Stable isotopes of oxygen and hydrogen in water, conventionally reported as δ¹⁸O and δD, are important tracers of the global, regional, and local hydrologic cycle. The significance of the isotopes tracer aspect in water management has long been recognized by the International Atomic Energy Agency (IAEA) that maintains a global network of sampling stations. Surprisingly, however, there are no stations on the entire U.S. Gulf Coast. Consequently, baseline isotope data for ground water resources in this important region are presently absent.

In order to determine the Gulf Coast hydrologic cycle we analyzed rainfall and ground water from the vadose and phreatic zones in central Alabama. Rainfall δ¹⁸O and δD were measured at weekly resolution since June 2005 from water collected with a rain gauge located at the University of Alabama campus. Vadose ground water was retrieved from drip water in DeSoto Caverns (Childersburg, Alabama), while phreatic ground water was extracted from a nearby well.

Isotopic composition of weekly precipitation varies widely and range from -12.5 to 1.9 per mille (‰) Vienna standard mean ocean water (V-SMOW) and -78.3 to 17.8 ‰ V- SMOW for δ¹⁸O and δD , respectively. The weighted mean for precipitation δ¹⁸O is -4.7 ± 1.6 ‰ (n = 117) with winter precipitation being slightly more negative than summer (-5.1 ± 1.7 ‰, n = 53, versus -4.4 ± 1.4 ‰, n=64). Anomalously negative isotope values (-7.0 ± 2.0 ‰, n = 6) were determined during hurricane-driven rainfall events in 2005. Both vadose and phreatic ground waters show little temporal variability in isotopic composition and yield mean values (-5.1 ± 0.2 ‰ and -4.9 ± 0.2 ‰) that match that of mean winter infiltration and hurricane-related rainfall events. The relationship between δ¹⁸O and δD for all water measured obeys Craig's meteoric water line equation. The observations suggest that (i) the principal source of ground water recharge is from rainfall infiltration, and (ii) the regional ground water reflects a strong “winter” rainfall signal and occasional hurricane-related rainfall flooding in the summer.