A surficial hydrogeologic framework for the Mid-Atlantic Coastal Plain

Abstract

A surficial hydrogeologic framework was developed for the Mid-Atlantic Coastal Plain, from New Jersey through North Carolina. The framework includes seven distinct hydrogeologic subregions within which the primary natural physical factors affecting the flow and chemistry of shallow ground water and small streams are relatively consistent. Within most subregions, the transport of chemicals from the land surface to ground water and streams can be described by a fairly uniform set of natural processes; some subregions include mixed hydrogeologic settings that are indistinguishable at the regional scale. The hydrogeologic framework and accompanying physiographic and geologic delineations are presented in digital and printed format. The seven hydrogeologic subregions that constitute the framework were delineated primarily on the basis of physiography and the predominant texture (typical grain size) of surficial and (where surficial sediments are particularly thin) subcropping sediments. Physiography for the Mid-Atlantic Coastal Plain was constructed by standardizing and extrapolating previously published interpretations for the Coastal Plain of South Carolina and New Jersey, based on similar work in the other States. Surficial and subcropping geology were similarly compiled from previous publications by resolving inconsistencies in nomenclature, interpretation, and scale, and interpolating across unmapped areas. A bulk sediment texture was determined for each mapped geologic unit on the basis of published descriptions. Fundamental differences among the seven hydrogeologic subregions are described on the basis of hypotheses about surficial and shallow subsurface hydrology and water chemistry in each, as well as variable land use, soils, and topography. On the regional scale, the Coastal Lowlands (Subregion 1), the Middle Coastal Plain - Fine Sediments (Subregion 3), the Middle Coastal Plain - Sands with Overlying Gravels (Subregion 4), and the Inner Coastal Plain - Upland Sands and Gravels (Subregion 5) are relatively homogeneous in terms of hydrogeology, although an examination of results from small-scale studies within the Coastal Plain demonstrates that even these areas are quite variable, locally. Moderate topographic relief and primarily permeable surficial sediments promote good drainage of the land surface in Subregion 4, for example, but drainage is commonly poor in the Coastal Lowlands (Subregion 1) due to flat topography and low elevations. Agriculture is common in both subregions, although artificial drainage is typically required to support cultivation in Subregion 1. Important physiographic differences are evident among the remaining three subregions, although sediment textures within the Middle Coastal Plain - Mixed Sediment Texture (Subregion 2), the Inner Coastal Plain - Dissected Outcrop Belt (Subregion 6), and the Alluvial and Estuarine Valleys (Subregion 7) are variable even at the regional scale.