39-1 Hydrogeochemical and Structural Controls on Groundwater Flow in the Coastal Karst Aquifer of the Northern Yucatán Peninsula

Session: Understanding Karst Hydrology and Karst Aquifers Using Innovative Tracers and Other Technologies

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The coastal karst aquifer of the northern Yucatán Peninsula, Mexico, is among the most complex carbonate systems globally, where groundwater flow is strongly influenced by secondary porosity developed along faults, fractures, and karst conduits. This study integrates fracture mapping, hydrochemical monitoring (2018–2025), and isotopic tracer data to evaluate how the geologic structure of the Ring of Cenotes and seasonal recharge variations control groundwater movement and contaminant accumulation. Fracture orientation data from four sites confirm alignment with the Chicxulub impact structure, suggesting fault-guided flowpaths from inland recharge zones to coastal discharge points. Water samples collected from wells, cenotes, springs, caves, submarine groundwater discharge (SGD) sites, and surface waters were analyzed for redox-sensitive parameters, major ions, nutrients, and isotopes (²²²Rn, ²²⁴Ra, ²²⁶Ra, Sr). Geochemical trends reveal a hydrochemical divide across the Ring. On the left side, radon activity was highest in springs, particularly in 2023, indicating gas accumulation in recharge zones. Radium (²²⁴Ra, ²²⁶Ra) levels were elevated in wells and springs, reflecting moderate residence times and groundwater–rock interaction. Nitrate concentrations were also highest on the left, especially in springs and cenotes, suggesting active recharge and nitrification under oxic conditions. On the right side, salinity, ammonium, and strontium concentrations were significantly higher in wells and SGD zones, especially during the dry year of 2023, pointing to longer residence times, limited flushing, and enhanced geochemical mobilization. Caves exhibited the highest radon activities on the right, particularly in 2023, consistent with groundwater dominance and minimal dilution from runoff. Radium levels (especially ²²⁶Ra) were also elevated in right-side wells and SGD, indicating sustained water–rock interactions. These findings emphasize that both structural controls and hydroclimatic variability shape flowpaths, geochemical evolution, and contaminant vulnerability in this dynamic karst system. Ongoing work will apply geophysical methods and tracer studies to further resolve the connectivity and discharge mechanisms governing this aquifer’s response to natural and anthropogenic stressors. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9678

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