U.S. Geological Survey
National Water-Quality Assessment Program

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Surficial Hydrogeologic Subregions

The surficial hydrogeologic framework defines 7 distinct subregions in which the depositional history and texture of surficial sediments that govern the occurrence and transport of contaminants to shallow ground water and streams are relatively consistent (Ator and others, 2000). The seven hydrogeologic subregions were delineated primarily from physiography and the bulk texture (average grain size) of surficial sediments. Because ground-water surveys of the NAWQA program have generally focused on unconfined aquifer systems that are susceptible to anthropogenic contamination, the surficial hydrogeologic framework is an important component for regional stratification purposes. The 7 surficial hydrogeologic subregions are defined as follows:

1) Coastal lowlands – Low-relief platform of the Outer Coastal Plain and margins of the major estuaries. Area is very flat and low lying, with poorly developed stream drainage and numerous tidal wetlands and pocosins with perched water tables. Streams are low gradient and largely tidal. Sediments were deposited primarily in estuarine and near-shore marine environments during the Holocene and Pleistocene, and are primarily fine grained, except for linear ridges of sand associated with ancient shorelines. Soils of swamps and marshes contain abundant organic matter. Soil types reflect chronic poor drainage and poor oxidation.

2) Middle Coastal Plain, mixed sediment texture – Broad platform of the Middle Coastal Plain created by Pleistocene sea level high stands. Borders are typically scarps or other time-indicator elevation breaks, such as the Suffolk and Princess Anne Scarps and equivalent scarps in Delaware that were formed during sea level high stands during the Pleistocene. Land surface is moderately dissected by streams; local relief generally ranges from 25 to 30 feet. Sediments were deposited in near-shore marine and estuarine environments. Sediment texture varies laterally and vertically with changes in depositional systems from marine littoral to deltaic. Sediment grain sizes are mixed, and range from coarse sands associated with shorelines to clays and silts deposited in back-barrier lagoons and estuaries.

3) Middle Coastal Plain, fine sediments – Dissected inner portion of the Middle Coastal Plain with predominantly fine-grained sediments at land surface. Local relief ranges from 45 to 150 feet. This unit is associated with Pliocene and Miocene-age estuarine deposits of the Bacons Castle Formation and fine-grained Miocene-age marine sediments of the Chesapeake Group (primarily of the Calvert Formation).

4) Middle Coastal Plain, sands with overlying gravels – outcrop belt of marine inner-shelf sands of the inner Middle Coastal Plain, overlain by upper delta plains of fluvial sands and gravels in New Jersey and on the Delmarva Peninsula . Sediments are Pliocene and Miocene age and largely include units of the Chesapeake Group. The original broad flat upland surface has not been completely dissected by developing stream networks. Local relief is less than 100 feet. Greater incision occurs near major tributaries that cut across the Middle Coastal Plain.

5) Inner Coastal Plain, upland sands and gravels – Deeply dissected innermost Coastal Plain adjacent to Fall Line, including sand and gravel caps on adjacent Piedmont hills. Local relief ranges from 100 to 150 feet. Sediments, which range in age from Miocene to Pliocene, are dominated by coarse fluvial sands and gravels overlying marine sands or saprolite of crystalline rocks. Surficial units are completely incised and there is no connectivity between upland surfaces on adjacent interfluves.

6) Inner Coastal Plain, dissected outcrop belt – outcrop and subcrop belt of lower Tertiary and Cretaceous formations with 250 to 300 feet of relief. Locally, older units may be covered with younger sands and gravels, particularly in the northern Delmarva Peninsula . Tertiary and Cretaceous units are generally deeply weathered where exposed. Some units are leached and oxidized to depths of tens of feet. There is widely contrasting variability in the permeability and geochemistry of units. These contrasts affect aquifer recharge and water-quality characteristics. The landscape is deeply dissected and streams typically cut into underlying units.

7) Alluvial and Estuarine Valleys – Incised valleys of major rivers that cut across the Coastal Plain, commonly perpendicular to regional strike. Scarps along the river valleys are connected to regional coastal scarps. Sediments range in age from Pliocene to Holocene, most are Pleistocene. Coarse-grained alluvial sediments fill deeper parts of the valleys. Upper portion of sequence is typically composed of fine-grained, organic-rich sediments deposited in alluvial flood plains or estuarine environments. In larger rivers there is a rich suite of minerals where sediment originates in the Piedmont and Appalachian Physiographic Provinces. Valleys in North Carolina are broader with greater volumes of alluvial fill than are valleys to the north that drain to the Chesapeake and Delaware bays, which are more deeply incised. Weathering is dominated by physical processes to the north and chemical processes to the south.

A preliminary evaluation of the surficial hydrogeologic framework is provided by comparison to available regional ground-water-quality data (Ator and others, 2000). As predicted by the framework, concentrations of major ions and nutrients typically differ significantly between Subregions I and IV. For example, phosphorus and ammonia plus organic nitrogen concentrations are higher in Subregion I, while nitrate concentrations are higher in Subregion IV. Water quality in Subregions II, VI, and VII is typically more variable and intermediate between those in Subregions I and VI. Limited data were available from Subregions III and V.