The groundwater-derived nutrient flux to estuaries and the coastal ocean can rival that of surface water inputs due to high groundwater nutrient concentrations. Groundwater inputs thus have an important impact on coastal water quality. Understanding patterns of variability in groundwater nutrient fluxes and the mechanisms generating this variability will help to inform coastal water resource management decisions. A suite of groundwater geochemical constituents has been monitored over the course of at least one year at two sites in coastal Georgia. Radium isotopes were used to determine the residence time of groundwater in these systems and estimate fluid fluxes. Two mechanisms for the observed variability in groundwater nutrient flux are highlighted here: tidal forcing and biological processes. Tidal cycling drives mixing of ground and surface water and export of high nutrient concentration groundwater. Nitrogen cycling in shallow sandy aquifers changes in phase with the spring/neap cycle such that NH₄⁺ concentration decreases and NO_x concentration increases at neap tide and the opposite occurs on spring tide.