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The National Water-Quality Assessment (NAWQA) Program

Waites Run Washington, DC Corn House

The Potomac River Basin and Delmarva Peninsula (PODL) Study Unit

The National Water Quality Assessment (NAWQA) is a national program of the U.S. Geological Survey designed to provide consistent and reliable information on the quality of streams, groundwater, and aquatic ecology in the United States. Primary goals of the NAWQA program include determining the status of streams and groundwater, identifying changes over time, and understanding interacting natural and human influences on observed status and trends. NAWQA activities are focused primarily in more than 50 study units across the Nation, including the Potomac River Basin and Delmarva Peninsula.

NAWQA has been active in the Mid-Atlantic region since 1987 when the Delmarva Peninsula was selected as a groundwater pilot project for the NAWQA Program. The pilot study on Delmarva continued until 1991, at which time study began in the Potomac River Basin as part of Cycle I of full NAWQA Program implementation. High-intensity NAWQA activities continued in the Potomac River Basin through 1996. The NAWQA program returned to Delmarva as a Cycle I study for a special abbreviated project from 1999-2001. In 2001 the Potomac River Basin and Delmarva Peninsula (PODL) study units were combined into one study unit as part of Cycle II of the NAWQA program.

PODL Study Unit

The Potomac River Basin and Delmarva Peninsula

The PODL study unit covers 20,728 square miles in the Mid-Atlantic region and includes parts of Maryland, Delaware, Virginia, West Virginia, Pennsylvania, and the District of Columbia. The Potomac River Basin portion of the study unit covers 14,760 square miles, including 11,600 square miles above the Fall Line at Washington, D.C. The Coastal Plain portion of the study unit is drained by a series of predominantly tidal streams. The non-tidal portions of these streams drain, on average, less than 25 square miles. The largest non-tidal stream on the Delmarva Peninsula is the Choptank River, which drains a watershed of 119 square miles. Most of the study unit drains into Chesapeake Bay, the largest inland estuary in the United States and an important ecological and economic resource. The eastern part of the Delmarva Peninsula drains into Delaware Bay and directly to the Atlantic Ocean.

Physiographic Provinces

The PODL is geologically diverse, including portions of seven physiographic provinces or subprovinces: the Appalachian Plateau, Valley and Ridge, Great Valley, Blue Ridge, Piedmont, Triassic Lowlands, and Coastal Plain. The Appalachian Plateau, Valley and Ridge, Great Valley, and Triassic Lowlands are underlain primarily by folded sedimentary rocks. These rocks are mainly siliciclastic, although carbonate rocks dominate the Great Valley and some narrower valleys to the west. The Piedmont and Blue Ridge Provinces are underlain primarily by older crystalline rocks, although some carbonate rocks also occur in the Piedmont near Frederick, Maryland. The Coastal Plain is underlain by a wedge of unconsolidated sediments that thickens from 0 feet at the Fall Line to more than 8,000 feet in places along the Atlantic Coast. These sediments are texturally diverse, and form a vertical sequence of alternating aquifers and associated confining units.

About half (51 percent) of the PODL is forested; other important land uses include agriculture (37 percent) and urban areas (5 percent). Six percent of the area is classified as wetlands. Forested areas occur throughout the study unit, but are concentrated on the steep slopes of the Appalachian Plateau, Valley and Ridge, and Blue Ridge Provinces, as well as in portions of the Coastal Plain west of Chesapeake Bay. Major agricultural areas are located in the Great Valley, Piedmont, Triassic Lowlands, and the Coastal Plain, particularly on the Delmarva Peninsula (Table 1). The most productive agricultural areas often coincide with the most permeable surficial geologic materials: including carbonate rocks and unconsolidated sands and gravels. Major agricultural products include corn, soybeans, and poultry. Wetlands are concentrated in tidal and non-tidal areas of the Coastal Plain and along major river valleys in other physiographic provinces. The Washington, D.C. metropolitan area is the major urban center; Wilmington, Delaware is the next largest. Both cities straddle the Fall Line. Urban areas associated with Washington, D.C. are located in the Potomac River Basin in both the Piedmont and the Coastal Plain. The urban area associated with Wilmington, Delaware is located in the Coastal Plain in the northern part of the Delmarva Peninsula. Both areas are experiencing rapid development and growth. The PODL includes several other smaller cities that are also undergoing urban growth and industrialization to varying degrees.

Table 1. Major Land Uses in the physiographic provinces and sub-provinces of the PODL study unit.
Province or Subprovince Urban (%) Agricultural (%) Forest (%)

Appalachian Plateau




Valley and Ridge (not including the Great Valley)




Great Valley




Blue Ridge








Triassic Lowlands




Atlantic Coastal Plain




Both streams and groundwater are used for drinking-water supplies in the PODL. The distribution of water use closely follows population. About 76% of the total drinking water is supplied by surface waters. Most of this water is withdrawn directly from the Potomac River for use in urban areas around Washington, D.C. Groundwater supplies the remaining 24%, including 100% of the drinking water for private domestic use. Groundwater is the predominant source of supply in the Coastal Plain and the Great Valley, which are underlain by the most productive aquifers (permeable sediments and carbonate rocks, respectively). Surface water from the adjacent Piedmont also provides drinking water to nearby areas of the Coastal Plain, and some drinking water in the Great Valley is withdrawn from the Shenandoah River.

Contact Information:

Scott W. Ator, Hydrologist
Chief, PODL Study Unit

U.S. Geological Survey
5522 Research Park Drive
Baltimore, MD 21228

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Page Last Modified: Wednesday, January 18, 2017