Upper District warns golden mussel threatens imported deliveries; Monrovia staff say local supplies remain stable

The Monrovia City Council heard a presentation on Dec. 2 from Upper San Gabriel Valley Municipal Water District General Manager Tom Love about the state of imported water deliveries and the invasive golden mussel.

Love said Metropolitan Water District has ample storage following high allocations in recent years, and local groundwater levels have recovered from historic lows. However, he told council that LA County Public Works turned off deliveries to some San Gabriel Valley delivery points on Sept. 24 because of concerns about the golden mussel, which was first detected in October near Stockton and has moved through the State Water Project system.

Love described the operational challenge: the mussel’s microscopic veliger larvae are difficult to detect, ocean‑going vessels currently lack decontamination stations in California, and large‑scale filtration for high conveyance flows is impractical. He outlined mitigation measures being pursued—pretreatment options (chlorine, ultraviolet, copper‑based molluscicides), containment and post‑delivery cleaning—and said the district and Metropolitan are coordinating control plans for each delivery point.

On timing, Love said the district hopes to have county and state agencies comfortable with control plans and to resume deliveries after winter rains, targeting April–May, but cautioned that a dry sequence could extend the timeline and that some basin areas would be more vulnerable if deliveries remain off for multiple years. He estimated imported water comprises about 30% of San Gabriel Valley supply and called the deliveries essential to long‑term reliability.

City members asked about contingency options. Love said adult mussels can be screened out at some points, but veligers are microscopic; filtration at the required flow rates would require very large facilities and routine backwashing that create disposal challenges. He said the district is preparing a technical memorandum with pretreatment recommendations to submit to county staff and is working with five local agencies and Metropolitan to finalize control plans.

Council members and staff stressed continued conservation messaging despite the near‑term supply being described as “good.” The city manager and public works director said they would continue coordination with the district and county if additional operational changes are proposed.