Effectiveness of natural treatment in a wastewater irrigation district of the Mexico City region: A synoptic field survey

Untreated wastewater from Mexico City has been used for decades to irrigate the Mezquital Valley, Hidalgo, Mexico. A synoptic survey of the natural tr eatment systems was carried out using thr: criteria of 24 trace metals, 67 target base/neutral/acid (BNA) semivolatile organic compounds, nontarget BN A semivolatile organics, nitrate, 23 chlorinated pesticides, and a 20 conge ner polychlorinated biphenyl (PCB) suite. Data suggest the irrigation regio n is acting as a huge open-system slow sand filter, the main reservoir as a large waste stabilization lagoon, and the canals as extremely long, narrow stabilization channels. The BNA levels in surface water (SW) after reservo ir retention were much lower than before it, while levels in groundwater (G W) were significantly lower than SW. All GW nitrate levels exceeded drinkin g water standards and were greater than those in SW. Metal levels in GW wer e below drinking water standards, and SW levels exceeded them for only a fe w metals. Low to moderate levels of organochlorine pesticides and PCBs were found.