Geohydrology of Buried Triassic Basin at Savannah River Plant, South Carolina

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A buried Triassic basin with possible potential for storage of high-level radioactive wastes was discovered beneath the southern third of the Savannah River Plant site near Aiken, South Carolina. Investigation into the characteristics of the basin was aimed at understanding the geohydrology to determine its compatibility with the safe storage of waste. Before it was completed, the investigation was deferred pending more detailed evaluation of alternative concepts of waste storage and management. However, some of the geohydrologic results of the investigation are reported herein. 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 inferred 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 two wells within the basin, heads above land surface have been measured that cannot be explained by connection with a recharge area. Possible explanations are tectonic compression, temperature increase, and osmotic-membrane phenomena. Whether the high head is general over the entire basin or only in segments of it is unknown.