HYDROLOGIC AND GEOCHEMICAL DYNAMICS OF THE KARSTIC AQUIFER SUPPLYING LAGUNA BACALAR, YUCATAN, MEXICO

Laguna Bacalar, located within a graben in the karstic Yucatan Peninsula, is the second largest lake in Mexico. Recently, the lake and the city of Bacalar have gained international attention because of the unique presence of large freshwater stromatolites. While fueling a growing tourism industry and economy, this attention raises the potential for accelerating human impacts that can threaten the health of this pristine freshwater ecosystem. This study uses δ¹³C, δ¹⁸O, δ²H isotopes, and major ion chemistry to provide for the first time insight into the overall chemical and physical hydrology of the lake. The lake averages 10m in depth and inflow is entirely supplied by groundwater upwelling out of deep cenotes found along the shoreline. Outflow is dominated by a surface water outlet in the southern portion of the lake. Lake water is strongly Ca-SO4 in nature and PHREEQC modeling indicates that it is at saturation with gypsum (SI=-0.11) and amorphous silica (SI = -0.06). The primary hydrochemical processes controlling lake chemistry include influx of high alkalinity/high CO2 groundwater in the southern portion, CO₂ evolution and a resultant pH rise and calcite precipitation. δ¹³C, δ¹⁸O, δ²H and chloride data all indicate that the northern portion of the lake is dominated by evaporation with no apparent groundwater influx.

The lake occupies a topographic low that breaches the groundwater and as such serves as an access point to provide insight into the dynamics of groundwater flow through this karstic system. A minimum of 2.3E8 +1.9E7 m3/yr (7.3 + 0.6 m3/sec) of groundwater exits from a series of 5 cenotes. The largest cenote precipitates 4.0E7 kg/yr of calcite and exsolves 4.5E7 kg/yr of CO2. ¹³C isotopes indicate that 49% of this CO₂ is modern (recycled) and 51% is fossil CO₂. Residence times within the lake are bimodal with times in the southern, well-flushed portion on the order of 120 days whereas the more stagnant northern portion has residence times of several years.