NGWA Event and Education Recordings Archive

Local groundwater contamination by chromium occurred in Northern Mexico City, when a chromite-processing plant injected resulting wastewater directly into a semi-confined aquifer at depths ranging from 50 to > 300 m. To a lesser extent, chromium lixiviation also took place from plant solid wastes disposed elsewhere on surface without protecting measures. The plant operated from 1958-1978 and, after identifying local contamination, it was closed and almost destroyed. Then, an industrial cemetery was built in that site by 1982. With time, cemetery partially collapsed, and the Federal Environmental Authority decided to demolish it, transporting all solid wastes and most of contaminated soil to regulated confinement sites in Northern Mexico (2008-2011). Remaining contaminated soil was remediated to a maximum depth of 2 m (2011-2012). Contaminated drinking-water wells were relocated and intensive groundwater monitoring campaigns were implemented (1976 through 1982) within a radius of 3 km from the plant. Some industrial wells, mainly from a power plant, provided a partial groundwater remediation (1958-2008) through declining-with-time pumping rates. This incidental action limited the expansion of chromium contamination, in spite of the rapid grow of suburban areas that increased abstraction of groundwater and drawdown: aquifer changed from semi-confined to unconfined conditions. From 2004-2015 the Federal Water Authority, with some support of the German government, implemented new groundwater monitoring campaigns and conducted contaminant characterization studies that included: geophysical surveying, core drilling, construction of both monitoring and pumping wells, pumping tests, water quality sampling at different depths, definition of contaminant plume geometry, flow and transport modeling, and pilot water treatment essays. Results showed that chromium is distributed into different plumes within three distinct hydrogeological units with subsurface structural geology controlling plume geometries. Currently, a cost-benefit study for the implementation of groundwater remediation is under development.