Showing papers in "Ground Water in 1987"

Characterization of Fracture Permeability with High‐Resolution Vertical Flow Measurements During Borehole Pumping

Abstract: The distribution of fracture permeability in granitic rocks was investigated by measuring the distribution of vertical flow in boreholes during periods of steady pumping. Pumping tests were conducted at two sites chosen to provide examples of moderately fractured rocks near Mirror Lake, New Hampshire and intensely fractured rocks near Oracle, Arizona. A sensitive heat-pulse flowmeter was used for accurate measurements of vertical flow as low as 0.2 liter per minute. Although boreholes were spaced at intervals ranging from 10 to 50 meters, acoustic televiewer logs showed little direct continuity of individual fractures from borehole to borehole in either the moderately fractured rocks or intensely fractured rocks. Results indicated that nearly all inflow and outflow to boreholes occurred by means of one or two discrete fractures in both cases. These fractures did not appear very different from other prominent fractures indicated on televiewer and resistivity logs for these boreholes. Hydraulic connections between boreholes apparently were composed of conduits formed by the most permeable portions of intersecting fractures. Most flow in the moderately fractured rocks occurred at isolated fractures at a depth of about 45 meters indicating a nearly horizontal zone of fracture permeability composed of orthogonal, steeply dipping fractures. Previous studies have identified a zone of horizontal permeability in the lower part of the boreholes in the intensely fractured rocks, but flowmeter tests indicated that flow also entered and exited individual boreholes by means of one or two steeply dipping fractures. These results indicate zones of fracture permeability in crystalline rocks are composed of irregular conduits that cannot be approximated by planar fractures of uniform aperture, and that the orientation of permeability zones may be unrelated to the orientation of individual fractures within those zones.

Seismic Reflection Study of a Shallow Aquifer During a Pumping Test

Abstract: Seismic reflection surveys were used to follow the drawdown in a shallow aquifer during a pumping test. Using severe analog low-cut filters and 1/4-m geophone spacings, 335 Hz reflections were obtained from the top of the saturated zone 2.7 m deep. The reflections moved down in time as the saturated zone dropped in response to pumping. The dominant frequency and bandwidth both dropped during pumping indicating a more diffuse reflecting boundary. Slight pullups of reflectors at specific locations on the CDP sections may indicate a higher elevation of the capillary fringe and therefore finer sediments in those locations. Other potential applications of this technique include mapping cones of depression and detecting and delineating perched-water tables.

Reverse Pathline Calculation of Time-Related Capture Zones in Nonuniform Flow

Abstract: A simple to understand and easy to implement numerical technique, based on reverse calculation of ground -water flow paths, is applied to delineation of time-related capture zones around water-supply wells. The areal extent of a t-year capture zone in a two-dimensional regional flow domain is approximated by a large set of reverse pathlines all with t-year travel times. The numerical approach is suitable for calculation of time-related capture zones in steady, nonuniform flow with inhomogeneous, anisotropic aquifer conditions. The example applications show the sensitivity of this technique to the influence of multiple wells in a block inhomogeneous flow regime.

On Methods of Determining Specific Yield

Abstract: A comparative discussion of several methods for the determination of specific yield is presented. The discussion relies on a pumping test conducted recently by Nwankwor and coworkers from the University of Waterloo in an unconfined aquifer at Borden, Ontario, Canada. We show that a water-balance method promulgated by these workers and used earlier by others, including Wenzel in 1942 and 1946, is invalid because it overlooks a major component of the water budget. This method suggests erroneously that specific yield increases with time during a pumping test. The method can therefore lead to arbitrarily large specific yield values which may lack physical significance. By relying on this water-balance method and on laboratory drainage experiments, Nwankwor and coworkers conclude incorrectly that type-curve methods, such as those proposed by Prickett in 1965 and Neuman in 1975, give unreasonably low specific yield values. We show instead that these specific yield values are consistent with water-balance considerations when all the components of the water budget are properly taken into account. We further point out that whereas the larger specific yields usually obtained from laboratory drainage experiments may be well suited for the evaluation of ground-water reserves that can potentially be recovered from an unconfined aquifer over long periods of time, they are not directly relevant to the problem of relating ground-water level variations to pumpage which is characterized by a shorter time scale. This is especially true when specific yield is taken to be the difference between water content at saturation and residual water content at high suctions as done by Nwankwor and coworkers. The rate at which ground-water levels drop or fluctuate in response to pumpage is controlled by the smaller specific yield that one obtains from time-drawdown analyses.

Theis' Solution by Nonlinear Least-Squares and Finite-Difference Newton's Method

Abstract: A method using the nonlinear least-squares and finite-difference Newton's method to determine the aquifer parameters via a pumping test in a homogeneous and isotropic confined aquifer system is proposed. The nonlinear least-squares is used to find the values of transmissivity and storage coefficient such that the sum of the squares of differences between the predicted drawdowns and observed drawdowns is minimized. The finite-difference Newton's method is used to solve the system of nonlinear least-squares equations for transmissivity and storage coefficient. Comparisons of the results between the proposed method and graphical methods including the Theis, Cooper-Jacob, and Chow methods are discussed in detail, showing data of a 6-hour pumping test. The proposed method has the advantages of high accuracy and quick convergence for most initial guesses.

DBCP, EDB, and TCP Contamination of Ground Water in Hawaii

Abstract: In recent years, several pesticide-related contaminants have been detected in the ground waters of the State of Hawaii. Two soil fumigants previously used by pineapple growers, 1,2-dibromo-3-chloropropanc (DBCP) and 1,2-dibromoethane or ethylene dibromide (EDB), have been detected in several wells on Oahu and Maui. An impurity of the soil fumigant DD, 1,2,3-trichloropropane (TCP), also has been detected in a number of wells. DBCP, EDB, and TCP are of particular concern to State public health officials due to known and possible unknown health effects associated with these compounds. The locations of the contaminated well sites on Oahu appear to be correlated with the areas of past and present pineapple cultivation when the ambient ground-water flow pattern is taken into consideration. In addition, several large fuel pipeline leaks are located in the vicinity of the EDB contamination. In general, TCP contamination is more widespread than either DBCP or EDB contamination. TCP levels appear to be declining in Oahu water wells while EDB levels in the contaminated portion of southern Oahu appear to be increasing. The direction of temporal trends in DBCP concentration at present varies according to site.

Diffusivity of Glacial-Outwash Aquifer by the Floodwave-Response Technique

Abstract: Aquifer diffusivity (transmissivity divided by storage coefficient) was calculated for three sites in a glacial-autwash valley aquifer near Cortland, New York from water-level fluctuations induced by rises in stream stage. The observed response data were analyzed through use of a one-dimensional floodwave-response model to calculate the theoretical head response in the aquifer generated by a floodwave in the stream, and then matched to the observed head response. Diffusivity values computed from sharply peaked flood rises ranged from 6.08 to 8.68 ft2. The closest match between observed and calculated heads was obtained from a site where the aquifer is confined and the saturated thickness (and thus the diffusivity) remains constant with the passage of a floodwave. Arrival time of the observed flood-peak seems to be the most useful criterion for curve matching, especially under unconfined conditions, where a match to the rising limb and floodpeak is difficult because of changes in the saturated thickness. A transmissivity value of 14,700 ft2/d for the glacial-outwash aquifer, calculated from specific capacity data from a nearby industrial well, combined with diffusivity calculated by the floodwave-response method, indicates a storage coefficient between the normal ranges for unconfined and confined aquifers. Calculated storage coefficients were 0.015, 0.027, and 0.034 at the three sites. The floodwave-response method of calculating aquifer diffusivity is relatively inexpensive compared to standard aquifer tests and is effective in glacial-valley fill aquifers that are hydraulically connected to a major stream or river.

Statistical Characteristics of Ground‐Water Quality Variables

Abstract: The emerging problem of contamination of ground-water resources has created a need for information which can be supplied by properly designed ground-water quality monitoring programs. The effective design of monitoring programs and the subsequent utilization of data obtained depends upon an understanding of the general statistical characteristics of ground-water quality variables. In order to provide some background information on these characteristics, a synthesis of current literature and a statistical analysis of existing ground-water quality data were conducted. Specifically, the purpose of the study was to determine whether ground-water quality variables: (1) are normally distributed, (2) exhibit seasonal patterns, and (3) are correlated in time. The results of the investigation suggest that many ground-water quality variables: are not normally distributed, but have skewed right distributions; can exhibit seasonal fluctuations of various shapes and magnitudes, especially in shallow or highly permeable aquifers; and can exhibit significant serial correlation when samples are collected quarterly.

Statistical Methods for Characterizing Ground‐Water Quality

Abstract: The benefits from ground-water quality monitoring ultimately depend on the statistical methods used to analyze data. The methods must match both the information expectations of users and the characteristics of the water variables to which they are applied. The primary objective of regulatory ground-water monitoring is detecting changes in quality. To select appropriate statistical tests for change, one must know whether the water quality variables of concern are seasonal, normally distributed, and serially dependent. This paper provides guidance in analyzing limited background data sets to determine these three characteristics. Recommended procedures to detect seasonality were periodograms, Student's t-test, Mann-Whitney test, analysis of variance, and Kruskal-Wallis test. To test for normality, the skewness coefficient is recommended. To detect serial dependence, sample auto correlation coefficients may be tested for significance.

Monitoring Well Installation, Purging, and Sampling Techniques - Part 1: Conceptualizations

Abstract: Commonly employed techniques for the installation, purging, and sampling of monitoring wells are examined. The degree to which hollow-stem augering disturbs the near-borehole environment, and how this may result in the movement of contaminated solids or fluids from one stratum to another is discussed. The desirability of driving temporary casing to shield one stratum from another, and to improve the general quality of the monitoring well installation (especially the effectiveness of the filter pack emplaced), is argued. A call for further testing of sampling devices, based on limited field comparisons and a lack of reports of conclusive tests elsewhere, is given.

Hydrogeology of an alkaline fly ash landfill in Eastern Iowa

Abstract: The hydrogeology and chemical quality of the shallow ground-water regime at a coal fly ash landfill was investigated near Montpelier, Iowa. An embankment dam retains the two-hectare deposit of silt-sized alkaline coal fly ash in an upland ravine. The fly ash landfill was operated between 1964-1973 and was subsequently capped with a thin loess layer and seeded to pasture. The ash is underlain by loess, over clay-rich till, over sandstone. Ground water now saturates the lower one-half of the ash. Nineteen shallow monitoring wells were installed around and within the landfill. Sampling in 1983-1984 was conducted for water levels, temperature, pH, alkalinity, specific conductance, calcium (Ca/sup 2+/), magnesium (Mg/sup 2+/), sodium (Na/sup +/), potassium (K/sup +/), sulfate (SO/sub 4//sup 2-/), chloride (Cl/sup -/), arsenic (As), and selenium (See). Calculated ion balances for 30 analyses had an error of less than 4%. Native loess-derived ground water of a calcium bicarbonate type enters the landfill, and then shifts to a calcium sulfate type. A distinct leachate plume presently extends at least 46 meters downgradient from the landfill, passing under and through the dam, then discharging into a small pond. SO/sub 4//sup 2-/ and Se concentrations in the plume exceed EPA drinking watermore » standards, and trace As was detected. Although ground-water pH increased after entering the landfill, bicarbonate alkalinity declined. Results of equilibrium solubility calculations suggest that this condition evolved from calcite supersaturation within the landfill, precipitating calcium carbonate. Dissolution of calcium and magnesium oxides on the glassy fly ash spheres sustains the highly alkaline leachate strength.« less

Monitoring Moisture Migration in the Vadose Zone with Resistivity

Abstract: Studies of moisture migration in the vadose zone were conducted at four field sites using a reverse Schlumberger resistivity array. Gravimetric moisture measurements on soil samples taken at each of the field sites were made to a maximum depth of 1.83 meters, and these results were correlated with resistivity values. Tensiometer measurements were also taken at two of the four sites. The soils at the sites have combined clay and silt content (less than .0625 mm grain size) which varies from 13% to 84%. Three of the sites are underlain by sand or gravel in the unsaturated zone. The fourth site has fractured but relatively impermeable shales and siltstones under the surface soils. Depths to water table varied from 1.58 m to 13.7 m. Resistivity and gravimetric moisture measurements were carried out prior to the addition of water to the surface, and following the application of water, either by watering of the site (at two sites) or during and after rainfall events (at two sites). Monitoring was carried out for periods of one to three weeks. Results indicate that (1) moisture is retained for long periods of time in clay/silt-rich soils; (2) moisture migration is slow below a moist soil zone and is not readily detected by surface resistivity measurements; and (3) near-surface moisture changes can be defined by surface resistivity.

Gaseous Behavior of TCE Overlying a Contaminated Aquifer

Abstract: Shallow soil gas (<2 meters deep) was collected and analyzed for trichloroethylene (TCE) to determine the relationship with ground-water contamination directly below. The gaseous TCE plume was mapped with 46 probes and spanned three orders of magnitude in concentration (<0.001 to 2 μg/1). TCE concentrations in water from five monitoring wells around the study site ranged from 4 to 2800 ppb and had a correlation coefficient (r) of 0.90 with TCE concentrations in shallow soil gas. Vertical borings were completed to the water table at four locations in order to obtain profiles of soil gas contamination, air porosity, and water saturation. Gaseous diffusion and air/water partitioning are probably the dominant mechanisms involved in transporting TCE from the ground water to the shallow soil gas.

Applications of Universal Kriging to an Aquifer Study in New Jersey

Abstract: This article describes the use of kriging for optimizing data collection and utility in a regional ground-water investigation of the Potomac-Raritan-Magothy aquifer system in central New Jersey. Kriging was used to (1) estimate the altitude of an aquifer surface, (2) estimate hydraulic conductivities from point data, and (3) estimate the associated kriged errors. The selection of locations for additional data collection, based on the kriged errors, was effective in terms of improving the aquifer surface data base. In another application, hydraulic conductivity values were kriged, first using equal weights, then unequal weights to account for the reliability of the data. The weighting values that were used for unequal weighting were estimated by an analysis of variance. Although inclusion of the data reliability in the kriging will increase the kriged errors, the accuracy of the range of uncertainty in the interpolated values also increases.

A Comparison of Field and Laboratory Methods for Determining Contaminant Flow Parameters

Abstract: In situ estimates of advective and dispersive properties of ground-water flow systems are essential for the adequate prediction of solute flow. Field tracer tests provide the best estimates of these properties; however, this approach is often precluded by the expense and time involved. Various laboratory techniques have been developed to circumvent these problems and provide convenient estimates of hydraulic conductivity and dispersivity. Although these techniques are used extensively in hydrogeological investigations, very few attempts have been made to compare their estimates with field values. A detailed investigation was performed on an unconfined sand aquifer using a combination of routine laboratory techniques and an innovative field tracer test. The extensive data set provided a unique opportunity to compare field results with those obtained by a variety of laboratory techniques. The hydraulic conductivity results from two different methods of grain-size analysis (Hazen, 1893; and Masch and Denny, 1966) were observed to be quite well correlated, although their absolute values did not correspond with the hydraulic conductivities determined from the tracer test. However, contrary to previous studies, the results from grain-size analysis were the same order of magnitude as the in situ values. In addition, the two grain-size methods appear to provide upper and lower boundaries within which lie the hydraulic conductivity estimates determined from the tracer test. Thus, grain-size analysis of moderately to well sorted sands may provide reasonable and convenient approximations of hydraulic conductivities of the aquifer. No correlation was observed between field tracer test longitudinal dispersivity values and those determined from laboratory column tracer tests. The latter, however, differed by only an order of magnitude from the in situ point measurements, a difference which is considerably smaller than has been previously reported. This is largely attributed to the accuracy of the sampling methods used in the tracer tests.

Beneficial Aspects of Sea-Water Intrusion

Abstract: The Permo-Triassic sandstone aquifer of northwest England has been a major source of ground-water supply since the early 19th century. Between Liverpool and Manchester, ground-water resource development has led to a considerable lowering of the water level inland and intrusion of sea water from the Mersey Estuary and Manchester Ship Canal. As part of a detailed investigation of resource management, a two-dimensional finite-difference flow model was developed to represent aquifer behavior for the period 1840 to 1980. Subsequently, this model was used to predict the effects of implementing various management strategies over a 40-year period ending 2020. Results indicate that ground-water withdrawals have exceeded natural replenishment by an average of 100% during the past 50 years. In this time the potentiometric heads have responded remarkably slowly to the flow imbalance, a resilience that is largely attributed to the inflow of sea water at rates only marginally less than recharge from meteoric sources. Careful development of the fresh ground-water resources has minimized water quality problems along the estuary and ship canal, and the model clearly shows how controlled inflow of saline water can considerably enhance the recovery of fresh ground-water reserves. Difficulties arise only when excessive exploitation accelerates water quality deterioration. Model calculations indicate that inflow of saline water would continue for over 10 years following complete cessation of fresh ground-water production.

Distribution Coefficient of Trichloroethylene in Soil-Water Systems

Abstract: The distribution coefficient of trichloroethylene (TCE) was obtained from field and theoretical methods. The field method was based on measuring TCE concentrations in the soil samples and in the adjacent ground water. The theoretical method was based on using the organic carbon content of the soil and the octanol/water partition coefficient for TCE. The average distribution coefficient for 19 field samples and four methods of calculation was 0.18 ml/g which is in agreement with literature data and octanol/water partition coefficients results. For soils containing greater than 0.1 percent organic carbon, the theoretical methods of calculating the distribution coefficient appear to be valid. For soils low in organic carbon content, laboratory determinations of the distribution coefficient can provide reasonable estimates for predicting actual migration rates. Field determinations of distribution coefficients are, however, preferred because they integrate the effect of various factors on partitioning of TCE.

Examples of Transient Sounding for Ground-Water Exploration in Sedimentary Aquifers

Abstract: Examples of the use of transient electromagnetic soundings for three ground-water exploration problems in sedimentary aquifers are given. The examples include: (1) estimating depths to water table and bedrock in an alluvium-filled basin, (2) mapping a confined fresh-water aquifer in bedrock sediments, and (3) locating a freshwater/salt-water interface in a glacial-outwash aquifer. The technique works quite well for these problems within the limitations imposed by the problem of equivalence. For thin conductive targets (S-equivalence) such as a salt-water lens, the ratio of the layer thickness to its resistivity can be determined, but not the individual layer parameters. The thickness of thin resistive zones (H-equivalence) can be resolved, but the resistivity of such layers is not well determined. The problem of H-equivalence is more severe than the S-equivalence. Equivalence imposes restrictions on the transient sounding method for some ground-water exploration problems. Model studies prior to field work can be used to assess the usefulness of the technique for a particular exploration objective.

Representative Sampling of Ground Water from Short-Screened Boreholes

Abstract: Boreholes terminated with a short (1–3 m) length of screen are often used in ground-water quality monitoring, although to obtain representative samples, pumping is often required to overcome the effects of stagnant casing storage. Methods for estimating pumping time prior to sampling from a pumped discharge, presented graphically and in summary equations, show that for most requirements, pumping time (ta) to purge well storage (where mixing of casing storage and ground water takes place within the casing) or to overcome casing storage effects (where little mixing occurs within the casing) is given by Here V is the volume of well storage, Q is the average pumping rate, and m is given by where Ct, Cg, and Cc are concentrations of some substance in pumpage, and ground water, and the initial concentration in casing storage, respectively. In boreholes which have been developed and which are regularly pumped and where a relative sampling error of 2.5% is considered to be acceptable, then m has a value of 0.1. Pumping time then equates to the time necessary to pump 2.3 volumes of well storage from the borehole. Laboratory and field evaluations using boreholes in transmissive and low permeability formations generally validate the theoretical approach. The theory can be used as a basis for estimating and optimizing pumping times for monitoring programs where ground-water samples are recovered from short-screened boreholes. Additionally, the theory allows the determination of an optimum pumping rate.

VOCs in Ground Water Influenced by Large Scale Withdrawals

Abstract: Ten wells were installed to monitor the water-table elevation and volatile organic compounds (VOCs) in ground water underlying Hastings, Nebraska. Trichloroethylene (TCE) was the most frequently detected VOC and had the highest concentration (1750 μg/1) in these monitoring wells. Perchloroethylene (PCE), 1,1-dichloroethylcne (1,1-DCE), 1,2-dichloroethane (1,2-DCA), and 1,1,1-trichloroethane (TCA) also occurred in one or more of the monitoring wells. The vertical and areal VOC distribution in both the contaminated monitoring wells and the city's contaminated municipal wells are largely related to withdrawal caused by heavily pumped, nearby municipal and industrial wells. A computer model was used to demonstrate the impact historic site changes in high capacity well withdrawal have on the local water-table configuration. Two sites of suspected VOC sources are designated for further investigation

Monitoring Well Installation, Purging, and Sampling Techniques — Part 2: Case Histories

Abstract: Three case histories are presented to illustrate the problems encountered using some monitoring well installation, purging, and sampling techniques. Hollow-stem auger-ing was found to pose problems during the installation of monitoring wells in terms of potential for cross-contamination of strata, the inability to emplace an adequate filter pack, the potential for smearing natural clays into sand and gravel strata, and the possible need for excessive purging of sediments that accumulate between sampling events. Cable tool drilling was found to allow construction of high quality wells, but was slow and expensive. Concern over the potential for increased emissions of noxious vapors from the borehole when drilling with the air rotary method prevented its use in situations where highly contaminated strata were likely to be encountered. A hybrid drilling technique, augering with temporary casing, was able to provide the advantages, and minimize the disadvantages, of the foregoing drilling methods. The installation of vertically separated clusters of monitoring wells lent important insights into the potential movement of a contaminant plume. Detailed aquifer tests provided needed information on the hydraulic behavior of the subsurface strata occupied by the plume so that potential pathways of contaminant transport could be more accurately identified. Obtaining reliable data from such tests depended greatly on the ability of each monitoring well to act as a high quality hydraulic observation well. The selection of drilling and installation techniques that ensured minimal disturbance of the near-borehole environment provided the best guarantee of such performance.

Generation of Ground‐Water Age Distributions

Abstract: Discrete-state compartment (DSC) models and their associated age distribution functions permit the quantitative interpretation of environmental radioisotope data such as carbon-14 ground-water decay ages. These mixing-cell models offer a means for constructing ground-water flow models that can be used to relate decay ages to ground-water mean ages. In addition, DSC models can also generate the entire distribution of ages in various subregions of a ground-water reservoir. A preexisting DSC model of a portion of the Tucson Basin alluvial aquifer is used as an example. Ground-water mean ages in this aquifer range from 100 to almost 15,000 years old, with the oldest waters about 40,000 years old. Since the ground-water ages are not normally distributed, means and medians are not equivalent. The results indicate that care must be used in interpreting both ground-water radioisotope decay ages as well as mean ground-water ages and that knowledge of the entire age distribution is preferable. Age distributions are especially useful in hydrogeologic studies in which mixing is important and may find use in paleohydrogeologic investigations.

A Horizontal Plane Source Model for Ground‐Water Transport

Abstract: A model was derived through the use of Green's functions to simulate three-dimensional contaminant transport from a horizontal plane source (HPS). This analytical model incorporates retardation and decay, and can simulate varying source emission rates. Appropriate uses of the model include simulations of contaminant transport from landfills, waste lagoons, land treatment facilities, and areas of pesticide application. Comparison between HPS and point source solutions indicates that for such simulations, the HPS model will provide more accurate results than the point source solution, particularly near the source. Representative model applications indicate the model's sensitivity to variations in retardation, decay, and temporal period of source emission.

Effect of Water‐Level Recoveries on Fault Creep, Houston, Texas

Abstract: Fault creep has stopped or slowed in the eastern part of the Houston, Texas, land subsidence area where reductions in pumping of ground water have allowed water levels to recover partially. Creep has continued across faults in the western part where heavy pumping causes water levels to continue to decline. The observations support the long-suspected relation between historical faulting and withdrawal of ground water and indicate that partial water-level recoveries can reduce fault movement and structural damage.

Water Movement in Till of East-Central South Dakota

Abstract: Results from the first phase of a study on water movement within the till of east-central South Dakota indicate that little to no water passes from the weathered till through the underlying unweathered till into underlying outwash deposits. Based on hydrographic analyses, field hydraulic conductivity measurements, and major-ion sampling at 22 study sites, discharge from the weathered till is hypothesized to be primarily from evapotranspirative losses during periods of high water table. Hydraulic conductivity of the unweathered till is typically 10 to 200 times lower than the weathered till. Water sampled from the unweathered till is up to three times higher in major-ion concentrations than water in underlying outwash deposits. Carbon-age dates of water from the buried outwash deposits suggest an age in excess of 9,000 years before the present, corresponding to the Late Wisconsinan glacial epoch.

Statistical prediction intervals for the evaluation of ground-water quality

Abstract: Factors for a normal distribution are given such that one may be 99% confident that the two-sided prediction interval chi-bar +- rs or the one-sided prediction several chi-bar + rs will contain all of the kappa future values, where chi-bar and s are the sample means and standard deviation obtained from n previous values. In the context of ground-water monitoring, the future samples may represent new monitoring values at each of kappa downgradient wells, and the n previous values might be the historical monitoring results for one or more upgradient wells. The Tables provided in this paper allow the computation of one-sided and two-sided 99% prediction intervals for previous sample sizes of n = 4 to 100 and future samples of kappa = 1 to 100. Modification of these intervals for log-normally distributed data is also presented.

Analysis of steady-state salt-water upconing with application at Truro well field, Cape Cod, Massachusetts

Abstract: Salt-water upconing describes the phenomenon where salt water is transported vertically upward under a well in response to pumpage in a fresh-water aquifer underlain by salt water. Sharp interface methods have been used successfully to describe the physics of upconing. A finite-element model is developed to simulate a sharp interface for determination of the steady-state position of the interface and maximum permissible well discharges. The model developed is compared to previous published electric-analog model results of Bennett and others (1968). Both methods are applied to a test case at Truro, Massachusetts, where maximum permissible discharges are determined by the finite-element model to range from 0.47 to 1.05 cubic feet per second for the Test Site No. 4 location.

Statistical Models for the Analysis of Volatile Organic Compounds in Waste Disposal Sites

Abstract: The occurrence of low-level hits of volatile organic priority pollutant compounds is statistically modeled as a Poisson process. Methods are developed to estimate the mean of the Poisson distribution for a random sample of volatile organic measurements as well as 99% prediction limits and 99% tolerance limits. The prediction limits provide an interval estimate that will include values obtained for the next k future measurements based on a sample of n previous measurements with 99% confidence. The tolerance limits provide an interval estimate for the n previous measurements that will contain 99% of the population of background measurements with 95% confidence. These methods are illustrated with measurements obtained from 61 field blanks, 56 trip blanks, and 162 samples obtained from 29 upgradient wells. Both prediction and tolerance limits yielded extremely similar results in all three examples.

Airborne Geophysical Exploration for Ground Water

Abstract: Geophysical methods have been used for many years to assist in hydrogeological exploration for ground water. For the direct detection of water-bearing formations, the electrical resistivity method has been effective but relatively slow and costly. More recently, surveys have been conducted successfully with electromagnetic prospecting apparatus developed primarily for the mineral industry. Such equipment has the advantages of greater speed and portability, improved subsurface resolution, and better potential discrimination of conductive aquifers. Electromagnetic methods also can be applied from moving platforms, including fixed-wing aircraft and helicopters. Detailed and regional surveys conducted in recent years for geological mapping and mineral exploration in Africa and elsewhere have revealed important information on the location and nature of water-bearing formations and structures. Simultaneous measurements of the magnetic field have been used also to map faults and dikes of potential ground-water significance. New, wide-band, digital airborne equipment and processing systems are capable of carrying out rapid, inexpensive studies of subsurface formations to depths of at least 200 meters. Computerized interpretation techniques can produce maps of depth and electrical conductance of a variety of buried aquifers. This information can be of direct assistance to the hydrogeologist or hydrologist in the identification and development of underground-water supplies.