Showing papers in "Ground Water in 1983"

Enumeration and Characterization of Bacteria Indigenous to a Shallow Water-Table Aquifer

Abstract: Regions of the earth below the root zone of soil have conventionally been considered void of life. However, widespread pollution of ground water in the U.S. by organic compounds has kindled interest in the numbers of microbes that might be found in aquifers and associated regions of the deeper unsaturated zone, and in their ability to degrade organic pollutants. Newly developed techniques in microbial ecology allow the direct enumeration and examination of soil microbes without recourse to their cultivation on growth media. These techniques reduce many uncertainties associated with the use of culture media, such as the growth of contaminants or the failure of indigenous forms to grow on a particular medium. Samples were recovered aseptically from depths of 1·2, 3·0, and 5·0 meters (m) at the margin of a small floodplain near Lula, Oklahoma. Depth to the water table was 3·6 m; depth to bedrock was 6·0 m. Numbers of microbes were surprisingly similar at all three depths (3 to 9 × 106/g dry material). Although both Gram-positive and Gram-negative bacteria were encountered, small Gram-positive coccoid forms predominated. No protozoa, yeasts, or other fungi were seen. The indigenous bacteria could rapidly degrade toluene. Chlorobenzene was degraded in material from the vadose zone, while bromodichloromethane was degraded in material from the saturated zone. There was no detectable degradation of 1,2-dichloroethane, 1,1,2-trichloroethane, trichloroethylene, or tetrachloroethylene.

Survival of Pathogenic and Indicator Organisms in Ground Water

Abstract: A comparative study was undertaken on the decay rates of three bacterial types (S. typhimurium, E. coli and S. faecalis), an enterovirus (poliovirus type 1) and a bacterial phage (f2) in ground water maintained under laboratory conditions. Except for f2 phage, all the microorganisms tested were relatively stable in ground water. S. faecalis survived best among all the bacteria tested and its decay rate was similar to that of poliovirus type 1. Under field conditions, bacterial indicators were also found to be stable in the ground-water environment. The decay rate for fecal streptococci was lower than for fecal or total coliforms in the shallow wells.

Aqueous Geochemistry and the Exchangeable Cation Composition of Glauconite in the Aquia Aquifer, Maryland

Abstract: The cation composition of water in the Aquia aquifer of southern Maryland changes systematically in the direction of ground-water flow. In the upgradient parts of the aquifer, calcium is the dominant cation in solution. Between 10 and 20 miles downgradient, calcium and magnesium are dominant. Between 20 and 35 miles downgradient, magnesium and potassium are dominant, and, beyond 40 miles downgradient, sodium is the most abundant cation in solution. The exchangeable cation composition of glauconite in the Aquia aquifer changes systematically in the direction of flow in a manner similar to the changes in water chemistry. These observations are interpreted as evidence that cation exchange reactions are processes that simultaneously alter water composition and glauconite composition along the flowpath. Exchange parameters K’and n for glauconite in the Aquia aquifer were determined for the reactions: The exchange parameters for each reaction were tested by an indirect method. Molar ratios of exchangeable cations or glauconite were calculated from water chemistry data using the derived k’and n values. In each case, the trend of calculated molar ratios compared favorably with measured molar ratios. The calculated exchange parameters demonstrate that the exchange process is not linear for the Ca2+ -Na+ and Mg2+ -Na+ ion pairs. The sum of exchangeable cations on glauconite samples is significantly lower in the outcrop area than downgradient. Corresponding to this, the concentration of dissolved silica and dissolved oxygen in Aquia water is high near the outcrop area and decreases downgradient. This pattern suggests that incongruent dissolution of glauconite is an additional process that modifies glauconite composition near the outcrop area but is less important downgradient.

An Examination of Scale-Dependent Dispersion Coefficients

Abstract: Many hydrologists have observed that dispersion coefficients, when measured in the field, turn out to be scale-dependent. That is, the greater the travel distance in a tracer test used to measure dispersivity, the larger the dispersivity value that is calculated. Recently, Guven et al.(1983) presented a study which contains a basis for understanding the phenomenon of scale-dependent dispersion within a deterministic framework. The results of that study are used as a basis for defining a scale-dependent macro- dispersion coefficient for unidirectional flow in a stratified aquifer. Theoretical expressions are then obtained for the macrodispersivity, its various components, and its small time and large time limits. Using the data of Pickens and Grisak (1981a, b), numerical values are calculated for the macrodispersivity, and estimates are made of the travel distance required in order to reach Fickian conditions. Asymptotic large time macrodispersivity values of 49.5 m to 990 m and travel distances of 5.4 km to 109 km result for the particular aquifer studied. Based on these results it appears that Fickian conditions will seldom apply in practice to porous media flow. This study and previous studies show that the primary physical mechanism that causes spreading of a solute near the source is different advection rates at different elevations in the aquifer. Present results and comparisons with field data indicate that this phenomenon is not represented well by a scale- dependent dispersion coefficient. In modeling dispersion phenomena, it appears that more emphasis should be placed on field study and the accurate determination of hydraulic conductivity variations and other nonhomogeneities, and less on incorporating somewhat arbitrary dispersion coefficients into complex mathematical models.

The Practical Application of Surface Electrical Resistivity to Detection of Ground-Water Pollution

Abstract: One of the primary problems in field investigations of ground-water pollution is locating the contaminant plume. Drilling of sampling holes on a hit-or-miss basis is both time-consuming and expensive. Under many subsurface conditions, surface electrical resistivity profiling can quickly and cheaply locate the general position of the plume and identify areas most feasible for sampling and monitoring. Many contaminants contain an ionic concentration considerably higher than the background level of native ground water. When such a contaminant is introduced into an aquifer, the electrical resistivity of the saturated soil is reduced. Surface electrical resistivity profiling across a suspected area can identify this reduced resistivity zone as an anomaly. The sensitivity of the method depends on relative uniformity of geology and topography as well as minimal extraneous electrical interferences. It is also essential that the “A” spacing used in the profiling procedure be carefully selected. If the resistivity contrast between contaminated and uncontaminated ground water is high, detection of at least the central part of the plume is likely within expected geologic variation. The method has been successfully used in the location of plumes from contaminants including brine, uranium reprocessing liquid wastes and landfill leachate in glacial deposits of New England.

Salt‐Water Intrusion in the Nile Delta

Abstract: Egypt is presently involved in a comprehensive program of land reclamation. Surface water has already been exhausted in meeting the urgent needs of agricultural expansion. If irrigation efficiency were increased, large amounts of surface water would be saved. However, this entails complex social, technical and political problems outside the scope of this paper. Ground-water development has priority over increasing irrigation efficiency (which would take a longer time). This paper deals with identification of the intruded salt-water wedge in the huge artesian Delta aquifer. The case is unique because most of this aquifer is invaded by salt water, and the major portion of its annual ground-water recharge is derived from the direct seepage from the Nile River and the huge net of irrigation canals serving about 3 million acres (∼ 11,561 km2) of fertile land, as well as the infiltration of excess irrigation water. The annual overall ground-water recharge to the aquifer was estimated as 6·40 km3/yr as explained later. Methods of salt-water control and various techniques of water resources management are discussed in this paper. Because of the great variation in the depth of the aquifer, two unconventional methods for identifying the salt-water wedge are also presented.

Experimental Study of Some Hydrogeophysical Properties of Unconsolidated Porous Media

Abstract: Laboratory investigations have been carried out on one- and two-component artificial sand samples in order to estimate the relations between formation factor (F), porosity (φ), and permeability (K). The measurements were carried out by varying grain size and size distribution while keeping constant shape and packing; in our experimental conditions the measured formation factor approximated the intrinsic formation factor. The relationship between formation factor and porosity (F = a ·φ−m) is independent of grain size and size distribution; the coefficient values are: a = 1·15, m = 1·42. The relationship between permeability and formation factor has the general form K = r · F−s, where s is constant, and r is a function of the mean size d of the small component of the samples. The K-F relation can be established only if d is constant; in these conditions the relationship is an inverse one, and the negative trend is related to the variations in size distribution.

Major Ion Characterization of Coastal Saline Ground Waters

Abstract: The ages and origins of saline ground waters in coastal aquifers can be important indicators of local aquifer flow characteristics. Unfortunately, attempts to classify such waters using established trilinear diagram techniques are frequently inconclusive due largely to the dominance of sodium and chloride ions. In a study of saline ground waters from the Chalk limestone of eastern central England, an alternative, less conventional, dilution diagram procedure is employed which reveals previously unrecognized differences in local major ion chemical character. These differences are interpreted in terms of the origins of the saline ground waters and their histories of mixing. Significantly, the interpretations reinforce and, in some cases, refine earlier interpretations based on notably less extensive minor ion and environmental isotope data. It is concluded that the saline ground waters are associated with at least three periods of saline intrusion during the past 120,000 years.

Evaluating System for Ground‐Water Contamination Hazards Due to Gas‐Well Drilling on the Glaciated Appalachian Plateau

Abstract: Recent drilling for natural gas in the Glaciated Appalachian Plateau area of northwestern Pennsylvania has caused limited, but increasing ground-water contamination. By evaluating hydrogeologic parameters at a proposed gas well site, such as the ground-water flow system, permeability of surficial sediments, and the presence of fracture zones, the contamination hazard of the site can be assessed. Three case studies document that the most hazardous sites are generally located on or near valley walls of major drainageways. The relatively steep hydraulic gradient, the frequent presence of highly permeable surficial sediments, and the low to moderate dilution of contaminants along the intermediate-length flow paths at these sites all contribute to a relatively high pollution hazard. In addition to locating gas wells in high-hazard hydrogeologic zones, allowing the annulus of gas wells to become pressurized is the other major factor contributing to aquifer contamination.

The Noordbergum Effect and Characterization of Aquitards at the Rio Maior Mining Project

Abstract: The planned exploitation of the Rio Maior lignite deposits will require a major dewatering program involving a thick leaky sand aquifer, and a heterogeneous and anisotropic aquitard complex. Diatomite, lignite, and clay are the main lithologies found in this aquitard complex. Characterization of the sand aquifer was accomplished by performing pumping tests with interpretations for steadystate and nonsteady-state conditions. Aquitards were characterized by using both laboratory and field tests to determine hydraulic conductivity. Field methods were either those commonly adapted to leaky aquifers using water-level data taken from observation wells tapping the underlying sand aquifer or those using water-level data taken from piezometers located at the same distance from the pumping well and screened in the aquitard and in the aquifer. Field values of hydraulic conductivity are greater than laboratory values. In several piezometers the Noordbergum effect was observed during pumping tests. Redistribution of the loads originated by the pumping in the underlying sand aquifer is considered to be responsible for the reverse water-level response in the overlying lignite-diatomite complex. The Noordbergum effect has some influence in the application of the ratio method. On a practical basis, it seems reasonable to use the Noordbergum effect to compute drawdowns in the aquitards, and this leads to somewhat higher values for vertical hydraulic conductivities.

Field Determination of Aquifer Thermal Energy Storage Parametersa

Abstract: Determination of the potential of a specific confined aquifer as an effective thermal energy storage medium requires thorough knowledge of the geochemical, thermodynamic, and hydraulic properties of the aquifer and its confining layers. A series of laboratory and field studies must be performed in order to determine the fundamental parameters. Procedures and analyses of a series of tests for a confined aquifer near Mobile, Alabama were completed prior to an aquifer thermal energy storage experiment. Parameters determined were: the regional gradient; vertical and horizontal permeabilities of the storage aquifer; horizontal dispersivity of the storage aquifer; vertical permeability of the confining layers; and thermal conductivities, heat capacities, and chemical characteristics of the aquifer matrix and native ground water.

Analysis of Flow to a Large‐Diameter Well Discrete Kernel Approach

Abstract: Unsteady flow to a large-diameter well has been analyzed by a discrete kernel approach, taking well storage into consideration. The variation of drawdown with time has been obtained at the well and inside the aquifer. The validity of the new method has been verified by comparing the drawdown at the well with the drawdown given by Papadopulos and Cooper (1967). The aquifer contribution to discharge at various times has been estimated.

Harmonizing Ghyben‐Herzberg Interface with Rigorous Solutions

Abstract: Ground-water management in coastal aquifers is becoming highly complex due to the increase in water demand coupled with the threat of salt-water encroachment. The salt-/fresh-water interface produced by the natural flow of the aquifer is distorted as a result of ground- water exploitation. The solution of these problems includes several simplifications of unknown effects. This paper deals with the first phase in ground-water management, namely locating the interface due to natural flow by simple equations which give results practically the same as the rigorous solution. During the last four decades, the use of the Ghyben-Herzberg (G-H) principle to determine the salt-/fresh-water interface in coastal aquifers was questioned by many investigators due to the neglect of the water movement. Some of the highly complex solutions which were later introduced are reviewed in this paper and compared with the G-H results for a very wide range of parameters. It has been found that the G-H principle is valid for practical purposes and that the differences between location of the interface predicted by the G-H principle and those determined by the rigorous solutions may be less than the differences resulting from small errors in the field records of water levels. For exceptionally high natural velocities and/or for more precision within the zones very close to the shore, two simple equations derived on the basis of the rigorous solutions are given. Each equation is written in terms of the practically popular G-H solution with a correction factor added. The results of these equations are tabulated in dimensionless values for practical use. Moreover, the extent and size of the fresh-water lens can be determined from the field records of one or two shallow observation wells. An illustrative example is given to clarify the procedure.

Velocity Plots and Capture Zones of Pumping Centers for Ground‐Water Investigationsa

Abstract: Nonpumping monitoring wells are commonly installed and sampled to delineate the extent of a contaminant plume and its chemical character. Samples from municipal and private pumping wells are frequently collected during ground-water-contamination investigations as well. Pumping wells are also employed for remedial actions. To properly interpret sampling data from monitoring and pumping wells and to estimate their potential effectiveness in remedial actions, it is important to clearly define the geometry of that portion of the aquifer contributing water to the well (the capture zone). Velocity-distribution plots by manual and computerized methods are illustrated and shown to be simple and of reasonable accuracy. 13 references, 14 figures.

Ground-Water Systems Simulation by Time-Series Techniques

Abstract: Time-series techniques have been widely used in economic and environmental contexts. This paper explores their use in ground-water systems. The basic techniques are considered in relation to hydrogeology and two case histories show their application to real systems. Water level fluctuations in a well field are shown to be dependent upon pumping rates and prior rainfall and can be simulated by a multiple linear regression model. The rate of dewatering of a mine is shown to be dependent upon pumping rates and prior rainfall and can be simulated by a multiple linear regression model. The rate of dewatering of a mine is shown to be dependent upon antecedent mine size and rainfall, and can also be simulated by a multiple linear regression model. Such models can be used for forecasting and control of the ground-water systems, and once formulated are ideally suited to the management of ground-water systems where more costly and complex methods cannot be used.

Drawdowns in Large‐Diameter Wells Due to Decreasing Abstraction Rates

Abstract: Due to characteristics of centrifugal pumps, most pumping tests in large-diameter wells exhibit a decreasing abstraction rate with increasing drawdown. Type curves are obtained for the case of abstraction rates which are functions of the well drawdown; values for the type curves are obtained from a numerical model. For the case of constant abstraction, these curves provide an alternative to the classical approach. By plotting the ratio of drawdowns at different times, it is possible to identify more easily when the aquifer begins to contribute to the well discharge. The practical application of the technique is illustrated by a field example.

Application of Resistivity Surveys to Regional Hydrogeologic Reconnaissance.

Abstract: Vertical electrical soundings using DC resistivity methods have been completed along over 60 miles of survey lines in southwest Florida. The resistivity soundings were obtained in order to outline major hydrogeologic features as part of a regional hydrogeologic investigation covering approximately 400 square miles. The two significant hydrogeologic features which can be effectively mapped on a regional scale by DC methods are the presence of shallow, high resistivity limestones associated with late Tertiary reef complexes, and the approximate depth to waters with TDS concentrations well above the potable water limits. The reef limestones commonly exhibit very high transmissivities and are potential sites for ground-water development. The approximate thickness of potable waters allows resource investigations to be limited to the most promising areas and a rough assessment of the total resource to be made for long-term planning. The interpretation and mapping of the resistivity section is accomplished through published resistivity inversion and computer graphics programs. This automated data processing produces resistivity maps and sections without requiring extensive geophysical training of the interpreter. The automatic interpretations compare well with more traditional master curve interpretation procedures, and have the same limitations with equivalence of solutions and geologic correlation, but are produced with considerably less effort.

Field Determination of Cation Exchange Properties for Calcareous Sand

Abstract: A technique has been developed for the determination of the exchangeable cation population of calcareous sandy material with cation exchange capacities of less than 1 meq/100 g. The technique involves the addition of exchange salt in the dry state to samples of porous media using original pore water as the exchange salt solvent. In applying this technique to samples from below the water table, the amount of pore water available for reaction is reduced by centrifuging in the field to bring the moisture content close to field capacity values. By utilizing the minimum amount of pore water, interferences during the exchange process due to calcite precipitation or dissolution are minimized. The extent of calcite dissolution or precipitation that occurs can be appraised by measuring alkalinity or total carbon on the pore water before and after addition of the exchange salt. Three salts, NH4C1, CsCl and LiCl were tested for their suitability for this technique. CsCl was found to be preferable because of its low tendency to dissolve carbonate and the preference of Cs+ on exchange sites. Application of the method to a field site in southern Ontario yielded a value of 0.51 ± 0·09 meq/100 g over 15 samples for the cation exchange capacity of a near surface glaciofluvial sand deposit. It is believed that this technique could be applied with reasonable accuracy and reproducibility to materials with exchange capacities of as low as 0·1 meq/100 g.

Seepage from Oilfield Brine Disposal Ponds in Utah

Abstract: Significant amounts of brine produced in conjunction with oil well operations in the Greater Altamont-Bluebell field have been disposed of to surface impoundments. Mass balance calculations indicate that about 93% of this brine, a total of 4,430 acre-feet (5·45 × 106 m3) over the ten-year period studied, is lost through seepage, introducing a total salt load of 200 million pounds (90 × 106 kg) into the shallow aquifer system. Estimates of salt-water plume movement indicate that potential adverse economic impacts on irrigators and other water users in the Duchesne River Valley could reach hundreds of thousands of dollars. Lining of disposal ponds and increased use of injection wells will reduce future contamination. A monitoring program should be set up to determine actual levels of contamination of domestic and agricultural water supplies.

The Rainfall/Water-Table Level Relationship of an Unconfined Aquifer.

Abstract: The soil parameters that determine the infiltration rates of a shallow unconfined coastal aquifer due to rainfall were obtained using the recursive least squares method. The parameters were predicted by minimizing the mean square error between the estimated and measured water-table levels in a bore hole. The parameters λt , a drainage factor, βt , a parameter which accounts for external disturbances, and αt , a set of parameters which determine the infiltration rates due to rainfall, were all treated as time-dependent variables. Their variation is traced by introducing a "forgetting factor" in the algorithm used to determine the parameters. The parameters were determined using rainfall and water-table levels for the year 1979 at a bore hole. The parameters βt and αt varied considerably over the period of time and λt remained constant. From the parameters obtained, it appears that most of the recharge due to rainfall takes place within the first two days of the event. After two days there is very little variation in the water-table level due to rainfall although there is a slight drop on the third, fourth and fifth day, presumably due to the escape of entrapped air beneath the water-table level.

A New Method for Simultaneous Measurement of Cl- , Br- , NO3- , SCN- , and I- at Sub-ppm Levels in Ground Water.

Abstract: Ion-exchange high performance liquid chromatography coupled with ultraviolet absorption detection is shown to be a rapid and sensitive method for analysis of some common anions in water. Sensitivity of measurement is approximately 50 ppb for NO-2 , NO-3 , Br- , I- , and SCN- while Cl- has a detection limit in the one to ten ppm range. Chromatograms require 8 to 13 minutes to complete. Analyses are performed on either of two stationary phases (Whatman SAX 10 μm or Brownlee anion exchange) depending on the anions of interest in the analysis and their interferences.

A Quantitative Model to Predict a Safe Yield for Well Fields in Kufra and Sarir Basins, Libya

Abstract: The Kufra and Sarir basins of Libya have huge ground-water resources. Three well fields-Sarir, Kufra, and Jalo-have been constructed for irrigation purposes. The Tazerbo and West Sarir well fields are under construction. Wells in these well fields have tapped only a small portion of the total ground water available. The hydrological behavior of the Sarir well field (determined by using analytical and numerical models) indicated a leaky artesian aquifer and a significant partial penetration effect. Long and short-term pump tests did not provide true estimates of the values of transmissivity and the coefficient of storage. A steady-state model of the Kufra and Sarir basins indicated a minimum of 80 m3 /s of underflow entering from Tibesti, Chad and Sudan. A transient model was constructed to pump a total of 120 m3 /s from 14 well fields. The simulation indicated that sufficient drawdown was available to last at least 50 years.

Experiments to Assess the Hydraulic Efficiency of Well Screens

Abstract: The hydraulic properties of commercial well screens were investigated to determine which screen design features affected head loss. The test program began with laboratory experiments and continued as a field-scale experiment with the installation of a well field in the Thames Valley Gravel Aquifer. The laboratory experiments indicated that, for all practical purposes, the head loss attributable to all of the screens tested was negligible. The experimental head loss, however, did vary from screen to screen, particularly at high intake velocities, and the screens could be ranked on the basis of their hydraulic efficiencies. This efficiency hierarchy is explained tentatively in terms of the screen construction methods and slot geometry. The field experiments showed that the hydraulic performance of all well screens is independent of screen design provided that the open area of the screen is above about 10%. The field experiments also indicated that the development capacity of a screen in a gravel aquifer is not wholly dependent on screen design, but that progressive development does increase the hydraulic efficiency of a well.

Rapid Solution of the Nonlinear Step-Drawdown Equation

Abstract: The step-drawdown test is frequently utilized by hydrogeologists as an aid in assessing well efficiency and approximate pumping capacity. Unfortunately, the analysis of step-drawdown data presently requires either a solution to a system of highly nonlinear equations or the application of tenuous assumptions relative to the solution form. A method for solution is derived which does not require limiting assumptions, type-curve methods or extensive computer facilities.

A Computer Program for a Trilinear Diagram Plot and Analysis of Water Mixing Systemsa.

Abstract: The Piper (1953) trilinear diagram has been widely used to graphically represent the dissolved constituents of natural waters and to test for apparent mixtures of waters from different sources. Because of the time required to plot points and calculate the proportional values of mixing, this treatment of data was often quite tedious, particularly in studies involving large numbers of chemical analyses. The PIPER program was written in BASIC to be run on a Hewlett-Packard desktop computer with an X-Y plotter. Data input is in ppm units. The program plots points in all three fields of the trilinear diagram, draws at each point within the central diamond field a circle with a radius correspondent to the concentrations expressed in meq/1, checks for points that fall on a straight line (or within a predetermined tolerance of a straight line) representing postulated mixtures with two end members, and/or within a triangle representing mixtures of three end members. Finally, the program does a numerical analysis of the mixing ratios of the constituents for postulated mixing systems according to the methodology as presented by Piper (1953).

Ground-Water Model for a Thick Fresh-Water Lens

Abstract: The Pearl Harbor aquifer, an important ground-water resource for southern Oahu, Hawaii, contains an enormous fresh-water lens, up to 300 m (1,000 ft) thick, floating above saline water. The upper boundary of the fresh-water lens is confined near the coast and phreatic inland, whereas its lower boundary is not confined. A finite-difference flow model was used to simulate the Pearl Harbor aquifer system. The location of the fresh-water/salt-water interface, which constitutes the lower boundary of the flow system model, was estimated by Hubbert's formula. Known geohydrologic boundaries and aquifer parameters were also simulated. First, formation of the lens by filling the aquifer with fresh water from natural recharge was simulated to reconstruct the initial conditions before exploitation of the lens and to partially calibrate the model against the hydraulic-head distribution in 1879. The model then was calibrated fully by simulating the lens’reponse to actual historical pumping patterns covering a time span of 100 years (1880 to 1980). Simulated heads compared favorably with the observed heads over time and space. The calibrated flow model may serve as a useful tool in the management of the Pearl Harbor aquifer.

A Numerical Model Study of Ground-Water Contamination from Price's Landfill, New Jersey— I. Data Base and Flow Simulation

Abstract: Toxic waste contamination is currently threatening the Atlantic City, New Jersey public water-supply wells. The geohydrologic data for this region are presented and organized into a format suitable for a numerical model study of the contamination problem. Presentation of the data in light of numerical work reveals the importance of good estimates of boundary conditions, historical pumping records, reliable water-quality data, accurate well logs, and reasonable parameter estimates. One set of measured head data is simulated.

Ground‐Water Modeling in a Southwestern Alluvial Basin

Abstract: A two-dimensional finite-difference computer code was used to model the ground-water flow system in an alluvial basin in southwest New Mexico. A three-step approach was used to determine the transmissivity distribution for the model. First, values of the natural logarithm of transmissivity (In T) were interpolated from existing data using the kriging technique. This interpolation scheme also produced a map of standard deviations of the kriging errors. Second, a conventional flow net was drawn from steadystate water levels with the aid of the kriged hydraulic head distribution. Third, the approximate transmissivity map, the standard deviation map of the kriged In T values, and flow net were used to select the segment of each streamtube where transmissivity was known with greatest certainty (smallest In T kriging error). Then, transmissivities in other segments of the stream tubes were calculated from Darcy's Law. This distribution, when input to the numerical model, did not have to be altered appreciably during the calibration for a steady-state and seven-year transient period. Most of the transient calibration was accomplished by adjusting storage coefficients. Considering the uncertainty in the available pumping data, very good agreement was found between observed and predicted water levels during a four-year model verification period.

Aquifer Tests and Well Field Performance, Scioto River Valley, Ohio: Part Ia

Abstract: Based on 13 aquifer infiltration tests at 11 sites, aquifer transmissivity ranged from 17,000 to 40,000 square feet per day, and the streambed infiltration rate ranged from 0·06 to 0·19 million gallons per day per acre along a seven-mile reach of the Scioto River in south-central Ohio. Despite these wide ranges, the sand and gravel aquifer, 40 to 65 feet thick, underlying 10–20 feet of river alluvium south of the glacial boundary, is fairly homogeneous due largely to the absence of till interbeds. Performance of a major well field designed on the basis of the tests has been about as predicted, validating the techniques of data collection and analysis. At each site, a well for pumping was drilled on the floodplain, 150 to 750 feet from the river, and an array of observation wells was installed on lines extending through the pumped well, one line perpendicular to the river and the other parallel. Values of drawdown measured in the observation wells at the end of constant-rate pumping periods, usually of three days duration, were used to determine line-source distance and aquifer transmissivity. The rate of streambed infiltration and the streambed-leakage factor, the latter term relating the infiltration rate to drawdown beneath the stream, also were determined at all but one site. Drive-point wells installed in the riverbed, or along the near bank, in the vicinity of the pumped well were used to determine drawdown beneath the river due to pumping.