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State Water Resources Control Board adopts pyrethroid TMDLs for Alamo and New Rivers
Summary
The board unanimously adopted a basin plan amendment establishing total maximum daily loads and numeric targets for six pyrethroid pesticides in the Alamo and New Rivers (Imperial County), implementing allocations via regional agricultural orders and NPDES permits and directing phased, adaptive implementation while coordinating with federal partners on transboundary pollution.
The State Water Resources Control Board on May 19 adopted a resolution to establish total maximum daily loads (TMDLs) and numeric targets for six pyrethroid pesticides in the Alamo and New Rivers in Imperial County, approving the Colorado River Basin Regional Water Board's July 9, 2024 resolution R7-2024-24. The motion passed unanimously on a roll-call vote.
Staff said water column samples collected from 2003 to 2020 frequently exceeded the numeric targets that prompted the 303(d) listing. Emma McCorquodale, a senior environmental scientist with the Colorado River Basin Regional Water Quality Control Board, told the board staff set allocations equal to the numeric targets and will implement them as waste-load and load allocations in irrigated lands general orders and NPDES permits to protect benthic and aquatic organisms, wildlife and human health.
The technical justification for the numeric targets relies on an updated UC Davis method, using the 2.5th percentile criteria to strike a balance between protection of sensitive species such as Hyalella azteca and analytical limitations. "The 2.5th percentile criteria is a good middle ground," McCorquodale said during her presentation.
Staff told members the TMDL's concentration-based approach requires each discharger to meet targets regardless of origin. The Colorado River Basin Water Board will coordinate with federal partners, including the U.S. International Boundary and Water Commission and U.S. EPA, to address pollution that crosses the international boundary with Mexico; nevertheless, staff emphasized implementation should not be deferred pending a binational solution.
Board members praised the region's work and asked about how this TMDL fits into other watershed and Salton Sea restoration priorities. One board member noted the benefit of a watershed approach and asked staff follow up on other priority TMDLs the region is pursuing. After brief discussion, a motion to adopt the resolution was made and seconded and the clerk called a roll-call vote. The record shows Board Member Laurel Firestone, Board Member Nicole Morgan, Board Member Maguire, Vice Chair Doreen D'Adamo, and Chair Joaquin Esquivel all voted "aye." The resolution was adopted unanimously.
The board and staff said next steps include implementing the allocations through permits and the region's irrigated lands regulatory program, monitoring effluent and receiving waters for pyrethroid compounds, and coordinating with federal partners on transboundary issues. Staff said the TMDL will proceed with a phased, adaptive implementation while binational efforts continue.
The board noted the TMDL complements ongoing Salton Sea work and that staff will return with implementation details and monitoring plans as permits and waste discharge requirements are amended to incorporate the new allocations.
