Fate of Emerging Contaminants During Infiltration of Untreated Wastewater in Mezquital Valley, Mexico

The Mezquital Valley is the world's oldest and largest example of usage of untreated wastewater for agricultural irrigation. North of Mexico City, the Mezquital Valley receives, since the 1950s, untreated wastewater through three different main sewage drains from Mexico City (an open canal and two deep sewage systems, with a third deep sewage close to completion). Currently, more than 45 ha are irrigated with approximately 50 m³/s of this untreated wastewater in the Mezquital Valley. Eighty one percent of the main canals, and 52% of secondary canals are unlined, with water infiltrating directly into the shallow aquifer. Because of the high artificial recharge in the area, groundwater is extracted for human consumption from the deeper aquifers.

This study analyzed 218 organic microcontaminants in wastewater canals, springs and groundwater from the Mezquital Valley. Five volatile organic compounds (VOCs) and 9 semi-volatile organic compounds (SVOCs) were detected in the wastewater used for irrigation. Only 2 SVOCs were detected in all the wastewater canals and groundwater sources, whereas no VOCs were detected in groundwater and springs.

Of the 118 pharmaceutically active compounds (PhACs) and 7 reproductive hormones measured, 65 PhACs and 3 hormones were detected in the wastewater. In groundwater sources, 23 PhACs were detected. Most of these compounds have low concentrations compared to those detected in canals. There were only few detections and at lower concentrations in the deeper aquifers. These results suggest that the subsurface acts as a filter, adsorbing and degrading most of the organic pollutant content in the infiltrated wastewater. A new wastewater treatment plant (PTAR Atotonilco) is being built to treat the wastewater prior to its release to the Mezquital Valley. The geochemical changes that this cleaner water will produce when infiltrated into the aquifer have not been assessed yet.