National Water-Quality Assessment--Potomac River Basin: Nitrogen Sources and Nitrate Loads in Major Watersheds of the Upper Potomac River Basin

Abstract

Estimates of nitrogen sources and nitrogen loads are needed for the development of nutrient management strategies that are an important component of the effort to restore water quality and ecological conditions in the Chesapeake Bay and the Potomac Estuary. Nitrogen sources and loads have been calculated for the upper Potomac River at Washington, D.C., but not for the major tributaries. Nitrogen inputs and corresponding nitrate loads calculated from 1970-90 data show large differences among major tributaries to the upper Potomac River, including the North and South Branch Potomac Rivers, Cacapon River, Conococheague Creek, Shenandoah River, and Monocacy River. During 1990, approximately 109,000 tons of nitrogen was input to the upper Potomac River watershed upstream from Washington, D.C.: 22.0 percent from atmospheric deposition, 38.5 percent from animal manure, 30.5 percent from commercial fertilizer, 6.5 percent from municipal and industrial discharges, and 2.5 percent from other nonpoint sources. Nitrogen inputs averaged 9.35 tons per square mile (ton/mi2) for the entire upper Potomac River watershed and ranged from 4.99 ton/mi2 in the North Branch Potomac River watershed to 14.2 ton/mi2 in the Monocacy River watershed. Atmospheric deposition was greatest in the forested South Branch Potomac and Cacapon River watersheds, where deposition comprised about 50 percent of the nitrogen input even though the total nitrogen input rate was low. Total nitrogen inputs were greatest in agricultural watersheds, although inputs from fertilizer and manure differed. Manure inputs were largest in the Great Valley watersheds, with manure comprising 56 percent of the nitrogen input to the Shenandoah River watershed. Fertilizer comprised 54 percent of the input to the Monocacy River watershed and 37 percent of the input to the Conococheague Creek watershed. Urban and industrial inputs were greatest to the North Branch Potomac River, but accounted for only 42 percent of the nitrogen input.

The long-term average-annual nitrate load of the upper Potomac River is 19,300 tons, or 1.65 ton/mi2. Major tributaries, however, contribute disproportionate nitrate loads because of differences in land use and nitrogen sources. The North Branch Potomac, South Branch Potomac, and Cacapon Rivers, three primarily forested tributaries, contribute proportionally small nitrate loads and together contribute 29 percent of the streamflow and only 17.5 percent of the nitrate load measured at Washington, D.C. Major tributaries in agricultural areas discharge greater nitrate loads than tributaries in forested areas. Shenandoah River discharges an average-annual nitrate load of 4,520 tons, Monocacy River discharges 2,330 tons, and Conococheague Creek discharges 2,320 tons. Together, these three watersheds contribute 36 percent of the streamflow and 47 percent of the nitrate load from the upper Potomac River. The relation between estimated nitrogen inputs and nitrogen loads in tributaries is complex, particularly in the agricultural watersheds. Nitrate yields are highest in agricultural watersheds, and are greatest where fertilizer inputs are high. Nitrate yield ranges from 0.9 ton/mi2 at Cacapon River to 4.7 ton/mi2 at Conococheague Creek. The large differences in nitrogen sources and loads among major upper Potomac River tributaries indicate that nutrient-reduction goals can best be achieved by implementing tributary-specific strategies.

Citation

Blomquist, J. D., and Fisher, G. T., 1994, Nitrogen sources and nitrate loads in major watersheds of the Upper Potomac River Basin [abs.] : EOS, Transactions of the American Geophysical Union, v. 75, n. 16, p. 165, Supplement with abstracts for the 1994 Spring Meeting, Baltimore, Md.