Nutrient and suspended-sediment trends, loads, and yields and development of an indicator of streamwater quality at nontidal sites in the Chesapeake Bay watershed, 1985-2010
The U.S. Geological Survey (USGS) updates information on loads of, and trends in, nutrients and sediment annually to help the Chesapeake Bay Program (CBP) investigators assess progress toward improving water-quality conditions in the Chesapeake Bay and its watershed. CBP scientists and managers have worked since 1983 to improve water quality in the bay. In 2010, the U.S. Environmental Protection Agency (USEPA) established a Total Maximum Daily Load (TMDL) for the Chesapeake Bay. The TMDL specifies nutrient and sediment load allocations that need to be achieved in the watershed to improve dissolved oxygen, water-clarity, and chlorophyll conditions in the bay. The USEPA, USGS, and state and local jurisdictions in the watershed operate a CBP nontidal water-quality monitoring network and associated database that are used to update load and trend information to help assess progress toward reducing nutrient and sediment inputs to the bay. Data collected from the CBP nontidal network were used to estimate loads and trends for two time periods: a long-term period (1985-2010) at 31 "primary" sites (with storm sampling) and a 10-year period (2001-10) at 33 primary sites and 16 "secondary" sites (without storm sampling). In addition, loads at 64 primary sites were estimated for the period 2006 to 2010. Results indicate improving flow-adjusted trends for nitrogen and phosphorus for 1985 to 2010 at most of the sites in the network. For nitrogen, 21 of the 31 sites showed downward (improving) trends, whereas 2 sites showed upward (degrading) trends, and 8 sites showed no trends. The results for phosphorus were similar: 22 sites showed improving trends, 4 sites showed degrading trends, and 5 sites indicated no trends. For sediment, no trend was found at 40 percent of the sites, with 10 sites showing improving trends and 8 sites showing degrading trends. The USGS, working with CBP partners, developed a new water-quality indicator that combines the results of the 10-year trend analysis with results from a greater number of sites (64 primary sites) where loads and yields of total nitrogen and phosphorus and sediment could be calculated. The new indicator shows fewer significant trends for the 10-year time period than for the long-term time period (1985-2010). For 2001-10, total nitrogen trends were downward (improving) at 14 sites and upward (degrading) at 2 sites; no trend was found at 17 sites. For total phosphorus, 12 sites showed improving trends, 4 sites showed degrading trends, and 17 sites showed no trend. For total sediment, most sites (21) did not exhibit a significant trend; 3 sites showed improving trends, and 10 sites showed degrading trends. Few significant trends were seen at the 16 secondary sites: improving trends for total nitrogen at 4 sites, improving trends for total phosphorus at 2 sites, and a degrading trend for sediment at 1 site. Total streamflow to the Chesapeake Bay was 20 percent higher in 2010 than in 2009 and is considered to be within the normal range of flow, whereas annual streamflow at 28 sites was greater in 2010 than in 2009. No trends in daily streamflow were detected at the 31 long-term sites. Combined loads for the farthest downstream nontidal monitoring sites (called "River Input Monitoring sites") increased 33 percent for total nitrogen, 120 percent for total phosphorus, and 330 percent for total sediment from 2009 to 2010. The large increase in phosphorus and sediment loads in 2010 was caused in large part by two large storm events that occurred during the spring in the Potomac River Basin. Yields (load per watershed area) of total nitrogen in the Chesapeake Bay watershed decreased from north to south (New York to Virginia). No spatial patterns were discernible for total phosphorus or sediment.