Monitoring of surface water quality in the San Quintín mining area during its restoration

San Quintín was a former galena (PbS) mine rich in Ag, and sphalerite (ZnS), whose extractive activity generated a set of mining wastes with high contents of heavy metals and metalloids, mainly present as primary sulfides and secondary sulfates, accompanied by significant amounts of pyrite (FeS₂). These wastes pose an environmental risk due to their dispersion into the surrounding natural environment. The presence of sulfides, such as pyrite, promotes the formation of acid mine drainage (AMD), generating waters with low pH and high electrical conductivity (EC).

The objective of this study was to assess the quality of water in streams and ponds in the San Quintín area through physicochemical parameters (pH and EC) during the restoration stage, comparing them with values obtained prior to the remediation actions. To this end, a network of sampling points was established along the stream system and in rain-formed ponds, prioritizing areas close to former waste rock dumps and tailings. Measurements were carried out both in situ and in the laboratory using calibrated equipment, allowing detailed monitoring of the physicochemical variability of temporary surface waters.

The results indicate that under drought conditions, which represent the critical load of acidity and ion concentration, partial recovery of pH and EC is observed in sectors associated with old ponds and waste rock dumps. During the rainy season, pH increases markedly due to dilution, while EC decreases near the deposits but increases at the confluence points of the stream system, where ions transported by runoff become concentrated.

As a provisional conclusion, considering the factors affecting the variability of physicochemical parameters, a partial recovery of pH and EC is observed in the temporary surface water system of the mining area, largely attributable to the neutralizing action of carbonate materials deposited during restoration. Nevertheless, continued monitoring of these parameters is still necessary to verify that conditions of near neutrality are ultimately achieved in the waters throughout the area.