Beach aquifers are hosts of dynamic mixing zones between fresh groundwater and saline seawater. Seawater, driven up the beachface by waves and tides, infiltrates into the aquifer and meets the seaward-discharging fresh groundwater, creating and maintaining a reactive intertidal circulation cell. The intertidal circulation cell is highly dynamic and has been shown to respond to hydrologic, geologic, and
topographic changes over various time scales (waves, tides, and seasons).
Spatial patterns of groundwater reactivity in an intertidal beach aquifer
Within the cell, land-derived nutrients delivered by fresh groundwater are transformed or attenuated. We investigated the relationship between physical flow and mixing processes with porewater samples and incubation experiments. Biogeochemical reactions within the circulation cell were highly related to flowpaths, with heightened oxic respiration near the infiltration zone and progressive dominance of denitrification near the discharge zone. The results of this project were publish in JGR Biogeosciences and selected for an EOS Research Highlight.
Migration of beach aquifer reaction zones : Transient seasonal patterns of porewater and sedimentary chemical conditions
Extensive characterization of porewater and sediments spanning two years revealed the seasonal migration of reaction zones. While oxic respiration closely followed the changes to groundwater flowpaths indicated by salinity patterns, spatial locations of denitrification was more variable. This was partially attributed to the heterogeneous distribution of reactive organic carbon within the beach, due to the filtration effect of beach sediments. A numerical model is being developed to explore this phenomena. Manuscripts detailing the outcomes are currently under preparation for submission to JGR Biogeosciences.
Short-term shifts in redox conditions
Salinity is a non-reactive, conservative tracer of groundwater flow. While changes to groundwater flow and subsequently salinity can induce geochemical shifts within the beach aquifer, fieldwork employing high-frequency, in-situ redox potential sensors revealed minute-scale redox fluctuations even under relatively static flow regimes.
Future research initiatives will investigate changes to biogeochemical conditions and ultimately groundwater quality in areas of complex hydrologic conditions. Interested frontiers with such hydrologic dynamics include: regions with aquifer compaction and subsidence, coastal zones with permanent and temporally-variable groundwater salinization, and aquifers with heterogeneous geochemical conditions.