Central Valley staff say Delta methylmercury TMDL will need rethinking; current targets likely unattainable with present tools

Central Valley Regional Water Board staff told the statewide water committee that the 2010 Delta methylmercury total maximum daily load (TMDL) and its one‑meal‑per‑week fish tissue goals pose serious technical and policy challenges and likely cannot be met with current tools or costs the region could reasonably bear.

Staff summarized the history: the methylmercury TMDL carved the Delta into nine subareas, set fish‑tissue targets for trophic‑level 3–4 predators and back‑calculated aqueous methylmercury goals and load allocations. Implementation was phased and relied on past technical work and decades of remediation and monitoring. Staff said subsequent research and monitoring show methylmercury “is far more stubborn” than originally expected and that even large reductions of controllable inputs would not achieve the one‑meal‑per‑week tissue targets.

Staff pointed to multiple reasons: legacy mercury in sediments from historical mining; upstream inputs from large watersheds; biogeochemical methylation processes in Delta sediments and water columns; and food‑web accumulation that produces high tissue burdens in top predators. Staff gave a notional example that reducing a small source (an illustrative 1 gram source) could cost more than a billion dollars while having negligible effect on Delta tissue burdens. The region also faces equity considerations: tribes and subsistence fishers rely on Delta fish; several communities (including tribal, Hmong and other subsistence fishing populations) asked for greater access to fish resources and would be affected by any permanent consumption advisories.

Staff said the region must now engage in ‘tough conversations’ with tribes, subsistence fishers, regulated parties and other stakeholders to reframe objectives, consider realistic risk‑management options and allocate responsibility for large, long‑term remediation work. They cautioned that there was no obvious engineering or policy quick fix, that remediation is costly and that the TMDL will require careful re‑examination to set feasible objectives and strategies compatible with equity considerations.

The presentation urged the board and staff to prepare for difficult decision‑making, emphasize transparency and culturally sensitive outreach, and weigh the limits of local authority (e.g., many mercury sources are historical or upstream) when communicating next steps.