Imaging Point Source Groundwater Discharges in a Confined Coastal Aquifer Using Electrical Resistivity.

Point-source submarine groundwater discharge (PSGD) delivers mass and solutes and locally reduces salinity along karstic coasts, yet the geometry of conduits conveying meteoric and mixed waters is rarely imaged. Here we present the first marine electrical resistivity tomography (ERT) characterization of PSGD along the northwestern Yucatán Peninsula and evaluate marine ERT across contrasting settings to detect and characterize PSGD conduits and the confining coastal aquitard under field conditions. Four dipole-dipole ERT profiles span an intact coastal aquitard on land (Sisal), a coastal lagoon perforated by PSGD (Dzul-Ha), and their offshore equivalents near the Xbuya-Ha vent. On land, a laterally continuous resistive horizon at -5 to -9 masl overlies low-resistivity units that host saline groundwater and is interpreted as a cemented coastal aquitard that confines the aquifer. Beneath Dzul-Ha, the inversion images an elongated conductive chimney that cross-cuts this horizon and coincides with the mapped PSGD. Seabed profiles near Xbuya-Ha resolve discrete resistive bodies embedded in a conductive matrix at the main and secondary PSGDs. Forward modeling shows that conduits remain detectable even when the salinity contrast generated by freshwater discharge is weak; however, anomaly amplitudes decrease with increasing sea-floor depth, following an exponential decay with a characteristic depth of ~8 m. Together, these results provide a resistivity-based framework for combining tracers and flow models to quantify PSGD fluxes. Because the study targets vigorous PSGD under favorable electrical contrasts, these performance estimates represent an upper bound and clarify the limits of marine ERT for detecting PSGD in comparable karstic coasts.