Water Chemistry and Lake Dynamics of Laguna Bacalar, Quintana Roo, Mexico

Abstract

Laguna Bacalar in the Quintana Roo region is the second largest lake in Mexico and contains freshwater derived solely from groundwater. Local geology on the Yucatan Peninsula is karstic and the southern shoreline of Laguna Bacalar is spotted with a handful of cenotes that contribute substantial amounts of inflowing groundwater to the lake. This is shown by sonde profile data taken in one of the largest cenotes in the area. Outflow is dominated by a surface water outlet in the southern portion of the lake and an unknown amount of outflowing groundwater. During January of 2017 through 2019, UWM researchers collected data on the physical flow to and from the lake, δ13C, δ18O and δ2H isotopes, and major ion chemistry in order to provide insight into the overall chemical and physical hydrology of the lake. The primary hydrochemical processes controlling lake chemistry include influx of high alkalinity groundwater in the southern portion, CO2 evolution and a resultant pH rise and calcite precipitation. Saturation indices modeled using PHREEQC indicate the water in Laguna Bacalar is oversaturated with calcite and at saturation with gypsum. The northern portion of the lake has no groundwater influx and is dominated by evaporative effects. Recently, the lake and the city of Bacalar have gained international attention and are attracting an increasing number of visitors. While fueling a growing tourism industry and economy, this raises the potential for accelerating human impacts that can threaten the health of this relatively pristine freshwater ecosystem. Understanding the basic hydrology of Laguna Bacalar will be a key element in managing water quality and preserving the truly unique hydrogeological and biological characteristics of the system.