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Ground-water samples collected from 97 wells in four subunits of the Potomac River Basin were analyzed for selected nitrogen and phosphorus species (nitrate, ammonia, organic nitrogen, and orthophosphate) as part of the National WaterQuality Assessment program of the U.S. Geological Survey. As essential nutrients, nitrogen and phosphorus are used in natural and synthetic compounds to enhance plant growth. When unused by plants, these compounds may seep into the ground water. Ground water containing elevated concentrations of nitrogen or phosphorus can cause health problems when consumed, and adverse environmental impacts when discharged to streams. Nitrate was detected in ground water in all four subunits, typically at concentrations indicative of anthropogenic sources. In each subunit, most samples contained detectable ammonia and about half contained detectable orthophosphate. Concentrations of ammonia and orthophosphate were typically lower than would be expected from anthropogenic sources. Of the 97 samples collected, only 1 contained detectable organic nitrogen. A network of 25 to 30 randomly selected wells was sampled within each of four subunits of the Potomac River Basin that were defined mainly by physiography and lithology. In the Valley and Ridge and Great Valley Carbonate subunits, agricultural areas were targeted exclusively. In the Piedmont and Triassic Lowlands subunits, the entire subunits were sampled, regardless of land use. The maximum concentration of each nutrient in natural ground water was estimated on the basis of statistical distributions of nutrient concentrations. Ground-water nutrient concentrations were compared to field-mapped land-use data within 0.25 mile of each sampled well and to countylevel agricultural census data for 1992. The concentrations of nitrate in ground water are higher in agricultural and urban areas of the Potomac River Basin than in forested areas, possibly because of anthropogenic sources in these areas. Among agricultural land uses, crop production contributes more nitrate to ground water than does livestock grazing, although both are probable nitrate sources. Ammonia concentrations are higher in forested areas, mainly in the Valley and Ridge subunit, than in agricultural or urban areas. This is possibly a result of the incomplete nitrification of ammonia that is formed naturally from organic nitrogen in leaves and other natural debris. Orthophosphate concentrations are typically low in ground water in the four sampled subunits and cannot generally be related to land use. Bedrock lithology affects ground-water flow in fractured media and is therefore important when considering the movement of dissolved nutrients through the saturated zone. In agricultural areas, ground-water nitrate concentrations are higher in carbonate areas than in areas underlain by other rock types. This may indicate that the relatively large solution cavities that typically develop in carbonate rocks can allow surficial contaminants such as nitrate to be transported more quickly and easily in those aquifers than in other fractured-rock systems; however, it may merely reflect the fact that more nitrogen is applied per unit area to carbonate areas of the Potomac River Basin than to other areas. In areas underlain by noncarbonate rocks, surface topography reflects the general direction of ground-water flow and should therefore be considered when attempting to relate land use to ground-water quality. Relation of Nitrogen and Phosphorus in Ground Water to Land Use 1

Relation of nitrogen and phosphorus in ground water to land use in four subunits of the Potomac River basin