Geology topics

Dean O. Gregg

Publications and source records attributed to Dean O. Gregg.

3 recordsLinked to original sources

USGS2006

Protective pumping to reduce aquifer pollution, Glynn County, Georgia

Water-level declines in the principal artesian aquifer have created a head imbalance between the aquifer and an underlying brackish-water zone containing up to 4,550 mg/1 chloride. The brackish-water zone leaks brackish water into the aquifer through several breaks in a confining unit. A relief well tapping the brackish-water zone was drilled near a suspected break and pumped at about 3,000 gpm to lower the potential in the zone and bring it into hydrostatic equilibrium with the aquifer. The pumping apparently succeeded in decreasing the rate of brackish-water leakage into the aquifer. Successive samples of water from a well tapping the aquifer and downgradient from the relief well showed a decrease in the chloride content. Several more relief wells may be necessary to ultimately control chloride contamination of the aquifer.

Georgia↗

USGS1975

A general outline of the water resources of the Toppenish Creek basin, Yakima Indian Reservation, Washington

Increasing demands for water supplies, plans for irrigating new lands, proposals to divert water from the Yakima River by users downstream from the reservation, and ground-water problems have made an accounting of the overall availability of water very important to water management on the reservation. This report, which broadly outlines the water resources of the Toppenish Creek basin, has been developed as part of the cooperative study by the Tribal Council of the Yakima Indian Nation and the U.S. Geological Survey. Precipitation averages about 20 inches a year over the basin but only about 2 inches fall during the April-September growing season. However, diversions from the Yakima River make the Toppenish Creek valley one of the best agricultural areas in the State. Ground water in the basin occurs in three distinct units: the basalt, old valley fill deposits, and young valley fill deposits. Each unit is capable of yielding more than 1,000 gallons per minute to properly constructed wells. Water levels in wells tapping the young valley fill are higher under present (1974) irrigation conditions than they were in the past, but some water-level declines have occurred in the old valley fill in response to pumping. Water levels in the basalt wells in some areas have been lowered as much as 80 feet by pumping during the past 15 years. Some additional pumping in these areas can be allowed without lowering the water level more than a few feet if the amount of pumpage and spacing of wells are carefully controlled. The young valley fill in the eastern part of the lower valley is the best source of ground water in the basin. The zones of coarse sand and gravel to depths of about 150 feet in this area can produce large quantities of water. Irrigation water, from the Yakima River which seeps into the ground, yearly builds up this ground-water supply. About 120,000 acre-feet of water goes into temporary storage each irrigation season in the young valley fill.

Washington↗

USGS1974

Geologic and hydrologic control of chloride contamination in aquifers at Brunswick, Glynn County, Georgia

Water from a brackish-water zone (1,050-1,350 ft) has concentrations as high as 2,150 milligrams per liter chloride, and concentrations are suspected to be higher than 3,000 milligrams per liter chloride. This brackish water has been identified as the source of the water that contaminates the upper and lower fresh-water-bearing zones of the principal artesian aquifer. The confining unit separating the fresh and brackish water seems to contain breaks that act as vertical conduits for the movement of brackish water into the fresh-water zones of the aquifer. Faults are suspected to be responsible for the breaks in the confining unit. The rate of upward movement of brackish water seems to be a function of the rate of water-level decline in the aquifer. There are two main areas of brackish-water intrusion. One area is near Bay and Prince Streets, and the other area is near Reynolds and Q Streets. Successive maps showing chloride ion concentration trace the movement of the chloride front northward in the Bay Street area at the rate of about 350 feet per year toward the center of pumping. An average of about 400 gallons per minute of water containing 2,000 milligrams per liter chloride invaded the upper water-bearing zone between December 1962 and December 1966. A like amount may have entered the lower water-bearing zone. Maximum chloride concentration in the upper water-bearing zone is 1,540 milligrams per liter in the Bay Street area and 640 milligrams per liter in the Reynolds Street area. In a few areas, where individual wells have been drilled deep enough to penetrate the confining unit over the brackish-water zone, the well furnishes a conduit for brackish water to recharge the fresh-water aquifer. Plugging the lower part of these wells usually reduces the chloride concentration of the water. The chloride concentration of water in the principal artesian aquifer can probably be reduced by use of interceptor wells, relief wells, or well-field spacing. Interceptor wells would prevent laterally moving brackish water from contaminating a well field. A relief well would tap and withdraw poor quality water from only the brackish-water zone to lower the head in that zone and decrease the rate of leakage into the fresh-water aquifer. Wider spacing of wells would prevent the development of a deep cone of depression and the steeper hydraulic gradients that accompany it. The brackish water pumped by the interceptor or relief wells could be used for industry, aquaculture, recreation, or for other processes in which the chloride content is not critical.

Georgia↗