SEDIMENT AND POREWATER OXYGEN DEMAND IN A SANDY BEACH AQUIFER, CAPE HENLOPEN, DELAWARE

The intertidal zone of sandy beach aquifers hosts spatially and temporally dynamic mixing zones between fresh and saline groundwater. Seawater, driven up the beachface by wave and tidal action, infiltrates into the sand and meets the seaward-discharging fresh groundwater, creating and sustaining a hydraulically-driven mixing zone. Seawater supplies oxygen and reactive organic carbon in dissolved and particulate forms, supporting oxic respiration in the aquifer. The geometry and position of mixing zones are influenced by the topography, geology, and hydrology of its setting. Therefore, groundwater flowpaths that determine the transport of reactive organic matter (dissolved, entrained, and entrapped) and dissolved oxygen to a given location within the mixing zone are altered on a number of characteristic timescales (tidal, spring-neap, seasonal). Samples were collected from multi-level wells and adjacent sediment cores along a transect perpendicular to the shore on a beach near Cape Henlopen, Delaware, to map oxic respiration of organic matter and to determine the relationship between respiration and the physical characteristics of the mixing zone. Within the mixing zone, unfiltered porewater incubations show higher in-situ oxygen consumption rates in response to elevated dissolved and entrained organic carbon concentrations. Sediment incubations, however, show less correlation between dissolved carbon concentration and respiration rates, suggesting that particulate carbon entrapped within the sediments can also contribute to and alter bulk reactivity. Because oxic respiration depends on the availability of both carbon and oxygen, understanding the divergent sources and flowpaths of gases and organic matter is important in characterizing reaction hotspots. A better understanding of the coupled physical and biogeochemical dynamics of the intertidal mixing zone will also improve understanding of nutrient fluxes to estuaries and the ocean through beaches.