What kind of water-quality information does the NAWQA Program provide? Water-quality assessments by a single program cannot possibly address all of the Nation's water-resources needs and issues. Therefore, it is necessary to define the context within which NAWQA information is most useful. • Total resource assessment-NAWQA assessments are long-term and interdisciplinary, and include information on water chemistry, hydrology, land use, stream habitat, and aquatic life. Assessments are not limited to a specific geographic area or water-resource problem at a specific time. Therefore, the findings describe the general health of the total water resource, as well as emerging water issues, thereby helping managers and decision makers to set priorities. • Source-water characterization-Assessments focus on the quality of the available, untreated resource and thereby complement (rather than duplicate) Federal, State, and local programs that monitor drinking water. Findings are compared to drinking-water standards and health advisories as a way to characterize the resource. • Compounds studied-Assessments focus on chemical compounds that have wellestablished methods of investigation. It is not financially or technically feasible to assess all the contaminants in our Nation's waters. In general, the NAWQA Program investigates those pesticides, nutrients, volatile organic compounds, and metals that have been or are currently used commonly in agricultural and urban areas across the Nation. A complete list of compounds studied is on the NAWQA Web site at water.usgs.gov/nawqa. • Detection relative to risk-Compounds are measured at very low concentrations, often 10 to 100 times lower than Federal or State standards and health advisories. Detection of compounds, therefore, does not necessarily translate to risks to human health or aquatic life. However, these analyses are useful for identifying and evaluating emerging issues, as well as for tracking contaminant levels over time. • Multiple scales-Assessments are guided by a nationally consistent study design and uniform methods of sampling and analysis. Findings thereby pertain not only to water quality of a particular stream or aquifer, but also contribute to the larger picture of how and why water quality varies regionally and nationally. This consistent, multiscale approach helps to determine if a water-quality issue is isolated or pervasive. It also allows direct comparisons of how human activities and natural processes affect water quality in the Nation's diverse environmental settings.

Evaluating long-term temporal trends in regional ground-water quality is complicated by variable hydrogeologic conditions and typically slow flow, and such trends have rarely been directly measured. Ground-water samples were collected over near-decadal and annual intervals from unconfined aquifers in agricultural areas of the Mid-Atlantic region, including fractured carbonate rocks in the Great Valley, Potomac River Basin, and unconsolidated sediments on the Delmarva Peninsula. Concentrations of nitrate and selected pesticides and degradates were compared among sampling events and to apparent recharge dates. Observed temporal trends are related to changes in land use and chemical applications, and to hydrogeology and climate. Insignificant differences in nitrate concentrations in the Great Valley between 1993 and 2002 are consistent with relatively steady fertilizer application during respective recharge periods and are likely related to drought conditions in the later sampling period. Detecting trends in Great Valley ground water is complicated by long open boreholes characteristic of wells sampled in this setting which facilitate significant ground-water mixing. Decreasing atrazine and prometon concentrations, however, reflect reported changes in usage. On the Delmarva Peninsula between 1988 and 2001, median nitrate concentrations increased 2 mg per liter in aerobic ground water, reflecting increasing fertilizer applications. Correlations between selected pesticide compounds and apparent recharge date are similarly related to changing land use and chemical application. Observed trends in the two settings demonstrate the importance of considering hydrogeology and recharge date along with changing land and chemical uses when interpreting trends in regional ground-water quality.

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Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32 Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8 Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88). Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). Altitude, as used in this report, refers to distance above the vertical datum. Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (µS/cm at 25°C). Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or micrograms per liter (µg/L).

This report is an element of the comprehensive body of information developed as part of the NAWQA Program. The program depends heavily on the advice, cooperation, and information from many Federal, State, interstate, Tribal, and local agencies and the public. The assistance and suggestions of all are greatly appreciated.