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S. A. Leake

Publications and source records attributed to S. A. Leake.

29 records · Page 2Linked to original sources
USGS1994

Documentation of a computer program to simulate transient leakage from confining units using the modular finite-difference, ground-water flow model

Transient leakage into or out of a compressible fine-grained confining unit results from ground- water storage changes within the unit. The computer program described in this report provides a new method of simulating transient leakage using the U.S. Geological Survey modular finite- difference ground-water flow model (MODFLOW). The new program is referred to as the Transient- Leakage Package. The Transient-Leakage Package solves integrodifferential equations that describe flow across the upper and lower boundaries of confining units. For each confining unit, vertical hydraulic conductivity, thickness, and specific storage are specified in input arrays. These properties can vary from cell to cell and the confining unit need not be present at all locations in the grid; however, the confining units must be bounded above and below by model layers in which head is calculated or specified. The package was used in an example problem to simulate drawdown around a pumping well in a system with two aquifers separated by a confining unit. For drawdown values in excess of 1 centimeter, the solution using the new package closely matched an exact analytical solution. The problem also was simulated without the new package by using a separate model layer to represent the confining unit. That simulation was refined by using two model layers to represent the confining unit. The simulation using the Transient-Leakage Package was faster and more accurate than either of the simulations using model layers to represent the confining unit.

Open-File Report↗
USGS1991

Documentation of a computer program to simulate aquifer-system compaction using the modular finite-difference ground-water flow model

Removal of ground water by pumping from aquifers may result in compaction of compressible fine-grained beds that are within or adjacent to the aquifers. Compaction of the sediments and resulting land subsidence may be permanent if the head declines result in vertical stresses beyond the previous maximum stress. The process of permanent compaction is not routinely included in simulations of ground-water flow. To simulate storage changes from both elastic and inelastic compaction, a computer program was written for use with the U.S. Geological Survey modular finite-difference ground- water flow model. The new program, the Interbed-Storage Package, is designed to be incorporated into this model. In the Interbed-Storage Package, elastic compaction or expansion is assumed to be proportional to change in head. The constant of proportionality is the product of the skeletal component of elastic specific storage and the thickness of the sediments. Similarly, inelastic compaction is assumed to be proportional to decline in head. The constant of proportionality is the product of the skeletal component of inelastic specific storage and the thickness of the sediments. Storage changes are incorporated into the ground-water flow model by adding an additional term to the right-hand side of the flow equation. Within a model time step, the package appropriately apportions storage changes between elastic and inelastic components on the basis of the relation of simulated head to the previous minimum (preconsolidation) head. Two tests were performed to verify that the package works correctly. The first test compared model-calculated storage and compaction changes to hand-calculated values for a three-dimensional simulation. Model and hand-calculated values were essentially equal. The second test was performed to compare the results of the Interbed-Storage Package with results of the one-dimensional Helm compaction model. This test problem simulated compaction in doubly draining confining beds stressed by head changes in adjacent aquifers. The Interbed-Storage Package and the Helm model computed essentially equal values of compaction. Documentation of the Interbed-Storage Package includes data input instructions, flow charts, narratives, and listings for each of the five modules included in the package. The documentation also includes an appendix describing input instructions and a listing of a computer program for time-variant specified-head boundaries. That package was developed to reduce the amount of data input and output associated with one of the Interbed-Storage Package test problems.

Techniques of Water-Resources Investigations↗
USGS1991

Simulation of vertical compaction in models of regional ground-water flow

A new computer program was developed to simulate vertical compaction in models of regional ground-water flow. The program accounts for ground-water storage changes and compaction in discontinuous interbeds or in extensive confining beds. The new program is a package for the U.S. Geological Survey modular finite-difference ground-water flow model. Several features of the program make it useful for application in shallow unconfined flow systems. Geostatic load can be treated as a function of water-table elevation, and compaction is a function of computed changes in effective stress at the center of a model layer. Thickness of compressible sediments in an unconfined model layer can vary in proportion to saturated thickness. The new package was tested by comparison with an existing model of one-dimensional compaction.

Conference Paper↗
USGS1988

Documentation of a computer program to simulate aquifer-system compaction using the modular finite-difference ground-water flow model

The process of permanent compaction is not routinely included in simulations of groundwater flow. To simulate storage changes from both elastic and inelastic compaction, a computer program was written for use with the U. S. Geological Survey modular finite-difference groundwater flow model. The new program is called the Interbed-Storage Package. In the Interbed-Storage Package, elastic compaction or expansion is assumed to be proportional to change in head. The constant of proportionality is the product of skeletal component of elastic specific storage and thickness of the sediments. Similarly, inelastic compaction is assumed to be proportional to decline in head. The constant of proportionality is the product of the skeletal component of inelastic specific storage and the thickness of the sediments. Storage changes are incorporated into the groundwater flow model by adding an additional term to the flow equation. Within a model time step, the package appropriately apportions storage changes between elastic and inelastic components on the basis of the relation of simulated head to the previous minimum head. Another package that allows for a time-varying specified-head boundary is also documented. This package was written to reduce the data requirements for test simulations of the Interbed-Storage Package. (USGS)

Open-File Report↗
USGS1987

Distribution and movement of trichloroethylene in ground water in the Tucson area, Arizona

In 1981, investigations of groundwater quality around Tucson International Airport revealed a number of wells that pumped water contaminated with trichloroethylene. Subsequent investigations resulted in the delineation of three distinct areas of contamination, the largest of which encompasses about 5 sq mi of aquifer surface area. Most of the contamination is in the top 100 ft of the saturated groundwater flow system. A fine-grained confining layer that is present in much of the contaminated area significantly limits the vertical movement of the chlorinated groundwater. Within the contaminated area, measured trichloroethylene concentrations were as high as 3,100 mg/L in 1984. Measured concentrations are highly variable vertically as well as horizontally. Future quantitative studies of contaminant movement may benefit from additional data collection and experiments to determine which contaminant transport and groundwater flow equations are most appropriate.

Arizona↗
USGS1979

Maps showing ground-water conditions in the northern part of the Gila River drainage from Painted Rock Dam to Texas Hill area, Maricopa, Pima, and Yuma Counties, Arizona; 1978

The Gila River drainage from Painted Rock Dam to Texas Hill area includes about 3,000 square miles in southwestern Arizona. Ground-water development has taken place only in the northern part of the area, and only this part is included in the report. The southwestward-flowing Gila River drains the 1 ,900-square-mile northern part of the area. The main water-bearing unit is the valley-fill deposits. Since 1967, the estimated ground-water pumpage has exceeded 100,000 acre-feet per year, and in 1977 the ground-water pumpage was 210,000 acre-feet; the ground water is used mainly for irrigation. The ground-water withdrawals have resulted in general water-level declines in most of the area. Information shown on the maps includes change in water level, 1965-78 and 1973-78, and irrigated area; depth to water, altitude of the water level, and well depth; and specific conductance and fluoride concentration in the water. Hydrographs of the water level in selected wells and a table of historical pumpage also are included. Scale 1:250 ,000.

Arizona↗
USGS1979

Maps showing ground-water conditions in the Gila River drainage from Texas Hill to Dome area and in the western Mexican drainage area, Maricopa, Pima, and Yuma counties, Arizona; 1977

The Gila River drainage from Texas Hill to Dome and the western Mexican drainage areas include about 4,700 square miles in southwestern Arizona. The main water-bearing unit is the alluvium along the Gila River and its tributaries and in the valleys that separate the mountains. Most of the ground-water development has taken place in the Wellton-Mohawk area in the northern part of the Gila River drainage from Texas Hill to Dome area. The use of imported Colorado River water for irrigation caused the water levels to rise, and in the early 1970 's the water levels were within 6 feet of the land surface in most of the area. Since 1961, a network of about 70 wells has been pumping about 200,000 acre-feet of ground water annually for drainage of the waterlogged land in the area. The ground water in the Wellton-Mohawk area is of unsuitable chemical quality for most uses. Information shown on the maps includes depth to water , well depth, altitude of the water level, irrigated area, and specific conductance and Fluoride concentration in the water, A table of historical pumpage also is included. Scale 1:125.000. (Kosco-USGS)

Arizona↗
USGS1979

Relation between proposed developments of water resources and seepage from the All-American Canal, eastern Imperial Valley, California

A two-layer digital model designed for this study indicated that sealing of the Coachella branch of the All-American Canal would cause an eventual increase in seepage from the All-American Canal of about 15,000 acre-feet annually. Sealing of both the Coachella Canal and the segment of the All-American Canal between Pilot Knob and Drop 1 would result in a lessening of seepage rates from the All-American Canal of 57,000 acre-feet in 1985, but of only 39,000 acre-feet in 2030. Sealing both the Coachella and the All-American Canals would reduce the outflow to Mexicali Valley from 120,000 acre-feet in 1980 to less than 9,000 acre-feet in 2030. The model also indicated that if only the Coachella Canal were sealed, a little less than 40 percent of water pumped from proposed well fields near the All-American Canal ultimately would be derived from increased seepage from the All-American Canal; between 50 and 60 percent of the water pumped would be water that otherwise would flow to Mexicali Valley.

California↗