State Water Resources Control Board holds workshop on draft urban stormwater infiltration policy

State Water Resources Control Board staff held a public webinar to solicit feedback on a proposed statewide urban stormwater infiltration policy, outlining a framework that would guide where and how stormwater may be infiltrated to benefit groundwater recharge while protecting groundwater quality.

Board staff said the policy is still in early development and will be refined through outreach and public comment; written comments are due to stormswaterboards.ca.gov by 12 noon on Feb. 24. Staff said they plan continued engagement over the next eight months before issuing formal policy language and that implementation would likely occur through MS4 permits rather than creating a new Waste Discharge Requirement (WDR) or separate statewide permit.

Staff described three central aims of the project: provide consistent statewide standards for infiltration, clarify where infiltration is and is not appropriate, and reduce regulatory confusion that currently discourages some permittees from installing infiltration systems. A staff member said, "We don't want to require that stormwater meet MCL before it gets infiltrated," and emphasized the team is studying vadose-zone (soil) attenuation and where pretreatment or monitoring will be necessary.

Key provisions discussed included a tiered risk system that would assign lower requirements to residential and park settings and higher requirements to airports, heavy industrial sites, and locations with known PFAS or other contaminants of concern. Staff gave examples: low-risk land uses would include residential neighborhoods and parks; high-risk land uses would include heavy industrial properties and sites with known PFAS sources. Staff repeatedly said thresholds and numeric cutoffs (for drainage area, vertical setbacks, monitoring triggers and other specifics) remain under development and that the guidance will include minimum setbacks as a starting point but allow site-specific engineering analyses to propose alternatives.

Participants pressed staff on several technical and programmatic issues. Consultants and municipal staff asked how the policy would interact with existing MS4 permits, the Industrial General Permit (IGP) and Attachment I (industrial requirements), and whether measures would be duplicative. Staff replied they plan a crosswalk of existing MS4 permit language and are coordinating with permit writers; the working assumption is to incorporate the policy into MS4 permits where feasible to avoid creating new permitting burdens.

PFAS and other persistent contaminants were a recurring topic. Several commenters and staff noted limited data on PFAS behavior between stormwater and groundwater. Staff said they are compiling available sampling from drinking-water and PFAS investigations and reviewing the literature, but acknowledged data gaps and limited funding to collect new statewide data. They signaled that PFAS will be treated as a special case in the risk framework.

On monitoring and oversight, staff said monitoring requirements will likely scale with risk tier: higher-risk projects would face stronger sampling and reporting requirements, while small residential systems and rain gardens would not be targeted for additional burdens. Staff also said operations-and-maintenance plans and pretreatment to control trash and sediment clogging will be part of the guidance. A staff member said clogging risk will be considered in pretreatment and O&M requirements and that full capture of trash is expected prior to infiltration.

Several attendees asked about exemptions or special treatment for existing infiltration systems and large programs such as Caltrans retention/retention facilities. Staff said existing systems will be evaluated and are not automatically exempt; the goal is to avoid imposing overly onerous retroactive requirements but to include existing systems in implementation planning. Staff also said there is no single statewide inventory of existing infiltration sites; that information typically resides with individual MS4 permittees or regional boards.

Other scope items clarified by staff: the policy focuses on urban stormwater runoff and infiltration measures (dry wells, infiltration galleries, etc.), not on urban river restoration or separate stormwater storage/tank projects. Coastal issues such as sea-level rise and saltwater intrusion will be expected to be addressed in project design and geotechnical engineering at the local level rather than as prescriptive statewide policy language.

Staff emphasized coordination with regional boards and permit writers and noted the U.S. EPA underground injection control (UIC) program remains relevant: projects implementing infiltration measures should follow EPA registration or UIC requirements where applicable. Staff said they do not intend to require that infiltrated stormwater meet drinking-water maximum contaminant levels (MCLs) prior to infiltration, but will define circumstances where pretreatment or monitoring is required to protect groundwater beneficial uses and comply with the state's anti-degradation policy.

The workshop did not produce any formal decisions or votes. Staff closed by inviting continued engagement, additional technical data (especially on PFAS and vadose-zone behavior), and written comments to stormswaterboards.ca.gov by 12 noon on Feb. 24, with further public workshops planned during policy development.