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 treatment systems was carried out using thr: criteria of 24 trace metals, 67 target base/neutral/acid (BNA) semivolatile organic compounds, nontarget BNA semivolatile organics, nitrate, 23 chlorinated pesticides, and a 20 congener polychlorinated biphenyl (PCB) suite. Data suggest the irrigation region 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 reservoir retention were much lower than before it, while levels in groundwater (GW) were significantly lower than SW. All GW nitrate levels exceeded drinking water standards and were greater than those in SW. Metal levels in GW were below drinking water standards, and SW levels exceeded them for only a few metals. Low to moderate levels of organochlorine pesticides and PCBs were found.