In order to determine the Gulf Coast hydrologic cycle we analyzed rainfall and groundwater from the vadose and phreatic zones in central Alabama. Rainfall d18O and dD were measured at weekly resolution since June 2005 from water collected in a rain gauge located at the University of Alabama campus. Vadose groundwater was retrieved from drip water in DeSoto Caverns (Childersburg, AL), while phreatic groundwater was extracted from a nearby well.
Isotopic composition of weekly precipitation varies widely ranging from -12.5 to 1.9 (V-SMOW) and -78.3 to 17.8 (V-SMOW) for d18O and dD, respectively. The weighted mean for precipitation d18O is -4.8 ±1.6 (n=108) with winter precipitation being slightly more negative than summer (-5.2 ±1.7; n=52 vs. -4.4 ±1.4; n=56). Anomalously negative isotope values (-7.0 ±2.0; n=5) were determined during hurricane-driven rainfall events in 2005. Both vadose and phreatic groundwaters show little temporal variability in isotopic composition and yield mean values (-5.1 ±0.2 and -5.0 ±0.4) that match that of mean winter infiltration and hurricane-related rainfall events. The relationship between d18O and dD for all water measured obeys Craig's meteoric water line equation. The observations suggest that (1) the principal source of groundwater recharge is from rainfall infiltration, and (2) the regional groundwater reflects a strong winter rainfall signal and occasional hurricane-related rainfall flooding in the summer.