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Robert W. Maclay

Publications and source records attributed to Robert W. Maclay.

9 recordsLinked to original sources
USGS1995

Geology and hydrology of the Edwards Aquifer in the San Antonio area, Texas

The Edwards aquifer, which is the sole source of water for the city of San Antonio, is one of the most permeable and productive carbonate aquifers in the United States. The aquifer is composed of extensively faulted, fractured, and cavernous limestone and dolomite of Early Cretaceous age lying within the Balcones fault zone a series of normal en echelon strike faults that separate the Edwards Plateau from the Gulf Coastal Plain in south Texas. Along segments of some faults, the entire thickness of the aquifer is displaced vertically, and these faults then act as barriers to downdip ground-water flow. The large porosity and exceptional permeability of the unconfined part of the Edwards aquifer result from the dissolution of limestone by circulating ground water and development of a cavernous network along fractures. The large porosity and permeability of the freshwater part of the confined Edwards aquifer result primarily from dedolomitization. The small permeability of the saline-water part of the confined aquifer is caused by the limited interconnection between the pores in the rock matrix and by the lack of substantial dissolution along fractures. The large transmissivity of the Edwards aquifer is indicated by the hundreds of highyielding wells, small hydraulic gradients, and large spring discharges. The determined transmissivity throughout most of the confined freshwater aquifer ranges from 430,000 to 2,200,000 feet squared per day; the determined transmissivity of the unconfined aquifer generally is less than 430,000 feet squared per day. Faulting causes the aquifer to be highly anisotropic, and simulation indicates anisotropy ratios ranging from 0.0:1 to 1:1. The ground-water-flow system of the Edwards aquifer includes several components. These include a catchment area on the Edwards Plateau where the unconfined aquifer receives direct recharge, an area of confining beds crossed by streams draining the Edwards Plateau, a major recharge area within the Balcones fault zone where streams lose flow directly into the unconfined Edwards aquifer, and the confined Edwards aquifer that consists of the freshwater and salinewater zones. Water entering the Edwards aquifer in the Balcones fault zone moves downdip in a generally southeasterly direction into the confined parts of the aquifer. In the confined aquifer, flow is toward the east and northeast under low hydraulic gradients through fractured, highly transmissive limestone and ultimately discharges at large springs and wells. All of the base flow and some of the storm runoff of streams crossing the recharge area infiltrates to the unconfined aquifer. On the basis of streamflow losses, the average annual recharge for 1934-88 was 635,500 acre-feet. Freshwater discharges from the Edwards aquifer primarily from wells, springs, and seeps. Beginning in 1968, annual discharge from the aquifer has consistently exceeded average annual recharge largely because of a doubling of well pumpage. However, total springflow also increased because of greater-than-average recharge during most years since the late 1960's. The total volume of circulating freshwater in the Edwards aquifer is about 45 million acrefeet. Long-term hydrographs at San Antonio indicate no net decline in ground-water levels during 1911-87; thus, there was no net loss of water from storage in the freshwater zone of the Edwards aquifer during that long-term period, assuming the San Antonio hydrograph represents the entire aquifer. However, short-term changes in water levels result largely from the variability of precipitation as indicated by severe declines during the drought of the late 1940's to middle 1950's and by rises to record highs during the abnormally wet years in the 1970's and 1980's. The principal components of the groundwater budget (recharge, springflow, and pumpage) have varied greatly over 55 years (1934-88) of pertinent hydrologic records. Annual recharge varied from about 44,000 to 2,000,000 acre-feet. Annual springflow varied from about 70,000 acrefeet to about 580,000 acre-feet. Pumpage increased from about 100,000 acre-feet annually in the early 1930's to more than 500,000 acre-feet annually during some years in the 1980's. However, the average annual recharge of 635,500 acrefeet is about equal to the sum of the average annual springflow (359,500 acre-feet) and average annual pumpage (273,000 acre-feet), indicating no longterm decrease in ground-water storage because of springflow and pumpage.

Texas↗
USGS1988

Simulation of flow in the Edwards Aquifer, San Antonio region, Texas, and refinement of storage and flow concepts

The Edwards aquifer is a complexly faulted, carbonate aquifer lying within the Balcones fault zone of south-central Texas. The aquifer consists of thin- to massive-bedded limestone and dolomite, most of which is in the form of mudstones and wackestones. Well-developed secondary porosity has formed in association with former erosional surfaces within the carbonate rocks, within dolomitized-burrowed tidal and evaporitic deposits, and along inclined fractures to produce an aquifer with transmissivities greater than 100 ft2/s. The aquifer is recharged mainly by streamflow losses in the outcrop area of the Edwards aquifer and is discharged by major springs located at considerable distances, as much as 150 mi, from the areas of recharge and by wells. Ground-water flow within the Edwards aquifer of the San Antonio region was simulated to investigate concepts relating to the storage and flow characteristics. The concepts of major interest were the effects of barrier faults on flow direction, water levels, springflow, and storage within the aquifer. A general-purpose, finite-difference model, modified to provide the capability of representing barrier faults, was used to simulate ground-water flow and storage in the aquifer. The approach in model development was to conduct a series of simulations beginning with a simple representation of the aquifer framework and then proceeding to subsequent representations of increasing complexity. The simulations investigated the effects of complex geologic structures and of significant changes in transmissivity, anisotropy, and storage coefficient. Initial values of transmissivity, anisotropy, and storage coefficient were estimated based on concepts developed in previous studies. Results of the simulations confirmed the original estimates of transmissivity values (greater than 100 square feet/s) in the confined zone of the aquifer between San Antonio and Comal Springs. A storage coefficient of 0.05 in the unconfined zone of the aquifer produced the best simulation of water levels and springflow. A major interpretation resulting from the simulations is that two essentially independent areas of regional flow were identified in the west and central part of the study area. Flows from the two areas converge at Comal Springs. The directions of computed flux vectors reflected the presence of major barrier faults, which locally deflect patterns of ground-water movement. The most noticeable deflection is the convergence of flow through a geologic structural opening, the Knippa gap, in eastern Uvalde County. A second significant interpretation is that ground-water flow in northeastern Bexar, Comal, and Hays Counties is diverted by barrier faults toward San Marcos Springs, a regional discharge point. Simulations showed that several barrier faults in the northwestern part of the San Antonio area had a significant effect on storage, water levels, and springflow within the Edwards aquifer.

Texas↗
USGS1976

Progress report on geology of the Edwards aquifer, San Antonio area, Texas, and preliminary interpretation of borehole geophysical and laboratory data on carbonate rocks

This report describes the geology and porosity of the rocks of the Edwards aquifer, with particular attention to the eastern half of the San Antonio area. The data were obtained from geologic and geophysical studies of nine cored test holes, from laboratory analyses of samples of the aquifer materials, and from recent stratigraphic studies by other investigators. The Georgetown Formation and the Edwards Group or their equivalent stratigraphic units are extensively exposed in central Texas and they occur in the subsurface throughout southern Texas. They consist of a complex sequence of carbonate rocks deposited during Early Cretaceous time under different depositional conditions. Within the study area, the thickness of the carbonate rocks ranges from less than 400 feet (122 m) in the eastern part of the area to more than 600 feet (183 m) in the southwestern part. The depositional environments significantly affected the texture and the composition of the sediments. The Maverick Basin in the southwestern part of the study area was a euxinic basin surrounded by a barrier rudistid reef. Fine-grained carbonates and some associated evaporites occur within this basin. The San Marcos Platform in the eastern part of the study area was intermittently submerged by a transgressing continental sea. Much of the sediments are extensively reworked and burrowed by marine organisms. Several thick sequences of dolomites, evaporites, and associated collapsed deposits occur on the San Marcos Platform. The carbonates of the San Marcos Platform are extensively dissolved within the fresh-water zone. Much of the dissolution is selective of beds containing evaporites or porous dolomites. The dominant structural feature of the study area is a broad homocline that extends from nearly flat-lying beds on the Edwards Plateau to steeply dipping beds of the deep subsurface of the Gulf Coastal Plain. The homocline is interrupted by the Balcones Fault Zone, which is a zone of normal faults that extend across the study area. The intensity and complexity of faulting increases eastward as part of a progression from gentle flexures in the southwestern part of the study area to a complex pattern of tilted fault blocks on the San Marcos Platform.

Texas↗
USGS1970

Water resources of the Wild Rice River watershed, northwestern Minnesota

The Wild Rice watershed includes two general physiographic areas- the glacial Lake Agassiz Plain and a glacial moraine. The lake plain is extremely flat in the western part, sloping only a few feet per mile, but in the eastern part the plain is traversed in a north-south direction by long, narrow beach ridges up to 20 feet high. The moraine is an area of hills and depressions. The local relief is low in the western part of the moraine but increases to more than 200 feet in the east. Altitude of the land surface ranges from 800 feet above mean sea level where the Red River flows north out of the area to 2,040 feet near the southeastern edge of the watershed (one of the highest points in Minnesota). The area of the watershed is about 2,600 square miles and includes most of Mahnomen and Norman Counties and parts of Becker, Clay, Clearwater, and Polk Counties. The population of the area is about 37,000 people of which about 70 percent live on farms. The economy is based principally on farming. The area of lake clay and silt is used mostly for raising sugar beets and wheat; and potatoes are grown largely on the sandy soils. In the western part of the morainal area small grain, dairy, and cattle farming is the most common. The eastern part of the morainal area is important for forest products and recreation. Industries in the area are small and are based on agricultural processing and service.

Minnesota↗
USGS1969

Water resources of the Buffalo River Watershed, West-central Minnesota

The Buffalo River watershed includes two general physiographic areas – a glacial lake plain and an glacial moraine. The lake plain, which was formed by Glacial lake Agassiz more than 9,000 years ago, is extremely flat – sloping only a few feet per mile westward near the Red River of the North. The moraine is largely an area of gently rolling hills, but in the eastern “tail” of the watershed the relief locally is over two hundred feet. The watershed has an area of about 1,690 square miles and includes most of Clay County and parts of Wilkin, Otter Tail, Becker, and Norman Counties. The population of the area is about 45,500 – 31,000 urban and 14,500 rural. The economy is largely agricultural. Sugar beet and wheat farming are dominant in the areas of clay and silt; potato farming is confined mainly to the sandy areas. Dairying and stock raising is important in the morainal area. The larger industries are located in Moorhead and consist of sugar beet, potato, and dairy product processing. Water-based recreation consists mostly of fishing in the larger lakes in the morainal area, and water-fowl hunting in the area of prairie potholes – the western part of the morainal area.

Minnesota↗
USGS1966

Reconnaissance of the geology and ground-water resources in the Aurora area, St. Louis county, Minnesota

The Aurora area is a glaciated upland of drift-mantled slopes, channels, swamps, and glacial-lake plains. It covers about 24 square miles of the eastern part of the Mesabi Iron Range in northeastern Minnesota. A deep narrow channel along the Embarrass River, the principal outlet of a former large glacial lake north of the Embarrass Mountains, lies partly within the area. The deposits in the report area consist of bedrock and unconsolidated glacial drift. The 'bedrock belongs to the Animikie Group of late Preeambrian age and consists of taconite (an iron-rich silicate rock) in the northern part and slightly metamorphosed argillite in the middle and southern parts. Bedrock is exposed only in the open-pit iron mines. Large quantities of ground water are pumped from porous and permeable ore zones in the St. James Mine. Small quantities of ground water are obtainable from openings along bedding planes and fractures in the argillite. Unconsolidated deposits consisting of till and water-laid glacial and alluvial materials mantle the bedrock to depths ranging from about 20 feet in the north-central part of the Aurora area to more than 300 feet near the Embarrass River. Thick deposits of sand and gravel in the Embarrass channel are capable of yielding large quantites of water. At places along the Partridge River glaeiofluvial deposits (glacial sediments deposited in running water) could yield moderate to large quantities of water. Sandy to bouldery till yields small quantities of water to domestic wells. Well yields in the Aurora area range from less than 5 gpm (gallons per minute) to about 250 gpm from a well tapping an ore body. The specific capacity of wells penetrating ore zones ranges from about 7 gpm per foot of drawdown to 25 gpm per foot of drawdown. Although no attempt has been made to develop a high-yield well in the sand and gravel deposits of the Embarrass channel, more than 5,000 gpm is pumped from sumps which collect water from these deposits in the Embarrass mine. Most domestic wells yield about 5 gpm and are drilled and finished in sand or gravel in either the bouldery till or glaciofluvial deposits. Ground water from the unconsolidated deposits is hard and commonly contains large, undesirable amounts of iron and manganese. Water from the 'bedrock aquifers contains less iron and manganese than does water from the unconsolidated deposits.

Minnesota↗