DYNAMIC MIGRATION OF BIOGEOCHEMICAL REACTION ZONES IN AN INTERTIDAL BEACH AQUIFER

The intertidal zone of sandy beach aquifers hosts dynamic mixing zones between fresh groundwater and saline seawater. Wave and tidal action drive seawater up the beachface, which infiltrates into the sand and mixes with seaward-discharging fresh groundwater. This creates and sustains an intertidal circulation cell, defined by elevated salinity. The intertidal circulation cell is chemically reactive and has the potential to alter chemical characteristics of coastal discharge, impacting the coastal ecosystem. The geometry and position of the circulation cell are influenced by the topography, geology, and hydrology of its setting, in turn altering the groundwater flowpaths and delivery of reactive chemical species to a given location on a number of characteristic timescales (tidal, spring-neap, seasonal). Therefore, chemical reaction zones within the circulation cell are also transient and spatially dynamic. We studied the migration of reactive zones within the intertidal circulation cell over seasonal timescales by collecting porewater and sediment samples from multi-level wells and adjacent cores along a beach-perpendicular transect. Samples were analyzed for solute and particulate concentrations, and incubated to obtain rates of aerobic respiration and denitrification. Further, oxidation-reduction potential was monitored at a high frequency using platinum probes deployed along the transect to observe the migration of chemical reaction zones over short timescales. Results show a decoupled movement between the chemical patterns and salinity distributions, with different parts of the intertidal circulation cell responding to quicker hydrologic shifts whereas other parts responded to longer timescale changes. Understanding the spatiotemporal dynamics of beach aquifer reactivity and its potential impacts to the coastal environment in times of sea-level rise will be crucial in our ability to predict and manage nutrient fluxes to oceans.