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EMC hears director recommendation to convert four IMAACs to groundwater standards; commissioners seek more site-specific data
Summary
DWR staff presented an information item recommending conversion of four IMAACs for herbicides and a fungicide into permanent groundwater standards or to allow them to expire; commissioners requested additional data on site prevalence, plume delineation, remediation options and program uses before deciding whether to pursue rulemaking in January.
The commission received an informational presentation from Bridget Shelton, Groundwater Standards Coordinator at the Division of Water Resources, about a director's recommendation to initiate rulemaking to establish groundwater standards in place of four interim maximum allowable concentrations (IMAACs) established in January 2024.
Shelton said the IMAAC mechanism allows the director to set a health-based protective level when no groundwater standard exists; the IMAACs at issue were requested by a petitioner (ELM Site Solutions) for an inactive hazardous site and cover three herbicides (bentasone, boscalid, metolachlor) and one fungicide (fluometuron). She described the rule's process: an IMAAC can be established with public notice and a 30-day comment period, and within 12 months the director must recommend to the EMC whether to initiate formal rulemaking to adopt a permanent standard or allow the IMAAC to expire and revert to the practical quantitation limit (PQL).
Commissioners pressed staff for site-specific information before any decision to proceed to rulemaking: prevalence of detections across the state, how many sites could rely on the IMAAC if left in place, the current status of plume delineation at the petitioner site, and how remediation programs (risk-based approaches or land‑use controls) interact with an IMAAC. Michael Scott (Division of Waste Management) confirmed the site assessment is advanced and that remedial planning is underway; commissioners discussed the possibility that reverting an IMAAC to the PQL could dramatically expand the mapped plume and increase cleanup scope.
Why it matters: an IMAAC used for a single site can become de facto statewide policy if other parties rely on it; converting an IMAAC to a permanent groundwater standard affects permitting, remediation footprints and potentially many programs that use IMAACs to set screening or compliance thresholds. Commissioners asked staff to return in January with more detailed prevalence data, detection magnitudes (relative to PQLs), and options that might sit between a statewide standard and site-level remedies.
Next steps: staff will return to committee in January with additional data and a recommendation package that the EMC can act on within the 12-month window.
