Declining nitrate-N yields in the Upper Potomac River basin: what is really driving progress under the Chesapeake Bay restoration?

Abstract:

Reducing nutrient pollution of surface and coastal waters in the U.S. and elsewhere remains a major environmental and engineering challenge for the 21^st century. In the case of the Chesapeake Bay restoration, we still lack scientific proof that previous watershed-based management actions have been effective at reducing nonpoint-source nutrient loads from the land to this estuary in accordance with restoration goals. While the conventional wisdom is that implementation of best management practices (BMP’s) has turned the against nutrient pollution, we examined long-term (1986-present) nitrate-N trends in streams and major tributaries of the Upper Potomac River Basin (UPRB) and found that: 1) dramatic reductions in annual discharge-weighted nitrate-N concentrations and yields across the UPRB can be almost universally attributed to reductions in atmospheric N deposition as opposed to on-the-ground management actions such as implementation of BMP’s; 2) observed water quality changes comport with a modified kinetic N saturation model (MKNSM); 3) the MKNSM can separate the nitrate-N yield that is responsive to atmospheric deposition from a “legacy” yield; and 4) N saturation from atmospheric N deposition appears to be an inherently reversible process across most of the landscape. These unanticipated region-wide water quality benefits can be attributed to NO_x emission controls brought about by the 1990 Clean Air Act Amendments (and subsequent U.S. NO_X control programs) and reflect one of a very few water quality “success stories” in the Chesapeake Bay restoration; the results have important ramifications for the 2017 “mid-point assessment” that is part of the latest Chesapeake Bay Watershed Agreement.