Wastewater and groundwater has been used for irrigation in the Valsequillo District, east central Mexico, for nearly 50 years. The environmental impact of wastewater on groundwater in the unconfined shallow aquifer is evaluated by means of hydrogeological, microbiological, hydrogeochemical and isotopic evidences. The shallow aquifer consists of upper Tertiary volcano-sedimentary rocks with a calcite-rich matrix. Groundwater from wells near the wastewater canal had similar total coliforms concentrations as the wastewater (∼100 MPN 100 mL-1). The hydraulic head in near-canal wells had a recovery of 10 m until 1983, indicating shallow recharge from wastewater. A bicarbonate vs. calcium plot shows a well-defined mixing process between wastewater and unaffected groundwater. Stable isotopic data (δD and δ18O) show characteristic signatures for wastewater and non-impacted groundwater, and define a mixing line between those end-members and groundwater affected by wastewater infiltration. Tritium data indicate that non-impacted groundwater is ‘pre-atomic hydrogen bomb’ (>50 yr), whereas the wastewater has a ‘younger’ signature. Tritium data from wells inside the district clearly indicate a mixing process between waste and groundwater. These results demonstrate the interaction and hydrochemical processes between wastewater and shallow groundwater at the site.