Risk screening for exposure to groundwater pollution in a wastewater irrigation district of the Mexico City region

Untreated wastewater from the Mexico City basin has been used for decades t o irrigate cropland in the Mezquital Valley, State of Hidalgo, Mexico. Exce ss irrigation water recharges the near-surface aquifer that is used as a do mestic water supply source. We assessed the groundwater quality of three ke y groundwater sources of domestic water by analyzing for 24 trace metals, 6 7 target base/neutral/acid (BNA) organic compounds, nontarget BNA organics, 23 chlorinated pesticides, 20 polychlorinated biphenyls, and nitrate, as w ell as microbiological contaminants-coliforms, Vibrio cholerae, and Salmone lla. Study participants answered a questionnaire that estimated ingestion a nd dermal exposure to groundwater; 10% of the sample reported frequent diar rhea and 9% reported persistent skin irritations. Detection of V. cholerae non-01 in surface waters at all sites suggested a potential risk (surrogate indicator present) of diarrheal disease for canal and river bathers by acc idental ingestion, as well as potential Vibrio contamination of near-surfac e groundwater and potential cholera risk, magnified by lapses in disinfecti on. High total coliform levels in surface water and lower levels in groundw ater at all sites indicated fecal contamination and a potential risk of gas trointestinal disease in populations exposed to inadequately disinfected gr oundwater. Using chemical criteria, no significant risk from ingestion or d ermal contact was identified at the method detection limits at any site, ex cept from nitrate exposure: infants and young children are at risk from met hemoglobinemia at all sites. Results suggest that pathogen risk interventio ns are a priority, whereas nitrate risk needs further characterization to d etermine if formal treatment is needed. The risks exist inside and outside the irrigation district. The method was highly cost-effective.