Water-Quality Assessment of the Potomac River Basin: Water-Quality and Selected Spatial Data, 1992-1996

The U.S. Geological Survey (USGS) implemented the National Water-Quality
Assessment (NAWQA) Program to describe the status and trends in the quality of
the Nation's surface water and ground water and to provide scientific
understanding of the natural and human-induced factors that affect water
quality. In 1991, twenty study units were started. Additional study units began
in 1994 and 1997. Data collection and analysis in each study unit follows
national guidelines for the NAWQA Program, but also addresses the issues most
important to that study unit. NAWQA is designed to produce a wealth of
water-quality information to assist policy makers and managers at the national,
state, and local levels in making more informed decisions. In 1991, the
Potomac River Basin study unit began planning assessment activities. The basin
was subdivided into eight subunits based on physiographic and geologic
characteristics, which were determined to be the most influential natural
factors affecting water quality. Water-quality sampling began in 1992. The
first high-intensity data-collection phase of the study unit spanned from 1993
to 1995; much of the data collection focused on the Great Valley Carbonate,
Piedmont, Triassic Lowlands, and Valley and Ridge subunits. Hundreds of
water-quality characteristics were measured in different media during this
time, including ground water, streamwater, streambed sediments, and aquatic
biological tissues. Fish communities and stream habitat also were sampled. In
addition, spatial data such as geology, land use, hydrography, and other
watershed characteristics were compiled into a geographic information system
(GIS) to support the assessment. After 1995 the project entered a period of
less frequent sampling called the low-intensity phase.

This report presents data collected and compiled during the first
high-intensity phase of the Potomac NAWQA study unit. Data are presented from
138 ground-water wells and 125 stream sites. Ground-water measurements compiled
in this report include chemical, physical, and water-level data. Streamwater
measurements compiled include chemical, physical, streamflow, bed-sediment
contaminants, aquatic-tissue contaminants, fish community, and selected stream
habitat data. Quality-control chemical data are also presented. Basinwide and
site-specific spatial data compiled by the study unit are also distributed with
this report.

The study design and sampling methods developed for the Potomac NAWQA study
unit are consistent with the national NAWQA guidelines (Gilliom and others,
1995). The design permits an integrated assessment of conditions at local,
regional, and national scales. For more information refer to the report,
"Design and Implementation of a Sampling Strategy for a Water-Quality
Assessment of the Potomac River Basin" (Gerhart and Brakebill, 1996). Sampling
methods and protocols for the NAWQA Program are documented in a series of
guideline documents. Citations for these can be found in the Selected
references section of this report.