DETERMINING HYDROLOGICAL INPUTS INCLUDING SUBMARINE GROUNDWATER DISCHARGE, TO A SUBTROPICAL ESTUARY USING GEOCHEMICAL TRACERS, CELESTUN, YUCATAN, MEXICO

Geochemical mixing models were used to determine the dominant sources of water (fresh and saline) to Celestún Lagoon, a small subtropical estuary in the western Yucatán of Mexico. Celestún lagoon is underlain by a karstified limestone, and has numerous locations where groundwater can be seen discharging to form surface boils known as Ojos de Aqua. In this study, samples of precipitation, groundwater, and lagoon surface water were collected in April of 2008 and analyzed for salinity, stable isotopes of oxygen, and strontium concentrations. These geochemical tracers were used in two separate ternary mixing models to calculate the relative inputs of seawater, fresh groundwater (salinity = 0.5 to 1.5), and brackish northern creek (salinity = 21) water. Fresh groundwater had the lightest oxygen isotopic signature (-3.30 ‰) and the lowest Sr²⁺ concentration (0.03 mM) of the model end-members. Brackish creek water from the north end of the bay, which we hypothesized to originate from a different groundwater source north of Celestún in the Ring of Cenotes, had the most evaporated oxygen isotopic signature (+3.01‰) and the highest strontium concentration (0.12 mM). Local seawater from the Gulf of Mexico had an isotopic signature and a strontium concentration in between the two fresher sources (δ¹⁸O = 1.40 ‰, Sr²⁺ = 0.09 mM). The lagoon-wide results of the two mixing models (δ ¹⁸O versus salinity and Sr²⁺ versus salinity) agreed well (within 5%) and indicated a ratio of Brackish creek water-Fresh Groundwater-Seawater of 31%-26%-43% ± 2% for the Sr²⁺model and 35%-25%-40% ± 2% for the δ¹⁸O model. Assuming that the brackish creek water also had a groundwater source, then groundwater input could be up to 60% of the lagoon volume. Nutrient concentrations of nitrate and total phosphorus were found to be highest in the fresh groundwater, and the relativity large input of groundwater may be responsible for elevated concentrations of these nutrients found within the estuary.