Effects of drought and subtidal sea-level variability on salt intrusion in a coastal karst aquifer

Abstract

In many coastal regions, groundwater is the main water source for humans. However, because of population growth and sea-level rise, many coastal aquifers increasingly suffer from salt intrusion, especially in karstic areas where the high permeability and porosity of the rock favours salt penetration. We collected field data from a Mexican karst system to show that sea-level variability at low frequencies (subtidal) may induce salt penetration further inland and generate larger oscillations than those observed at tidal frequencies. Measurements of conductivity and pressure from inland wells (5 and 10 km) and from substantial (~1 m3 s–1) submarine groundwater discharge (SGD) at ~2-m depth entering a shallow ocean were analysed. We found that sea and piezometric levels co-oscillated at subtidal frequencies, with a correlation of 0.6 and a differential lag. Conductivity of the SGD resembled that of the aquifer. Intense droughts driven by the 2009 ‘El Niño’ event markedly increased the conductivity of the aquifer and its discharge. Our findings indicated that coastal land use and the consequences of climate change (i.e. sea-level rise and the alteration of rain patterns) on the Yucatan Penisula threaten water availability.