Hydrologic Dynamics of a Subtropical Estuary Using Geochemical Tracers, Celestún, Yucatan, Mexico

Abstract

Oxygen isotopes and strontium concentrations were used as geochemical tracers to discern the sources of water to Celestun Lagoon, a small subtropical estuary on the western side of the Yucatan Peninsula of Mexico. Celestun Lagoon is underlain by karstified limestone with numerous locations where groundwater is observed discharging directly to the lagoon. In this study, samples of groundwater, lagoon surface water, and seawater (SW) were collected in April 2008 and June 2009 and analyzed for salinity, stable isotopes of oxygen, and strontium (Sr2+) concentrations. These geochemical tracers were used in two tertiary mixing models to calculate the relative ratio inputs of fresh groundwater, brackish groundwater, and SW to the lagoon. Two sources of groundwater were found to contribute to the surface water in the lagoon; one fresh and the other brackish with an average salinity of 19 psu. The fresh groundwater had an oxygen isotopic signature (δ18O) and strontium concentration (Sr2+) of δ18O = -3.30‰ and Sr2+ = 0.03 mmol/l, respectively. The brackish groundwater observed in the northern end of the lagoon add a dissimilar oxygen isotopic signature and Sr2+ concentration of δ18O = 3.01‰ and Sr2+ = 0.12 mmol/l, respectively. Local SW had an isotopic oxygen signature and Sr2+ concentration between the two fresher sources (δ18O = 1.40‰, Sr2+ = 0.09 mmol/l). The lagoonwide results of the two tracer mixing models (δ18O and Sr2+) agreed well (within 5 %) and indicated a ratio of brackish groundwater–fresh groundwater– SW of 31 %–26 %–43 % (±5 %) for the Sr2+ model and 35 %–25 %–40 % (± 5 %) for the δ18O model. Brackish groundwater is dominant in the northern portion of the lagoon, while SW dominates the southern portion. Fresh groundwater discharge is a significant contributor of water along the entire eastern boundary of the lagoon where mangrove forests are the dominant vegetation.