Water reservoirs in open coal pits: A useful resource for NE of Mexico?

Coal mining remains an essential component of the energy matrix in many countries worldwide. In Mexico, the Sabinas Basin represents one of the most intensively exploited coal regions for several decades. As a result, the landscape has been significantly altered, leading to the formation of numerous water reservoirs within depressions created by mining activities. Despite their extent, these reservoirs have received limited attention from hydrochemical perspectives. In this study, we conducted a digital mapping of these reservoirs, characterized their physical attributes, and assessed water quality in eighteen reservoirs and two wells. Our findings reveal that the water exhibits a low quality, characterized by elevated concentrations of sulfates (up to 1982 mg/L), chlorides (up to 1197 mg/L), iron (up to 64.7 mg/L), total dissolved solids (up to 6432 ppm), and hardness (up to 3084 mg/L). Nevertheless, this water may be suitable for irrigation in crops or soils capable of assimilating high sulfate levels. The conceptual model summarizes the mechanisms and processes responsible for the progressive flooding of these mining-induced depressions, as well as the resulting physicochemical characteristics of the accumulated water. The hydrochemistry appears to reflect a combination of surface and groundwater interactions influenced by mining operations, including the disturbance of geological strata, natural processes (e.g., erosion, leaching, and landslides), hydrological and climatic factors, and water–rock interactions involving shallow aquifers and carbonaceous sequences containing sulfide, sulfate minerals, and salts. Excess sulfate can be treated by various methods, which would allow a significant volume of these resources to be available for various uses. Other ecological and environmental services should be evaluated in the future to take advantage of these vital spaces.