The new analysis reveals mixed news about progress toward reducing nutrients over the past 31 years, particularly during the last decade.
Since 2000, nitrogen has been decreasing in the Susquehanna and Potomac rivers, while levels are nearly unchanged in the James and Rappahannock rivers. During the same period, phosphorus levels changed minimally in the Susquehanna, while there were moderate decreases in the Potomac and measurable increases and the James and Rappahannock.
Looking back even farther, scientists found a substantial improvement in pollution loads from the Patuxent River since 1978. Phosphorus from the Patuxent declined by 75 percent from 1978-2000, while nitrogen declined by about 26 percent during the same time period and an additional 15 percent from 2000-2008. These improvements are likely due to large investments in advanced wastewater treatment facilities.
Conversely, there was a 53 percent increase in nitrogen from the Choptank River from 1978-2008. Much of the increase is attributed to groundwater flowing into the river from deep below the land surrounding the river.
The new method takes multiple factors into consideration: seasonality, variations in river flow, and long-term trends driven by human activities, such as wastewater treatment and land management.
“When we analyze long-term nutrient trends for the Chesapeake Bay or other major water systems, it’s important that we consider [river] flow variations,” said Robert Hirsch, a USGS research hydrologist who led the development of the new method. “This new method enables us to remove this source of variation from the data and get a much clearer picture of the effect of human activities, including nutrient-management actions, on nutrient delivery from these watersheds to the Bay.”
Methods that do not consider variations in river flow can paint a much different picture of long-term nutrient trends in the Bay.
For example, 1999-2002 were very dry years throughout the Bay watershed. As a result, nutrient delivery to the Bay was relatively low and conditions in the Bay appeared to be much improved.
These years were followed by extremely high flow conditions in 2003, and then a series of progressively drier years from 2004 through 2008. The 2003 data showed very poor conditions, but the subsequent years’ data suggest slow improvements from one year to the next.
“These apparent changes were largely the consequence of differences in flow,” said Hirsch. “This new method helps us to see past these random year-to-year changes and get at the underlying long-term changes taking place.”
“The new USGS method will allow the Chesapeake Bay partners to better assess progress toward reducing the delivery of nutrients and sediment to the Chesapeake Bay,” said Rich Batiuk, the Chesapeake Bay Program’s associate director for science. “This method, based on monitoring data, will improve accountability regarding the nutrient reductions needed to meet our restoration goals for the Bay.”
USGS, a Chesapeake Bay Program partner, works with other partners to collect data for the Bay Program’s Nontidal Water Quality Network and provides critical science to the Bay Program partnership. Learn more about USGS Chesapeake Bay activities at http://chesapeake.usgs.gov.
The new analysis is available online in a report from the Journal of the American Water Resources Association.