chapter  7
32 Pages

The Choptank Basin in Transition

Abstract .......................................................................................................................................... 136 7.1 Introduction .......................................................................................................................... 136 7.2 Study Site .............................................................................................................................. 137 7.3 Methods ................................................................................................................................ 139

7.3.1 Hydrology ................................................................................................................. 139 7.3.2 Surface Water Chemistry.......................................................................................... 139 7.3.3 Drainage Control Structures ..................................................................................... 141 7.3.4 Groundwater Piezometers ......................................................................................... 141 7.3.5 Excess N2, O2, N2O, and CH4 ................................................................................... 142 7.3.6 Estuarine Water Quality Data .................................................................................. 143

7.4 Results ................................................................................................................................... 145 7.4.1 Regional Hydrology .................................................................................................. 145 7.4.2 Water Quality Trends: Nontidal Monitoring Stations .............................................. 147 7.4.3 Water Quality Drivers of Change ............................................................................. 148 7.4.4 Agricultural N and P Reduction ............................................................................... 151 7.4.5 Estuarine Water Quality Conditions ......................................................................... 157

7.5 Discussion ............................................................................................................................. 158 7.5.1 Water Quality Status ................................................................................................. 158 7.5.2 Water Quality Improvement ..................................................................................... 159

Acknowledgments .......................................................................................................................... 162 References ...................................................................................................................................... 162

The Choptank Basin and Estuary are located on the Delmarva Peninsula in the mid-Atlantic coastal plain. The regional hydrology is characterized by nearly uniform seasonal rainfall but large seasonal variations in temperature, evapotranspiration, groundwater levels, and stream discharge. Water quality in nontidal streams is largely determined by agricultural land use and animal feeding operations, and nitrogen (N) and phosphorus (P) concentrations have been increasing for decades. Inputs from nontidal streams, together with increasing human populations and wastewater discharges, have resulted in degraded estuarine water quality, including increases in chlorophyll a in surface waters and declining oxygen in bottom waters. Attempts to reduce losses of N and P from nontidal streams in agricultural areas have met with limited success. One targeted watershed in the Choptank Basin showed stabilized concentrations of base flow N, along with small decreases in base flow P, a decade after extensive application of some best management practices (BMPs) in contrast to the nearby Greensboro watershed which was not targeted for BMPs and exhibited increases in base flow N and P. An attempt to improve water quality using increased stream buffers has yet to be successful, probably because new stream buffers represented only an 11% increase over existing ones.