Decadal scale recharge-discharge time lags from aquifer freshwater-saltwater interactions

Abstract

Coastal aquifers are an important freshwater resource for significant portions of the global population. Understanding the dynamic relationship between aquifer recharge, storage and discharge is fundamental for sustainable groundwater management. Here we present an expanded freshwater balance equation that considers the dynamic interaction between fresh groundwater and underlying saltwater through a respective storage term, which may explain significant time lags in long-term trends between recharge, storage and discharge. We demonstrate the presence of such a time lag and its effect on freshwater budget calculations with data from a major karst spring in Florida. We develop and validate a parsimonious aquifer model and show that our expanded water balance is consistent with an observed time lag on the order of 15 years between multi-decadal averages of rainfall and spring discharge. Long-term trends in precipitation are known to occur in many regions of the world, and we expect our findings to have direct relevance for coastal aquifers at a global scale.