PATTERNS IN NITRATE LOADS IN AN AGRICULTRUALLY-IMPACTED STREAM AND IMPLICATIONS FOR BEST MANAGEMENT PRACTICES (BMPS)

In order to meet State, Federal, and Chesapeake Bay Agreement goals, nutrient loading of streams in the Chesapeake Bay watershed (CBW) must decrease. Best management practices (BMPs) for reducing nitrate loads can be ineffective if there are insufficient funds for correct implementation or a lack of understanding of the source and delivery mechanisms of nitrates in individual watersheds. However by characterizing the watershed first and then using the most effective type and placement of BMPs, nitrate loading can be cost-effectively reduced.

Warrior's Mark Run (WMR) is an agriculturally-impacted stream in the CBW, originating in the Appalachians and flowing through the karst landscape of central Pennsylvania. We began investigating water quality of WMR in May, 2002, and by the spring of 2005, we had 10 sampling locations, from headwaters to mouth. At each sampling location, we measured physical characteristics of the stream and analyzed nitrate concentration approximately 75 times/year. Using these data, we calculated the total nitrate loading for 11 continuous stretches of the stream. For each stretch, we also performed GIS analysis to calculate areas for the sub-watershed and agricultural land use within the sub-watershed.

These analyses indicate that delivery of nitrates to the stream is primarily through groundwater. Over 83% of the nitrate loading on this stream is attributable to baseflow. Following storm events, there is typically a 1 to 4 day lag between end of precipitation and increased nitrate loads. Although nitrate loading is typically highest during the summer, there have been extended periods in winters and springs during which total nitrates exceeded average summer loads. Thus, BMPs for this watershed should address nitrate loading of groundwater throughout the year.

There are discrete stretches in the stream that have high nitrate loading patterns over multiple years. These may reflect nutrient management practices and/or small scale aquifer characteristics. For example during the summer of 2005, on WMR, approximately 75% of the nitrate loading occurred over three stretches of stream which account for 20.4% of the watershed area and 31.2% of the stream length. These areas that contribute a large proportion of the nitrate load should have high priority for the implementation of groundwater quality BMPs.