RELATIONSHIP BETWEEN THE PHYSICAL AND BIOGEOCHEMICAL DYNAMICS IN THE SHALLOW FRESHWATER-SALTWATER MIXING ZONE OF AN INTERTIDAL BEACH AQUIFER (CAPE HENLOPEN, DELAWARE)

Intertidal beach aquifers host a spatially and temporally dynamic mixing zone between fresh and saline groundwater. Seawater infiltrates vertically into the beach due to waves and tides and is driven seaward by upland gradients and mounding due to infiltration, forming a near-surface saline circulation cell (SCC). Seawater delivers oxygen and particulate carbon to support active biogeochemical cycling in the SCC. The geometry and position of the SCC is influenced by topography, geology, and hydrology of its setting, which in turn influences its biogeochemical reactivity by altering groundwater flowpaths, chemical gradients, and residence times on a number of characteristic timescales (tidal, spring-neap, annual). Multi-level porewater samplers were installed perpendicular to the shore across the beach near Cape Henlopen, Delaware, to determine the biogeochemical characteristics of the SCC and their relationship to the physical dynamics. Porewater was filtered to collect chlorophyll-a, particulate phosphorus, particulate nitrogen, and particulate carbon in order to determine the cross sectional distribution of these parameters and their relationship to flowpaths and the position of the SCC. Porewater samples were also incubated to determine changes in dissolved oxygen (an indicator of respiration) and nitrogen (an indicator of denitrification potential) using membrane inlet mass spectrometry (MIMS). Incubation experiments show higher in situ oxygen consumption rates in the SCC where seawater infiltration and chlorophyll-a (an indicator of reactive marine-derived organic matter) are highest. Because the supply of reactants is dependent on water, particle, and solute fluxes and mixing within the SCC, the location of reaction hotspots varies on tidal and seasonal time scales. A better understanding of nutrient dynamics in the SCC will improve understanding of nutrient fluxes associated with groundwater discharge to the ocean through beaches.