Oxygen and Hydrogen Isotope Time-Series Data In the Hydrologic Cycle of the Gulf Coast, USA

Stable isotopes of oxygen and hydrogen in water, conventionally expressed as d¹⁸O and dD, are important tracers of the global, regional, and local hydrologic cycle. The importance of the isotopes tracer aspect in water management has long been recognized by the International Atomic Energy Agency that maintains a global network of sampling stations. Surprisingly, however, there are no stations in the entire US Gulf Coast. Consequently, a baseline isotope data for groundwater resources in this important region of USA is presently absent.

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.