Direct determination of total and fresh groundwater discharge and nutrient loads from a sandy beachface at low tide (Cape Henlopen, Delaware)

Abstract

Total groundwater discharge from a sandy seepage site to the Delaware Bay at Cape Henlopen, Delaware, was determined together with the associated nutrient loads by trapping the discharge from a length of the shoreline in a tidal pond at low tide and measuring the discharge of the trapped water through a weir at steady state. Salinity was used to parse the total groundwater discharge from the beachface at low tide into a “recycled” estuarine component and a “new” fresh groundwater component. Based on 16 measurements over 18 months, average total discharge and average freshwater discharge at low tide were found to be 2.76 ± 1.08 and 0.87 ± 0.43 L min−1 m−1 of shoreline, respectively. The estuarine component of discharge varied with the maximum height of the immediately preceding high tide. Owing to the control of freshwater discharge by the average upland hydraulic gradient away from the beach, individual measurements of freshwater discharge at low tide represent a good estimate of this discharge component over time scales of hours to days. Based on the nutrient concentrations in the discharge waters, nutrient loads from the beach to the adjacent estuary at low tide were 148 ± 79, 7.5 ± 5.1, and 197 ± 101 µmol min−1 m−1 of shoreline for total dissolved N, P, and Si, respectively. Groundwater discharge and these nutrient fluxes contribute significantly to the unusually productive benthic communities that inhabit the seepage and nearby intertidal zones.