Biogeochemical reactions within intertidal zones of coastal aquifers, induced by the mixing between fresh groundwater and saline seawater, have been shown to alter the concentrations of terrestrial solutes prior to their coastal discharge. In organic-poor sandy aquifers, the input of marine organic matter from infiltrating seawater has been attributed to active biogeochemical reactions within the sediments. However, while the seasonality of surface water organic carbon concentrations (primary production) and groundwater mixing patterns have been well-documented, there has been limited speculation on the contributions of particulate organic carbon pools within the sediments that arise from transient hydrologic conditions. To understand the relationship between physical movements of the circulation cell and the seasonal migration of geochemical patterns, beach porewater and sediment samples from six field sampling events spanning two years were analyzed. While oxygen saturation, oxygen consumption rates, and silica distributions closely followed the seasonally-dynamic salinity, other chemically-reactive parameters (pH, ORP) and nutrient characteristics (N distributions, denitrification rates, reactive organic carbon distributions) were unrelated to contemporaneous salinity patterns. Particulate organic matter was distributed in pools within the aquifer due to the filtration effect of sediments, contributing to the divergence of chemical patterns from salinity patterns via nutrient release and leaching. Together, the results present the asynchronous movement of chemical conditions to salinity patterns due to the divergent transport pathways between solutes and particles arising from transient hydrologic forcing.
submarine groundwater discharge,coastal aquifer,diagenesis,organic matter,biogeochemistry,SGD,beach aquifer
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