ABSTRACT:

The intertidal zone of beach aquifers hosts biogeochemical transformations of terrestrially-derived nutrients that are mediated by reactive organic carbon from seawater infiltration. While dissolved organic carbon is often assumed the sole reactive organic carbon component, advected and entrapped particulate organic carbon (POC) is also capable of supporting chemical reactions. Retarded advection of POC relative to groundwater flow forms pools of reactive carbon within beach sediments that support biogeochemical reactions as dissolved solutes move across them due to transient groundwater flow. In this work, we simulate the contribution of POC to beach reactions and identify parameters that control its relative contribution using a groundwater flow model (SEAWAT) and reactive transport model (PHT3D). Results show transient contributions of POC to denitrification, as the spatial extent of the saline circulation cell varies over time due to changing hydrologic factors. A decrease in POC retardation and an increase in tidal amplitude during POC deposition resulted in POC expansion, which increased the relative contributions of POC to beach reactivity. Decreased hydraulic conductivity and increased tidal amplitude post-deposition decreased the utilization of POC for denitrification by allowing the oxic, saline water to completely encompass the pool of POC. Results highlight that POC is an intermittently-utilized source of carbon that displays complex spatial relationships with groundwater flow conditions and overall beach biogeochemistry. This work demonstrates that POC may be a periodically important, but overlooked contributor to biogeochemical reactions in carbon-poor beach aquifers. This resource contains the input and output files for the base case model.

ABSTRACT:

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.