gas bubbles are substantially higher in areas where the same contaminants occur in the ground water that discharges to the streams. Analyses of the bubbles located an area of previously unknown ground‐water contamination.The method developed for this study consisted of disturbing the bottom sediment to release gas bubbles, and then capturing the bubbles in a polyethylene bag at the water‐column surface. The captured gas was transferred either into sealable polyethylene bags for immediate analysis with a photoionization detector or by syringe to glass tubes containing wires coated with an activated‐carbon adsorbent. Relative concentrations were determined by mass spectral analysis for chloroform and trichloroe

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00523.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractFluorescent dyes are being used as adsorbing ground‐water tracers for conducting solute transport studies for adsorbing organic chemicals (e.g., pesticides). In this research, the ability of two fluorescent dyes (rhodamine WT and fluorescein) to mimic the adsorptive behavior of two herbicides (atrazine and alachlor) with alluvial aquifer sands was evaluated. Laboratory studies (batch and column) indicated the following order of increasing adsorption: fluorescein, atrazine, alachlor, and rhodamine WT. Thus, the use of fluorescein and rhodamine WT as adsorbing ground‐water tracers was observed to delimit the appearance of the atrazine and alachlor (fluorescein appeared before and rhodamine WT after the herbicides). Several adsorption characteristics of the fluorescent dyes differed from those commonly observed for pesticides. The levels of adsorption for the dyes were several orders of magnitude greater than predicted from empirical relationships based on Kowand foc. The presence of divalent cations was observed to increase the level of rhodamine WT adsorption. The rhodamine WT breakthrough curves were not of the conventional sigmoidal shape but instead leveled off at a C/CO value of 0.5 for a number of pore volumes prior to increasing again towards a C/CO value of 1.0. These characteristics indicate that the adsorptive mechanisms for the fluorescent dyes differed from the adsorptive mechanisms for most pesticides. These variations in fundamental adsorptive mechanisms raise concerns as to the transferability of the results of this research (fluorescein and rhodamine WT delimiting the appearance of atrazine and alachlor) to other subsurface me

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00524.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractGround water from wells in the Cambrian‐Ordovician sandstone aquifer of eastern Wisconsin often contains elevated concentrations of dissolved solids, chloride, and sulfate and locally approaches or exceeds the current drinking‐water standard for combined radium activity. Samples were collected from wells located along an approximate flow line in order to gain an improved understanding of the chemical evolution of ground water in this system. The concentrations of chloride, sulfate, and sodium increase where the aquifer is confined by the Maquoketa Shale, and the highest combined radium activity is also found in this area. Charge balance calculations and geochemical modeling indicate that dissolution of trace evaporite minerals and cation exchange are important mechanisms controlling major ion distribution. Isotopic equilibrium calculations and analysis of well cuttings from one well with alpha‐sensitive film indicate that the dissolved radionuclide activities are consistent with a uranium source occurring in shaly intervals of the sandstone aquifer at maximum local concentrations of

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00525.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractBoth response matrix and embedding techniques are used to compute optimal sustained‐yield ground‐water extraction strategies for confined aquifers. Historically, the response matrix method has been considered the most practical, because of numerical instability reported to occur when using the embedding approach for large aquifer systems. More recent studies have proven the embedding approach to be stable for large‐scale regional planning. Also, there is increasing emphasis on using microcomputers for ground‐water studies. Thus, it is appropriate to compare response matrix and embedding approaches to steady‐state problems in terms of computational efficiency and memory requirements.For the hypothetical study area, a steady‐state embedding model requires less processing time than a comparable response matrix model. In addition, the embedding model sometimes requires less memory than a response matrix model. Required computer memory is frequently a function of the number of nonzero values in constraint equations. For embedding models, the number of nonzeroes is fixed for a particular study area. For response matrix models, the number of nonzeroes can increase dramatically in proportion to the number of pumping cells and cells at which heads must be constrained. For the sample system, if more than 25 percent of the cells can pump, the response matrix approach requires more memory than does the embedding approach. Before selecting an optimization model for a particular study, one should, as illustrated, evaluate potential memory requirements of both embedding and response matrix approaches. If there is a high percentage of pumping cells, or if many heads must be constrained or computed within the optimization model, the embedding approach seems

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00526.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA nonsteady‐state model is presented for the in‐situ biostimulation of a microbial population in saturated porous media. The model includes basic processes of microbial growth, utilization of electron donor and acceptor, advective transport, dispersion, and sorption in porous media. Model simulations are compared with results from a series of controlled field experiments at the Moffett Naval Air Station, where the growth of an indigenous population of methane‐utilizing bacteria (methanotrophs) was stimulated by the controlled addition of dissolved methane and oxygen (DO) into a semiconfined aquifer. Simulations provide good matches to the observed transient uptake of methane and DO, demonstrating that the observed response resulted from the growth of methanotrophs in the test zone. Simulations duplicate results from alternate pulsed addition of methane (electron donor) and oxygen (electron acceptor), used as a means for distributing microbial growth throughout the test zone. The model permits estimation of changes in microbial population distribution at various stages of the two‐year experiment. Temporal changes in model‐fitted kinetic parameters indicate that the microbial population evolved to one that more effectively utilized the methane at lower concentrations. These analyses demonstrate that a relatively simple model, which includes basic microbial and transport processes, can be of use in the design and evaluation of in‐situ biotreatment processes. The model user, however, must provide judgment in the selection of appropriate input parameters, as well as being aware of model

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00527.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractA ground‐penetrating‐radar system was used to study selected stratified‐drift deposits in Connecticut. Ground‐penetrating radar is a surface‐geophysical method that depends on the emission, transmission, reflection, and reception of an electromagnetic pulse and can produce continuous high‐resolution profiles of the subsurface rapidly and efficiently. Traverse locations on land included a well field in the town of Mansfield, a sand and gravel pit and a farm overlying a potential aquifer in the town of Coventry, and Haddam Meadows State Park in the town of Haddam. Traverse locations on water included the Willimantic River in Coventry and Mansfield Hollow Lake in Mansfield. The penetration depth of the radar signal ranged from about 20 feet in fine‐grained glaciolacustrine sediments to about 70 feet in coarse sand and gravel. Some land records in coarse‐grained sediments show a distinct, continuous reflection from the water table about 5 to 11 feet below land surface. Parallel reflectors on the records are interpreted as fine‐grained sediments. Hummocky or chaotic reflectors are interpreted as cross‐bedded or coarse‐grained sediments. Other features observed on some of the radar records include the till and bedrock surface. Records collected on water had distinct water‐bottom multiples (more than one reflection) and diffraction patterns from boulders. The interpretation of the radar records, which required little or no processing, was verified by using lithologie logs from test holes located along some of the land traverses and n

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00528.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractTraveltime‐related capture areas of wells can be estimated in Hydrogeologie settings with predominantly two‐dimensional flow regimes using image‐well theory, the principle of superposition, an appropriate well‐hydraulics; equation, and particle‐tracking analysis. These analytical methods can be used to simulate the response of an aquifer to pumping stress by incorporating site‐specific values of hydraulic properties, the effects of geologic and hydrologie boundaries, nonuniform regional hydraulic gradients, and well interferences. This is done by developing a conceptual model of the flow system, incorporating an image‐well solution (if needed), computing drawdowns at the intersections of a rectangular grid superposed over the area of interest, and subtracting computed drawdowns from regional prepumping head values assigned to the grid intersections. The resulting flow model can be calibrated by comparing predicted heads with measured heads. The calibrated head distribution then is used in a particle‐tracking analysis to determine flowpath trajectories and traveltime‐related capture areas of wells.Analytical flow models based on the Theis equation and the Hantush‐Jacob equation were constructed from conceptual flow models based on geologic, hydrologie, and pumpage data from wellfields completed in a confined/unconfined stratified‐drift aquifer and in a leaky‐confined fractured‐carbonate aquifer. Predicted head distributions from the calibrated flow models were used in a particle‐tracking analysis to compute flowpath trajectories and traveltime‐related capture areas. The results compare favorably with known flowpaths from sources of contaminati

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00529.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractIn regions where ground water is generally younger than about 30 years, developing the tritium input history of an area for comparison with the current tritium content of ground water allows quantitative estimates of minimum ground‐water age. The tritium input history for central Wisconsin has been constructed using precipitation tritium measured at Madison, Wisconsin and elsewhere. Weighted tritium inputs to ground water reached a peak of over 2,000 TU in 1964, and have declined since that time to about 20‐30 TU at present.In the Buena Vista basin in central Wisconsin, most ground‐water samples contained elevated levels of tritium, and estimated minimum ground‐water ages in the basin ranged from less than one year to over 33 years. Ground water in mapped recharge areas was generally younger than ground water in discharge areas, and estimated ground‐water ages were consistent with flow system interpretations based on other data. Estimated minimum ground‐water ages increased with depth in areas of downward ground‐water movement. However, water recharging through thick moraine sediments was older than water in other recharge areas, reflecting slower infiltration through the sandy till o

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00530.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY

摘要:

AbstractThe St. Clair Plain in southwestern Ontario is underlain by extensive clayey till deposits which are generally 30 m to 40 m thick. The tills have vertical fractures near the ground surface, and this study investigates the depth of those fractures. Observations in test pits show that most weathering features along fractures reach a depth of only 2.5 m to 4 m, though isolated major fractures extend past the pit bottom depth of 5.6 m at two sites. This study also investigates the depth of active ground‐water flow, which is defined as the ground‐water flow in the clayey deposit which occurs in fractures. Water‐level response tests show bulk hydraulic conductivities of the fractured till ranging from>10−7to 10−8cm/s. Seasonal variations in hydraulic head profiles suggest that fractures influence ground‐water flow to maximum depths ranging from 5 m to more than 10 m at the 10 sites studied. The water table is usually within 2 m of the surface, but piezometers went dry to depths of 2 m to 4.9 m during dry periods in 1987 and 1988. Tritium levels greater than l T.U., indicating the presence of post‐1952 water, are found to depths of 7.5 m at all sites and to depths exceeding 12 m in two cases. Tritium has moved to the base of deep, open fractures by active ground‐water flow and has travelled 1 or 2 m beyond that depth by molecular diffusion. Observations of seasonal water‐level variations and tritium sampling show the most promise for determining the maximum depth of hydraulically active, vertical fractures. The maximum depth of active ground‐water flow in fractures varies widely, ranging from 5 m to more than 10 m at the 12 sites studied on

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1991.tb00531.x

出版商:Blackwell Publishing Ltd    

年代:1991

数据来源: WILEY