ASSESSING THE INFLUENCE OF URBAN LAND USE ON KARST DRINKING WATER QUALITY DURING WET AND DRY SEASONS, STATE OF YUCATAN, MEXICO

To help understand the impacts of dry and rainy seasons and urban land use on water quality in a karst system, twenty-one deep municipal water supply wells were sampled synoptically across the State of Yucatan, Mexico at the ends of a dry season and a subsequent wet season. Surrounding land uses included urban/suburban, crop agriculture and animal farming. Measured parameters included pH, temperature, specific conductance, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^-, SO₄^2-, alkalinity, NO₃^-, and fecal coliforms. These measurements were compared to a similar study of changes in water quality in (shallow) cenotes. The data were reduced using spatial analysis, multivariate statistics, various solute plots, Piper plots, and geochemical modelling. Results show that groundwater is diluted during the rainy season, but not for all parameters. Some land-use impacts on the geochemical structure of the groundwater is observed during the dry season, but obscured during the rainy season. For example, Ca²⁺ and Mg²⁺ concentrations are decoupled during the rainy season. Changes in most parameters differ between the cenotes and the wells, but fecal coliforms increase in both during the rainy season. We interpret these results to indicate that 1) rock-water interactions and hydrology may play a larger role in determining water quality for most of these parameters than land use and 2) municipal water quality is at risk in both dry (e.g., increased NO₃^-) and wet (e.g., increased fecal coliforms) seasons. We posit that changes in groundwater flow paths between seasons may explain these observations, including the differential responses between the cenotes and wells. We consider these results preliminary, but those that might guide future studies. Examples would be increasing the diversity of chemical and microbial measurements to better identify the influences of land use on water quality and testing the hypothesis that matrix flow is the main influence of water quality during the dry season and that conduit flow is the main influence on water quality during the rainy season. Clearly, understanding the impact of both urban land use and seasonal changes on groundwater quality in any karst environment remains challenging.