GEOCHEMISTRY OF A GULF COASTAL AQUIFER SYSTEM, BALDWIN COUNTY, ALABAMA

Coastal aquifers serve as important water resources in many regions of the world. Such aquifers are frequently subjected to the stresses of groundwater withdrawals to serve rapidly growing populations. In this study we determine the major ion and stable isotope geochemistry of groundwaters in Gulf Coastal Baldwin County, Alabama and use these geochemical data to discern much about the groundwater hydrology of the region. The Holocene–Miocene aquifer system of the upper, unconfined A2 aquifer and the deeper, confined A3 aquifer in Baldwin County, Alabama, is the major source of freshwater to southwestern Alabama and is paramount to the sustainability of both tourism and agricultural enterprises in the region. Groundwater withdrawals have been increasing in the county and will likely increase in the future. The calculated recharge rate of 39 cm yr^-1 to the upper, unconfined A2 aquifer provides an abundant freshwater resource, but this rapid recharge also serves the important function of acting as a barrier to saltwater intrusion. The aquifers' seaward flow must be maintained to preserve the freshwater resources. We have collected and analyzed a series of groundwater samples from these coastal aquifers over a multi-year period. Unlike many other southeastern Coastal Plain aquifers, these siliclastic aquifers contain primarily sodium chloride-rich groundwater with low titration alkalinities and low pH. Major cation and anion data suggest that the groundwater reflects a significant influence of marine aerosol input. Stable isotope data are similar to modern local precipitation but also suggest differences in timing or location of recharge for the two aquifers. Quartz and other silicate weathering is an important control of groundwater chemical composition, with a silicate addition rate to the groundwater of 1.7×10^-15 mol L^-1 s^-1. Radiocarbon ages of these groundwaters are as young as 375 years B.P. in the upgradient region of the shallower A2 aquifer and as old as 6630 years B.P. downgradient, near the Gulf Coast, in the deeper A3 aquifer. Geochemical differences among the groundwaters suggest that possible structural control of flow isolates the south Baldwin County groundwaters from the regional groundwaters.