Assessment of Physicochemical Groundwater Quality and Hydrogeochemical Processes in an Area near a Municipal Landfill Site: A Case Study of the Toluca Valley

Sanitary landfills are considered one of the main sources of contamination of water resources due to the generation of leachate with a high content of dissolved organic matter (DOM), inorganic material, and toxic elements. This study aimed to determine the influence of leachate on the physicochemical quality and hydrogeochemical processes which determine the chemical composition of groundwater in an area near a municipal sanitary landfill site. In situ parameters (pH, temperature, electrical conductivity, dissolved oxygen, ORP), physicochemical parameters (HCO(3)(−), PO(4)(3−), Cl(−), NO(3)(−), SO(4)(2−), NH(4)(+), Ca(2+), Mg(2+), Na(+), K(+)), and dissolved organic matter were analyzed. The content of dissolved organic matter (DOM) was determined by 3D fluorescence microscopy. The presence of Cl(−), NO(3)(−), NH(4)(+), PO(4)(3−), BOD, and COD indicated the presence of contamination. The significant correlation between NO(3)(−) and PO(4)(3−) ions (r = 0.940) and DOM of anthropogenic origin in the 3D fluorescence spectra confirm that its presence in the water is associated with the municipal landfill site in question. The type of water in the area is Mg-HCO(3), with a tendency to Na-HCO(3) and Na-SO(+)-Cl. The water-rock interaction process predominates in the chemical composition of water; however, significant correlations between Na(+) and Ca(2+) (r = 0.876), and between K(+) and Mg(2+) (r = 0.980) showed that an ion exchange process had taken place. Likewise, there is enrichment by HCO(3)(−) and SO(4)(2−) ions due to the mineralization of the organic matter from the leachate. The groundwater quality that supplies the study area is being affected by leachate infiltration from the sanitary landfill.