Submarine groundwater discharge along the northern coast of the Yucatán Peninsula

The peculiar intrinsic geological and hydrogeological features of karst aquifers enable an immense capacity of karst aquifers to store and carry contaminants from sources to potential discharge zones (e.g., coastal seas). In coastal settings, the function of karst aquifers is closely linked to the sea. Submarine groundwater discharge (SGD) is critical for coastal ecosystems and their services. This research aims to promote understanding of the spatial variability of SGD, and its potential role in delivering contaminants, along the nearshore Gulf of Mexico of the northern Yucatán Peninsula, Mexico using a combination of stable (oxygen-δ18O and hydrogen- δD) and radiogenic (radon-222Rn and radium-224Ra, 223Ra and 226Ra) isotope measurements. This region is unique because of the presence of a “ring of cenotes” that preferentially directs continental groundwater toward the ocean. Several submarine groundwater point sources or springs have been identified in the area where the cenotes ring and the coastline intersect. In this study, 2 underwater- and 4 coastal-springs were sampled. Continuous measurements of 222Rn along approximately 300-km of Yucatán State coast, yield spatially variable SGD rates, ranging between 1 and 18 m d-1. The largest SGD rates and lowest radium ages were measured in vicinity of the so-called “underwater labyrinth” Xbuya-Ha spring. Based on the observed correlations between temperature, δ18O and δD isotope ratio abundances and radium activity ratios, at least two distinct hydrologic environments were identified. Nearshore waters with (1) weaker groundwater signals have lower temperatures and more depleted δ18O and δD abundances and higher radium ages; (2) stronger groundwater signals have higher temperatures, more enriched δ18O and δD signatures, and lower radium ages. Nevertheless, the pervasive occurrence of SGD along the entire coast indicates that inland anthropogenic inputs of pollutants likely affect the ecosystem health and magnitudes of impact are expected to vary dependent on the different inputs and forms of groundwater discharge