The South El Monte Operable Unit (SEMOU) is generally characterized by shallow groundwater contamination that is mostly contained in the upper 100 feet of the aquifer; however some contamination in the northwest and southern portions of the operable unit has migrated below 100 feet into the intermediate zone aquifers currently used for potable supplies. Contamination in the SEMOU consists primarily of volatile organic compounds with perchlorate concentrations in certain wells exceeding the new state notification level of 6 ppb. Furthermore, cleanup has been complicated by the presence of low concentrations of 1,4-Dioxane in the OU.
The SEMOU has more Potentially Responsible Parties (PRPs) than any other operable unit. However, most of the them reached Ability To Pay (ATP) settlements with the United States Environmental Protection Agency, leaving a huge funding gap for cleanup. The San Gabriel Basin Water Quality Authority, and water purveyors have worked closely to encourage settlements with several of the non-ATP PRPs. Many of the required cleanup facilities have been constructed as a result of early action sponsored by the water purveyors and WQA.