Characterizing Temporal Dynamics of Isotope and Ion Chemistry in Groundwater Across a Barrier Island as Influenced by Rainfall and Tidal Cycles.

The Isle of Hope, near Savannah, Georgia, is a barrier island made up of sediments deposited during the beginning of the Pleistocene epoch. These sediments by the Atlantic coast in Georgia bear aquifer systems such as the Surficial Aquifer. This study was conducted to characterize spatial variability in the isotope and ion chemistry of groundwater on the island, and to evaluate spatial variability in the influence of rainfall recharge and tidal cycles on the same variables. We sampled groundwater in four wells along a transect that stretches from the west (most inland) to the east (closer to a marsh), respectively, as MW01, MW02, MW03, and MW04. The isotope chemistry of the samples was analyzed with Los Gatos Research IWA-45EP laser spectrometer, and the ion chemistry of the samples was analyzed with Thermo Fisher Aquion ion chromatographer. According to the isotope chemistry and the electric conductivity data, the groundwater of the four wells does not show the expected graduation or mixture of fresh water and saline water. The 18O and 2H of the wells increase to deplete in the order of MW01, MW03, MW02, and MW04. The electric conductivity of the wells increases in the order of MW04, MW01, MW02, and MW03. The readings of both high tide and low tide are nearly similar. The disorderly pattern of the groundwater chemistry can be explained by the variation of aquifer dynamics due to lithological variations laterally and vertically. In addition, human-made surficial irrigation systems across the island might have an effect as well. During rainfall events in winter, isotope values of the marsh and the well MW04 record similar values, possibly indicating groundwater discharge into the marsh.