Groundwater discharge and associated nutrient fluxes to the Delaware Bay at Cape Henlopen, Delaware
Date
2005
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Publisher
University of Delaware
Abstract
Groundwater discharge is a source of both fresh water and dissolved nutrients (nitrogen, phosphorus, and silica) to coastal waters. The fresh groundwater frequently carries higher nutrient concentrations than the receiving coastal waters and thus knowledge of the fluxes of nutrients from the fresh groundwater is important in defining mass balances in the coastal zone. This study focused on quantifying the total groundwater discharge at low tide from a seepage site at Cape Henlopen, Delaware, and on determining the nutrient fluxes from the groundwater to the adjacent Delaware Bay. The groundwater discharging from the beachface at Cape Henlopen is a mixture of “new” fresh groundwater and “recycled” estuarine water and thus should have characteristics intermediate to the two end-member waters, however, diagenetic reactions occurring within the beachface were found to modify the fluxes of some of the nutrient species. ☐ A novel technique for determining groundwater discharge was developed over the course of this project. Total discharge was collected from and section of the beach at low tide and measured and then parsed into freshwater and estuarine contributions using a salt balance. Average total discharge was 2.76 ± 1.08 L/min/m beachface at low tide and was highest in the fall and lowest in the summer. Freshwater discharge was 0.87 ± 0.43 L/min/m beachface and was at its minimum during the summer. While the estuarine component of discharge must vary due to inputs from tides and wave swash, fresh water discharge is thought to be constant over time scales of hours to days. ☐ The inputs of dissolved nutrients to the beachface (from fresh groundwater and recycled estuarine water) and the output from the beachface to the adjacent estuary were measured. Total dissolved nitrogen (TDN) fluxes to the beachface were frequently higher than the fluxes out of the beachface. Nitrate fluxes to the coastal waters were almost always less than the flux from the upland groundwater. The flux of ammonia from the beachface was always greater than the fluxes into the beachface. The fluxes of phosphate and total dissolved phosphorus (TDP) to the estuary were almost always less than the total dissolved input. The flux of silica from the beachface was frequently greater than the fluxes entering the beachface. ☐ Discrepancies between the input and the output fluxes are the result of the diagenetic reactivity of the beachface. Beachface diagenesis augmented or attenuated dissolved nutrient fluxes from upland groundwater and recycled estuarine water. The Cape Henlopen beachface plays a significant role in nutrient cycling and, therefore, the ecology of the intertidal and shallow subtidal zones of Delaware Bay at Cape Henlopen. ☐ The hydrological, geochemical, and biological interactions occurring within the beachface determine if, how, and when nutrients will be transported to coastal waters. It is clear that beachface dynamics must be included in studies of intertidal and coastal ecology, particularly in settings that support high rates of freshwater discharge.