Nutrient inputs to the Potomac River Basin are related to land use. Agricultural areas receive the largest amounts of nutrients because manure and fertilizer applications comprise 45 percent of nitrogen and 93 percent of phosphorus inputs. In most waters of the Potomac River Basin, concentrations of nutrients do not pose a threat to human health or wildlife. Elevated nitrogen concentrations in streams and ground water are common in areas of intensive row cropping, such as the northeastern part of the Potomac River Basin, and areas underlain by carbonate bedrock, such as the Great Valley. Organic nitrogen and phosphorus concentrations are typically low, except in streams during high flows .
Pesticides can make water unfit to drink and cause adverse ecological effects in streams.
Commonly used pesticides are present in ground water in the Potomac River Basin, but in most cases at concentrations that are not threatening to human health. More pesticides were detected in streams than in ground water, but only rarely at concentrations threatening to aquatic life. Pesticides were commonly detected in agricultural areas of the Potomac River Basin, particularly in areas of intense crop production such as the corn-producing northeastern counties and in the Great Valley. Samples from forested areas rarely contained detectable pesticides. Maximum concentrations of most pesticides occur in streams during the spring and early summer months, coincident with their application to fields, although atrazine and metolachlor are present year round in streams in agricultural areas.