The South El Monte Operable Unit (SEMOU) addresses about 2 ½ square miles of contaminated groundwater underlying portions of the cities of South El Monte, El Monte, and Rosemead.

It 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 site has migrated below 100 feet, into the intermediate zone aquifers currently used for drinking water supplies.

Contamination in the SEMOU consists primarily of volatile organic compounds with perchlorate concentrations in certain wells exceeding the state notification level of 6 parts per billion. Cleanup has been complicated by the presence of low concentrations of 1,4-dioxane.

The SEMOU has more Potentially Responsible Parties than any other operable unit. However, most of them reached Ability To Pay settlements with the EPA, leaving a huge funding gap for cleanup. The San Gabriel Basin Water Quality Authority (WQA) and water suppliers have worked closely to encourage settlements with the remaining parties. Many of the required cleanup facilities have been constructed as a result of early action sponsored by the water purveyors and the WQA.

Additional information:

Appendix A in WQA’s Groundwater Quality Management and Remediation Plan (406 Plan)