SOUTHEASTERN UNITED STATES PIEDMONT HYDROGEOLOGY: IMPROVING OUR UNDERSTANDING OF GROUNDWATER FLOW IN SAPROLITE AND IN FRACTURES OF PARTLY WEATHERED AND UNWEATHERED CRYSTALLINE METAMORPHIC AND IGNEOUS ROCKS: OCCURRENCE AND MIGRATION OF GROUNDWATER IN CRYSTALLINE ROCKS, FRACTURES AND IN SAPROLITE
Complex processes of groundwater flow (GF) in Piedmont rocks is not entirely understood even after 60 years of study by many prominent hydrogeologists and geologists. GF is near chaotic because time and spatial interrelated and intra-related ever changing, extremely complex, almost indecipherable and indeterminable variables. Changes from surface soils to underlying clay-rich saprolite to partially weathered rock to fractured unweathered rock yield a highly transitory complex system. Saprolite and residual soils are present in over 95% of the surface area and to depths of 20 - 80 feet in most areas.
The mineral composition of the parent rock, the degree of weathering and its thickness, mineral grain orientation, presence of veins, joints, and shear zones, the orientation of the foliation, differential flow, and other factors determine specifics of seepage and water flow.
Surface and near-surface materials are divided into:
1.Regolith zone (alluvium, soil a and b horizons) is unconsolidated or semi consolidated mixture of clay and fragmental material ranging in size from silt to cobble size. The saturated regolith provides the water storage most groundwater system and supplies water to any interconnected fractures.
2.Transition/saprolite zones serve as conduits for rapid movement of groundwater, saprolite merges below with partly weathered rock and underlying unweathered fractured crystalline bedrock. Crystalline rocks have principal openings along fractures, that decrease with depth.
3.Saprolite/partially weathered rock. Saprolite deposits retain the texture from the parent rock, the chemical composition, is altered to a clay-rich material derived from in-place weathering of bedrock.
4. Bedrock may be unweathered, gradational and may be mixed partly weathered with unweathered core stones. Clay filled bedrock joints and fractures, may serve as channels for fluid flow and have a greater directional permeability than saprolite. Piedmont bedrock has very few fractures at depths greater than 400 feet (LeGrand, 1967).