Geohydrology of Buried Triassic Basin at Savannah River Plant: ABSTRACT

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At the Savannah River plant near Aiken, South Carolina, as at other locations where there are chemical-separation plants for the processing of nuclear fuels, the high-level radioactive wastes are stored in concrete-and-steel tanks buried just beneath the surface of the ground. This waste is of such activity and longevity that it cannot be dispersed into the environment, but it must be contained for periods of time extending at least into hundreds and perhaps thousands of years. One concept for the terminal containment of this waste is to store it in excavated chambers within the bedrock, which is covered by about 1,000 ft of coastal plain sediments at the plant site. As part of the safety evaluation of this concept, the geology and hydrology of both the coastal plain sedi ents and the bedrock have been studied. Intensive investigation of bedrock waste storage now has been postponed indefinitely while other concepts of waste storage and management are being investigated. A buried Triassic basin that might have potential for waste storage was discovered beneath the southern third of the plant site. Investigation into the characteristics of this basin was started in 1971. This was not an engineering on design study but was aimed at understanding the geohydrology of the Triassic basin to determine its compatibility with the safe storage of waste. Seismic surveys, gravity and magnetic surveys, and the drilling of several exploratory wells indicate that the Triassic basin is about 30 mi long, 6 or more mi wide, and perhaps 5,300 ft thick. One well penetrated the Triassic border, a second was in the center of the basin, and a third investigated an intrabasin fault. The rock is predominantly mudstone of very low permeability, with a few lenses of poorly sorted gritty sand. The water yield of all the exploratory wells is extremely low, and water-transmitting fractures are virtually nonexistent. In 2 wells within the basin, heads above land surface have been measured that cannot be explained by connection with a recharge area. Ten possible explanations have been evaluated: aquifer head, fossil head, tectonic compression, rapid loading and compaction of sediments, water derived from igneous intrusions, infiltration of gas, precipitation of minerals, phase changes, temperature increase, and osmotic membrane phenomena. Systematic evaluation, particularly of the time for dissipation of the elevated head to the head of its surroundings, eliminates most of these explanations. Those that remain as possible explanations are: tectonic compression, temperature increase, and osmotic membrane phenomena. It is not known at present whether the high head is general over the entire basin or nly in segments of it. End_of_Article - Last_Page 1601------------