Subsurface hydrogeologic systems underlying the Savannah River Plant (SRP) were studied to determine the origin and age of the contained fluids. Three distinct systems exist beneath SRP: the Coastal Plain sediments, crystalline metamorphic basement rock, and a Triassic rock basin surrounded by the crystalline rock. The water in the Coastal Plain sediments is low in dissolved solids (approximately 30 mg/l), acidic (pH approximately 5.5), and comparatively recent. Water in the crystalline rock is high in dissolved solids (approximately 6000 mg/l), alkaline (pH approximately 8), and approximately 840,000 years old as determined by helium dating techniques. Water in the Triassic rock is highest in dissolved solids (approximately 18,000 mg/l) and is probably older than the water in the surrounding crystalline rock; a quantitative age was not determined. The origin of the water in the crystalline and Triassic rock could not be determined with certainty; however, it is not relic sea water. A detailed geologic-hydrologic history of the SRP region is presented.