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S. G. Robson

Publications and source records attributed to S. G. Robson.

11 recordsLinked to original sources
USGS2005

Preparation of specific-yield logs for clastic bedrock aquifers

Specific yield is the principal aquifer characteristic needed to estimate the volume of recoverable ground water in storage in an aquifer. Determination of specific yield can be difficult and costly, particularly in deep, confined aquifers where core drilling and core analyses may be needed to define specific yield. A method has been developed for preparation of specific-yield geophysical logs that could greatly ease the determination of specific yields in such aquifers. Three geophysical logs that were investigated as potential indicators of specific yield were the free fluid index log, the effective-porosity log, and the apparent grain-density log. The free fluid index log did not accurately represent conditions at the test site in central Colorado and may not be suitable for application in other shallow and permeable aquifers. The effective-porosity and apparent grain-density logs were each used in least-squares linear regressions to correlate log response to specific yield measured in core samples. The resulting regression equations have coefficients of correlation (R) of 0.84 and 0.90, and were used to successfully prepare specific-yield logs from the effective-porosity and apparent grain-density logs.

Groundwater↗
USGS1995

Ground-water resources of the Florida Mesa area, La Plata County, Colorado

Rapid population growth in La Plata County, Colorado, has increased the demand for ground water in the Florida Mesa area. This report was prepared in cooperation with La Plata County to provide needed information about the geology, extent, thickness, and depth of the aquifers in the area; sources of ground-water recharge and discharge; direction of ground-water movement; water-level changes; and water quality in the alluvial and bedrock aquifers. Ground water in the study area is present in bedrock formations and in terrace deposits on Florida Mesa. Porous or fractured sandstone beds that contain bedrock aquifers are present near land surface along the northern margin of the study area and are present at depths less than 3,000 feet throughout the study area. Terrace deposits as much as 200 feet thick and consisting of gravel, sand, silt, and clay are present on Florida Mesa. The terrace deposits and the upper part of the underlying Animas and Nacimiento Formations form the principal aquifer under the mesa. Ground water under the mesa is supplied from precipitation and irrigation water. A small part of the precipitation and irrigation water on the mesa percolates to depth in the soil and recharges the aquifer. Irrigation water is the largest source of this recharge. Water levels in the aquifer can decline because of a reduction in irrigation recharge, or because of an increase in well pumping. Because irrigation recharge is so much larger than pumping, changes in recharge can have a much larger effect on ground-water levels than can changes in pumping. Factors that tend to increase ground-water recharge and thereby increase or maintain ground- water levels include: maintaining large rates of surface-water diversion onto Florida Mesa, reducing surface flow off the mesa, increasing use of ponds and spreading basins to promote infiltration, and irrigating by use of flood irrigation. The general direction of ground-water movement on the mesa is from the northern part of the mesa to the south, southwest, and southeast. Most ground water discharges from the mesa to the Animas and Florida Rivers through seeps and springs along the margin of the mesa. Winter water levels in wells generally are lower than summer water levels because of the lack of irrigation recharge during the fall and winter. Potable water of low dissolved-solids concentration is present in the shallow parts of most aquifers.

Water-Resources Investigations Report↗
USGS1993

Data from core analyses, aquifer testing, and geophysical logging of Denver Basin bedrock aquifers at Castle Pines, Colorado

This report contains data pertaining to the geologic and hydrologic characteristics of the bedrock aquifers of the Denver basin at a site near Castle Pines, Colorado. Data consist of a lithologic- description of about 2,400 ft of drill core and laboratory determinations of mineralogy, grain size, bulk and grain density, porosity, specific yield, and specific retention for selected core samples. Water-level data, atmospheric-pressure measurements, aquifer-compression measurements, and borehole geophysical logs also are included.

Open-File Report↗
USGS1993

Techniques for estimating specific yield and specific retention from grain-size data and geophysical logs from clastic bedrock aquifers

Specific yield and specific retention are aquifer characteristics that are important in determining the volume of water in storage in an aquifer. These characteristics can be determined by laboratory analyses of undisturbed samples of aquifer material. However. quicker, less costly alternatives to these laboratory analyses are needed. This report presents techniques for estimating specific yield and specific retention based on grain-size analyses, and based on interpretation of borehole geophysical logs.

Water-Resources Investigations Report↗
USGS1985

Proposed work plan for the study of hydrologic effects of ground-water development in the Wet Mountain Valley, Colorado

Large-scale development of groundwater resources in the Wet Mountain Valley, Colorado, could adversely affect other water rights in the valley or in the Arkansas River Basin. Such infringement on senior water rights could severely limit development of additional water supplies in the valley. A work plan is presented for a study that is intended to define the hydrologic system in the valley better, and to determine the extent that the quantity and chemical quality of both surface and groundwater in the valley might be affected by proposed development. (USGS)

Open-File Report↗
USGS1984

Bedrock aquifers in the Denver basin, Colorado; a quantitative water-resources appraisal

The Denver metropolitan area is experiencing a rapid population growth that is requiring increasing supplies of potable water to be pumped from bedrock aquifers in order to meet demand. In an effort to determine the ability of the aquifers to continue to meet this demand, the Colorado Department of Natural Resources, the Denver Board of Water Commissioners, and Adams, Arapahoe, Douglas, Elbert and El Paso Counties joined with the U.S. Geological Survey in undertaking a hydrologic evaluation of the ground-water resources of the basin. This involved mapping of aquifer extent, thickness, structure, hydraulic characteristics, and water-level and water-quality conditions. This enabled ground-water modeling techniques to be used to simulate aquifer response to various pumpage estimates and ground-water development plans. The Laramie-Fox Hills aquifer (the deepest aquifer) underlies the 6,700-square-mile study area and is overlain by the more permeable Arapahoe aquifer, the Denver aquifer, and the Dawson aquifer, which crops out in the southern part of the study area. It is estimated that 260x10 6 acre-feet of recoverable ground water are in storage in these four bedrock aquifers. However, less than 0.1 percent of this volume of water is stored under confined conditions. The larger volume of water stored under unconfined conditions will be available for use only when the water levels in the confined aquifers decline below the top of the individual aquifer, allowing water-table conditions to develop. Annual precipitation on the Denver basin supplies an average of 6,900 cubic feet per second of water to the area; about 55 cubic feet per second of this recharges the bedrock aquifers, principally through the Dawson Arkose. The direction of ground-water movement is generally from ground-water divides in the southern part of the area northward toward the margins of the aquifers. Pumpage has ranged from about 5 cubic feet per second in 1884 to about 41 cubic feet per second in 1978. Pumpage exceeds recharge in the metropolitan area and has caused water-level declines (1958-78) to exceed 200 feet in a 135-square-mile area of the Arapahoe aquifer southeast of Denver. A quasi-three-dimensional finite-difference model of the aquifer system was constructed and calibrated under steady-state and transient-state conditions. Steady-state calibration indicated that lateral hydraulic conductivity within the aquifers is about 100,000 times larger than the vertical hydraulic conductivity between the aquifers. Transient-state calibration indicated that between 1958 and 1978, 374,000 acre-feet of water was pumped from the aquifers, producing a 90,000-acre-foot net decrease in the volume of water in storage in the aquifers. During this time, pumpage also changed the rates of interaquifer flow, induced additional recharge, and caused capture of natural discharge. Three 1979-2050 pumpage estimates were made for use in simulating the effects of various ground-water development plans. Simulations using each of these pumpage estimates indicate that by the year 2050 large water-level declines could occur, particularly in the deeper aquifers. Maximum water-level declines of 410, 1,700, and 1,830 feet were produced using the small, medium, and large pumping rates. Four plans for supplementing the Denver water supply include pumping a satellite well field, pumping a municipal well field, pumping to irrigate parks, and injecting water during periods of low demand for later use during periods of peak demand. Model simulation of these plans indicates that the satellite well field will yield twice as much water as the municipal well field, but will produce larger and more widespread water-level declines in the four aquifers. The municipal well field would not significantly affect water levels in the Dawson aquifer. Pumping the Arapahoe aquifer to supply irrigation water to selected parks was shown to produce only small water-level declines in the aquifer. Results of simulating injection-pumpage well fields at two locations indicate that simulated injection rates could range from 1.7 to 10 cubic feet per second, depending on the choice of site. The volume of water that could be stored in the bedrock aquifer is, thus, sensitive to the hydrologic characteristics of the chosen site. More study is needed to evaluate water-chemistry compatibility of native and injected water.

Colorado↗
USGS1983

Hydrogeologic data from parts of the Denver basin, Colorado

This report presents hydrogeologic data collected and compiled during 1956-81 as part of a comprehensive hydrogeologic data collected and compiled during 1956-81 as part of a comprehensive hdryogeologic investigation of the Denver basin, Colorado, by the U.S. Geological Survey in cooperation with the Colorado Department of Natural Resources, Division of Water Resources, Office of the State Engineer. The data, in tabular and graphic form, consist of records for 870 wells which include water-level data for 158 wells and water-quality analyses for 561 wells; geophysical logs from three wells which include resistivity, self potential, and natural gamma logs; and gain-and-loss data of streamflow measured at 54 sites.

Colorado↗