The Choptank Coastal SEES Project: Biogeochemical monitoring at three spatial scales
The Choptank Basin is dominated by agriculture (60-80%) and has water quality issues in most of its smaller watersheds. The approach that the EPA Chesapeake Bay Program has adopted to improve water quality in streams is the application of agricultural and residential Best Management Practices (BMPs) to reduce transport of N, P, and sediment to estuarine waters. Motivated by the fact that few BMPs have been adequately tested at the watershed scale, we are testing whether the widespread application of BMPs within small (8-14 km2), agriculturally dominated watersheds will result in increased water quality. We have monitored 15 Choptank watersheds for chemistry and discharge from 2003-present and we have recently focused efforts in four of these sub-watersheds which are similar in land use. Stream chemistry has high baseflow total nitrogen (TN, 1.6-6.3 mg N L-1) and high stormflow total phosphorus (TP, 0.1-1.0 mg P L-1). The four watersheds are divided into three test watersheds (BMP classes of soil, water and nutrient) and one control. We are conducting monthly baseflow sampling at 3 scales: watershed, meso (within watershed), and field, and we are sampling storms 4-8 times per year at the watershed scale.
After 1.5 years, we have completed baseline data collection for the three sampling scales. Hydrology at the watershed scale is characterized by baseflow discharges of 0.1-0.2 m3s-1 with 20-30 storm events y-1with peak discharges of 1-7 m3s-1. The initial concentrations at the watershed scale were 1.6-6.3 mg TN L-1 and 0.01-0.05 mg TP L-1, whereas the mesoscale sites had TN ranging from 1.7-8.1 mg TN L-1 and TP was 0.09-0.20 mg P L-1. Samples taken on individual farm fields (field scale) had TN/NO3- values of 2.8-17.2 mg N L-1, and TP/PO4 ranged from 0.07-0.28 mg P L-1. Stormflow TN (8 events) at the outlet was 2.7-5.1 mg N L-1, and TP was 0.19-0.23 mg P L-1. Analysis of monthly TN and TP data for the 15 watersheds from 2003-2014 showed varying trends. TP did not increase significantly over this time period for any sites, whereas the TN data exhibited increasing concentrations over the entire timeframe, with a few sites increasing from 2003- 2008 and then slightly declining afterwards. The baseline data provides a good basis against which to test our hypotheses once widespread BMPs are applied throughout these areas.