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The transition zone between bedrock and regolith: Conduit for contamination?

Proceedings of a Conference on Ground Water in the Piedmont of the Eastern United States
By Douglas A. Harned and C.C. Daniel, III

Abstract

Preliminary results of an investigation of the ground-water system in the regolith and fractured bedrock in the North Carolina Piedmont indicate that the transition zone at the base of the regolith is a primary lateral transmitter of ground water and, potentially, of ground-water contaminants. A zone of weathered rock fragments, residual boulders, and lesser amounts of saprolite is generally present between the base of the saprolite and the top of the bedrock. Saprolite is clay-rich, decomposed bedrock. Careful augering of three wells in Guilford County, North Carolina, indicated that the transition zone was approximately 15 feet thick. This transition zone has been observed in Georgia, Maryland, and North Carolina by several investigators and is described as being the most permeable part of the system, even slightly more permeable than the soil zone. This observation is substantiated by reports from well drillers of so-called "first water," "sand," and boulders at the base of the regolith.

The high permeability of the transition zone is probably due to less advanced weathering relative to the upper regolith. Chemical alteration of the bedrock has progressed to the point that expansion of certain minerals causes extensive fracturing of the crystalline rock, yet has not progressed so far that formation of clay has clogged these fractures.

The transition zone may serve as a preferential channel for rapid movement of contaminants to nearby wells, or to streams that have cut their channels into or through the zone to bedrock. The presence of a high-permeability zone between the bedrock and saprolite may create a favorable zone of rapid flow within the ground-water system. Because the distance from the point at which a molecule of water or waste enters the flow system in the Piedmont to where that molecule may discharge to a stream is commonly less than half a mile, ground-water contamination can rapidly become surface-water contamination. How rapidly any contamination moves through the system may be largely a function of the characteristics of the transition zone.

The customary practice in construction of water-supply wells in the Piedmont is to set casing through the regolith into the weathered bedrock. This practice has the benefit of supplying water clear of the fine-grained sediment (clay, silt, and sand) found in the overlying zones. At the same time, this practice makes identification of potential contamination within the transition zone difficult and complicates scientific study of the characteristics of the transition zone.

Harned, D.A., and Daniel, C.C., III, 1992, The transition zone between bedrock and regolith: Conduit for contamination?, in Daniel, C.C., III, White, R.K., and Stone, P. A., eds., Ground water in the Piedmont, Proceedings of a Conference on Ground Water in the Piedmont of the Eastern United States, Charlotte, N.C., Oct. 16-18, 1989: Clemson, S.C., Clemson University, p. 336-348.