Investigating Storm Impacts on Springs in a Karst Region Using Oxygen and Hydrogen Isotopes

ABSTRACT Groundwater contamination is a growing concern in many regions around the world. Karst aquifers are particularly vulnerable to contamination because surface runoff can rapidly enter the subsurface through sinkholes and conduits. This study analysed time‐series oxygen and hydrogen isotope data collected before and after Hurricane Idalia to assess storm impacts on two nearby springs with different discharge rates–Sally Ward Spring (~0.6 m 3 /s) and Wakulla Spring (~13 m 3 /s)–in a karst region of North Florida in the US. The δ 18 O and δ 2 H values of rainwater from Hurricane Idalia (event water hereinafter) were −15.2‰ and −108.5‰, respectively, considerably lower than those of normal rainfall in the region (δ 18 O = −4.1‰ ± 1.6‰ and δ 2 H = −20.2‰ ± 12.2‰), providing a distinct tracer for storm‐derived water. Event water appeared at Sally Ward Spring within one day and again at 12–19 and ~30 days post‐storm, while it took 10–12 days to appear at Wakulla Spring. These relatively short transit times are in general agreement with previous dye‐tracer‐based estimates, reflecting rapid flows through sinkholes and conduits. Isotopic mass balance calculations, however, reveal that event water contributed no more than ~7% and ~1% of the discharge at Sally Ward Spring and Wakulla Spring, respectively. These results demonstrate that despite the marked differences in hydrodynamics, discharge at both springs is dominated by pre‐event aquifer water, even following a major storm. The data further suggest that surface water bodies (e.g., lakes and ponds) within the springshed are important groundwater sources, contributing a substantial amount (> 20%) of water to base flow at both springs. These findings highlight the complex recharge dynamics of the karst aquifer system and the limited direct contribution of event water to spring discharge. This study also demonstrated that the unique isotopic signatures of tropical cyclones can serve as powerful natural tracers in regional hydrological investigations.