The main aim of this research was to determine the environmental effects of wastewater irrigation on the subsurface at two locations in Mexico; Leon and the Mezquital Valley. Freshly-drilled subsurface sediment samples (both sites) and groundwater samples (Mezquital Valley) were taken at various depths. These were analysed for a range of physico-chemical and microbiological parameters using standard methods. Elevated concentrations of faecal and total coliforms were recorded at both sites indicating that faecal bacteria had been transported through the subsurface. The level of subsurface contamination was affected by both spatial and temporal factors with higher coliform concentrations occurring at shallower depths (<10 m below the surface) and at the top of the water table, and during the wet season. Elevated nitrate concentrations were found in all of the Mezquital Valley groundwaters. Thus, wastewater irrigation appears to have had a negative impact on groundwater quality. In order to mitigate this, treatment of wastewater prior to irrigation and careful irrigation management is recommended. The potential effects of groundwater degradation on public health, and the need for guidelines to protect groundwater quality, are discussed.

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