Assateague Island

Objectives

The proposed project is a three-year fixed-station monitoring study to estimate the nitrate load transported by streamflow discharge, and the seasonal variability of that load to the coastal bays system.

Statement of Problem

The lack of information regarding ground-water nutrient loads has been identified repeatedly as a top priority need by the scientific and resource management community (Maryland Department of the Environment, 1993; Beider et al., 1996; Bohlen et al., 1997; Bohlen and Boynton, 1998). Recent studies suggest that the stream baseflow portion of the groundwater nitrate load to the coastal bays may be the most significant component of ground-water nutrient loads that affect the health of the bays. Dillow and Greene (1999) have estimated that 76 percent of the nitrate load to the coastal bays from ground-water discharge (196,000 lbs/yr as N) comes from stream baseflow, with the remaining ground-water nitrate load being derived from direct ground-water discharge to the bays.

Site-specific information is needed to describe the nitrate load in local streamflow discharge. The lack of these data restricts the ability of NPS, local, and state resource managers to propose and implement specific, effective guidelines or policies for best-management practices aimed at reducing nitrate inputs. Park management must be able to refer to direct cause-and-effect relations between land use and impacts on park resources supported by data.

Ongoing park monitoring programs include a limited effort to assess water quality in Sinepuxent and Chincoteague Bays and to monitor SAV distribution and health. Ongoing USGS projects have measured water quality in wells and non-tidal streams in the Chincoteague and Newport Bays watershed. Further efforts to characterize direct ground-water nutrient discharge to the aquatic environment of ASIS through real-time water quality sensing and ground-penetrating radar were carried out during spring and summer 2001. All of these programs provide useful information, but none of them provide the much-needed information on the total nitrate load from stream baseflow for any of the coastal bays tributaries. Information from such a study could be combined with historical and contemporary data and interpretations from coastal bays monitoring and research programs to enhance the accuracy of predictions of nitrate loads throughout the area and to provide insights into the pathways and processes of nitrate delivery to the bays.

Strategy and Approach

Previous seasonal studies of surface-water nutrient concentrations on Delmarva have found highest nitrate in winter, associated with high base flow and low uptake, and high phosphate in summer, associated with runoff from flashy storm events and active remineralization. An effective design for continuous flow monitoring and periodic water-quality sampling should capture the range of seasonal variability.

Bassett Creek drains directly into Chincoteague Bay. Equipment to continuously monitor stream stage, water temperature, specific conductance, pH, and dissolved-oxygen content will be installed and calibrated on Bassett Creek at the Maryland Route 113 crossing. Data from these monitors, along with periodic nutrient sampling and streamflow measurement, will be used to calculate the total nitrate load, and its distribution over time, from the gaged portion of the watershed. The Bassett Creek site will be instrumented with a suite of sensors that will continuously measure and record stream stage, water temperature, specific conductance, pH, and dissolved-oxygen content at 15-minute intervals. Periodic measurement of streamflow will be used to develop a stage-discharge relation for the site so that streamflow can be computed. Water samples will be periodically collected and analyzed for nutrient concentration. The resulting nutrient concentration values will be related to concurrent values of other measured parameters to estimate nitrate concentrations in the stream water at 15-minute intervals (Christensen et al., 2000, Christensen, 2001). The load for each time interval will be calculated by multiplying the estimated nitrate concentration by the discharge for the interval computed from the stage-discharge relation established for the site. Total loads for any given period of time can be calculated by summing the loads computed for each measurement interval during the specified period. The accuracy of the continuous waterquality sensors will be checked on a monthly basis and corrected for any detected drift. Probe maintenance will also be performed monthly, including inspection and cleaning to remove organic films and sediment.