ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00840.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe extensive lacustrine Chalco Plain in the southeastern part of Mexico City is underlain by an aquitard up to 300 m thick composed of a layered sequence of very porous (80‐90%) fine‐grained, organic‐rich Quaternary deposits, with thin horizontal interbeds of volcanic sand (“Capas Duras”). The aquitard overlies a thick sequence of alluvial‐pyroclastic sediments which form a highly productive regional aquifer. The Chalco Plain was a shallow lake until the 1940s when it was drained for agricultural use and human habitation. Historic information indicates that the Chalco Plain was an area of ground‐water discharge prior to the onset of heavy ground‐water extraction from the semiconfined aquifer. Due to aquifer pumping, however, the hydraulic gradient has reversed throughout the full thickness of the aquitard in areas where the aquitard is thin (<100m), and recharging conditions now prevail. Where the aquitard is thick, the hydraulic head data show a progressive decline with time even though the hydraulic gradient still indicates upward flow in at least the upper part of the lacustrine sequence. Between the early 1960s, when major ground‐water extraction began beyond the periphery of the aquitard, and the onset in 1982 of heavy pumping from aquifers beneath the aquitard, the land surface subsided approximately 3 m. An additional subsidence of 2 m occurred between 1984 and 1989, causing a shallow lake to form and gradually expand. If the present rate of ground‐water withdrawal from the Chalco Basin continues, total land subsidence in the middle of the plain will probably continue to a rate of about 0.4 m per year for many years, and could eventually reach a total subsidence of tens of meters in the thickest part of the Chalco Plain. Pore water in much of the aquitard is saline; however, release of salts and other chemical constituents to the underlying aquifer has not yet significantly impaired the aquifer water quality. A better understanding of the behavior of the aquitard under the influence of aquifer pumping is needed to assist in long‐term management of ground water and land us

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00841.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractSulphur hexafluoride (SF6), a volatile, nontoxic, inorganic compound, behaved identically to bromide in a laboratory column tracer experiment with saturated sandy media. A simple system was used to achieve an SF6injection concentration of approximately 7 mg/1, roughly 25% of its solubility. While this system requires more equipment and time than that involved in preparing ionic tracer solutions, certain benefits are realized. The detection limit for SF6was approximately 5 μg/l using a simple headspace extraction technique and a gas Chromatograph with an electron capture detector. Thus, the tracer could be followed for a three order of magnitude decrease in concentration. For similar resolution, bromide had to be injected at a concentration over 1000 mg/1 (typical detection limit approximately 0.5 mg/1). When the required higher concentrations of the ionic tracers may lead to significant geochemical changes or density effects, SF6may be the preferable tracer. Furthermore, unlike ionic tracers, the highly volatile SF6may be readily removed from water. High volatility does, however, require that some care be taken to limit the potential loss of mass during injection, sampling, and analysis. Since others are developing procedures for analyzing SF6in ground water with extremely low detection limits (on the order of 1 ng/1), it appears likely that measurable concentrations could range over six to seven orders of magnitude. This range is considerably wider than typically possible with ionic tracers, and may offer an important advantage in some applications

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00842.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractUncertainty is associated with interpretation of the subsurface, and stochastic simulation techniques are incapable of accounting for all the uncertainty if only a single deterministic semivariogram model is utilized. Jackknifing the data bounds the limits of model semivariograms but typically indicates that a large number of simulations must be conducted to consider the full distribution of possible semivariograms. Latin‐hypercube sampling, particularly when combined with expert opinion, reduces the number of simulations that must be created and evaluated. For small data sets, where there is significant uncertainty, this process provides for a more complete assessment of the potential variability of the subsurface and of flow paths for contaminants, given the available data. Such assessment can be used to guide the data collection program and the decision‐making proc

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00843.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractA diffusive sampler was developed to provide a passive in situ method for long‐term monitoring of volatile organic compounds (VOCs) in the ground water at hazardous waste sites and waste disposal facilities. The sampler was constructed of a sorbent tube that fits inside a specially designed sampling chamber equipped with a diffusional membrane that is not permeable to water but has a high permeability rate for organic vapors. VOCs in the ground water, having relatively high Henry's Law constants, enter the chamber by molecular diffusion and are collected by the sorbent. After a predetermined exposure period, the sorbent is easily retrieved for laboratory analyses by gas chromatography/mass spectrometry (GC/MS), or qualitative concentrations can be determined in the field directly using specific‐indicator detectors in place of the sorbent. The measurement approach was evaluated in the laboratory by exposing the device to a solution of 22.5 μg/1o‐xylene before testing at a field site where the ground water was contaminated by trichlorethylene (TCE). Results of the laboratory and field studies indicate the sampling approach is viable for monitoring relatively low concentrations of VOCs over a one‐day sampling period, as well as providing longer monitoring periods up to

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00844.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractChina's first sea‐water intrusion observation network is in the Laizhou Bay region. The data obtained for four consecutive years are presented. There is usually a wide transition zone in the study area, but in some regions, a relatively distinct and sharp interface is formed. A detailed analysis has shown that there are at least two types of salt‐water/fresh‐water interface. The interface in the study area had moved about 140 m along the main observed section in a period of three years and three months due to the effect of pumping wells. Salt water may invade via the riverbed deposits but usually changes its invading direction due to the united effect of the pumping wells. Sea‐water intrusion was caused by the excessive pumping of ground water from the coastal aquifer. The interface was generally formed on the inland side not far away from the powerful discharge pumping center, and sea‐water intrusion stops there. The monitoring data suggested that if the powerful discharge pumping center moves landward, salt water will start advancing landward until a new equilibrium is reached, and even if a new pumping well, whose discharge is very small, is drilled, the equilibrium would be upset, thus causing the interface to advance inla

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00845.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractLong‐term exposure assessments regarding four commonly used pesticides are presented in this paper. Alachlor, aldicarb, lindane, and parathion have been detected in ground‐water monitoring wells. A multimedia transport model is employed to predict the long‐term fate of these chemicals in the environment. Concentrations in each environmental compartment are calculated and used to determine the dominant exposure pathways and hence dose rate.Aldicarb is shown to move from the upper soil compartment, through the lower soil compartment, and accumulate at a rate which is several orders of magnitude greater than the other pesticides. Lindane moves faster to the biota compartment than the other pesticides. The dominant exposure pathway for aldicarb and alachlor is drinking water ingestion for both adults and children. For lindane and parathion, the dominant exposure pathway is biota ingestion for children and fish ingestion for adults. The methods presented in this paper can be used as screening tools for designing remediation, mitigation, or interdiction measures and can serve as a basis for evaluating risk management strat

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00846.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractTo estimate aquifer parameters from pumping test data obtained from large diameter wells, one must consider the recovery data. This is particularly important for low permeability formations. Observations on the recovery should be continued to the maximum possible extent for an accurate estimation of these parameters. Analysis of data from a few representative case studies for granitic and basaltic rocks are presented here to establish a criterion to decide how long observations should be taken during recovery in the well.

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00847.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThis paper describes a field experiment involving the release of 230.9 liters of tetrachloroethylene (PCE) below the water table in a naturally occurring, unconfined sand aquifer. The release was executed in a 3 m X 3 m X 3.4 m deep, scalable‐joint steel sheet‐pile cell anchored into an underlying clay aquitard. After allowing 28 days for redistribution, excavation of the upper approximately 0.9 m of the cell revealed PCE pools and residual to be present in relatively coarser grained horizons, with substantial degrees of lateral flow having taken place. This lateral flow was observed in laminations and lenses ranging in thickness from a few mm to a few cm, with only subtle variations in texture separating individual migration pathways. Detailed sampling during the excavation procedure and subsampling of three cores extended down to the clay aquitard revealed a spatially variable distribution of PCE with saturations ranging from 1% to 38% of pore space. Laboratory measurement of a fully hysteretic capillary pressure curve demonstrated that the degree of nonwetting phase residual is a function of the maximum saturation attained along main drainage during the initial infiltration process. Various models for consolidated petroleum reservoir materials did not fit the experimental data well. The theory governing pool formation in heterogeneous porous media is also presented, and it is demonstrated that pools can form in homogeneous media exhibiting a distinct entry press

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00848.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThis paper discusses the results of a geological, hydrogeological, and hydrological analysis of the catchment boundaries and area of the Sv. Ivan karst spring. The underground watershed has been determined by geological and hydrogeological methods. The control used was the hydrologie water budget analysis appropriate for karst basins with limited data (Turc, 1954). The Sv. Ivan spring includes one main spring and several intermittent springs. The water in the main spring penetrates the flysch layers which limit the spring's discharge; therefore, the discharge of the main spring is fairly uniform. The ratio between minimum and maximum yearly discharges ranges from 1:3.3 to 1:12.8. Only a part of the water flows through the main spring while the other springs in the zone are overflows. The catchment area of Sv. Ivan spring zone is defined as 65 km2.

ISSN:0017-467X    

DOI:10.1111/j.1745-6584.1993.tb00849.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY