TRACING GROUNDWATER INPUTS AND COMPOSITION NEAR CORAL REEFS ALONG A STRETCH OF THE CARIBBEAN COAST

The unique geologic and hydrogeological features of karst formations make them capable to store and transmit large amounts of groundwater and contaminants from sources of recharge to potential discharge zones (e.g., coastal seas). Because karst aquifers are closely linked to the sea in coastal settings, groundwater discharge, as either underwater springs or coastal springs, also referred as submarine groundwater discharge (SGD), is critical for coastal ecosystems and their services. This research aims to understand the connection between groundwater inputs to the nearshore Caribbean Sea and the health of coral reefs using a combination of stable (oxygen-δ¹⁸O and hydrogen- δD) and radioactive (radon-²²²Rn and radium-²²⁴Ra, ²²³Ra and ²²⁶Ra) isotope measurements. Several submarine groundwater point sources or springs have been identified in the area in proximity to the coral reefs. In this study, 4 cenotes (or sinkholes), 5 underwater springs and 15 seawater samples were collected in fall 2018. Continuous measurements of ²²²Rn along multiple transects between the shoreline and near the reefs identified several locations that are likely affected by groundwater inputs, but high wind conditions limit our ability to quantify SGD fluxes using the mass balance method. Nevertheless, the highest ²²²Rn activities, and ²²⁶Ra, were measured in the cenotes and near some of the springs. But, cenotes, considered to be the groundwater sources, did not always have higher activities than the underwaters springs which may indicate that groundwater inputs via submarine springs may be associated with deeper flowpaths, thus higher ²²²Rn and ²²⁶Ra activities. Based on the observed nonlinear relationships between δ¹⁸O/δD isotope ratio abundances and radium activity ratios, the most significant deeper groundwater inputs are not near the most degraded coral reefs. Preliminary analyses indicate that the highest nitrate concentrations were found near reefs and in areas where the deep groundwater input indicators had the lowest presence. On the other hand, ammonium was mostly associated with higher activities of ²²⁶Ra and more depleted stable isotope signatures and mostly at distance from the reefs. Further analyses are conducted to constrain other types of nutrients and their association with the coral reefs or nearshore inputs.