Our MS4 Permit

The Los Angeles Regional Water Quality Control Board (Regional Board) adopted a new regional permit, Order R4-2021-0105 (Regional Permit), on July 23, 2021, that covers Ventura and Los Angeles Counties and all the incorporated cities therein. As of its effective date, September 11, 2021, the regional permit superseded the three Orders that covered the Permittees in the region: R4-2010-0108 (Ventura County), R4-2012-0175 (Los Angeles County except for the City of Long Beach), and R4-2014-0024 (City of Long Beach).

The Regional Permit, while similar to the 2012 Los Angeles Permit and 2014 City of Long Beach Permit, differs significantly from the 2010 Ventura County Permit.  

One of the most significant differences in the Regional Permit was the option to achieve compliance by developing a Watershed Management Program (WMP), which examines water quality priorities and utilizes computer modeling to identify the actions Permittees can take to meet applicable water quality limits. The goal of a WMP is to forecast the future water quality after the most cost effective structural and non-structural best management practices have been installed or implemented. The Ventura County Permittees worked together to develop WMPs for each major watershed in Ventura County. The WMP incorporated the latest data, science, and technical methodologies to help Permittees better understand current watershed conditions.

The Regional Permit also included the option to develop a Coordinated Integrated Monitoring Program (CIMP) to specify approaches for meeting the objectives of the Permit’s Monitoring Reporting Program. The  guidelines for the CIMP allowed water quality monitoring for Regional Permit and TMDL (Total Maximum Daily Load) activities to be combined to eliminate duplicative or wasteful efforts. The Ventura County Permittees worked together to develop a CIMP to detail the receiving water monitoring, stormwater outfall-based monitoring, non-stormwater outfall-based monitoring, and all TMDL monitoring requirements, either by incorporation or by reference.

The WMPs and CIMP were submitted to the Regional Board by the due date of September 11, 2023. The WMPs outlined actions the Permittees would take to comply with the Permit for each watershed, and the CIMP provided details of the monitoring approach, including monitoring locations, constituents, and frequency.

The Regional Board approved the WMP for the Malibu Creek watershed on May 12, 2025. The remainder of the WMP was not approved. For those Permittees outside of Malibu Creek Watershed, the WMP will serve as a guide and implementation tool to target pollutants that cause and contribute to receiving water impairments from the MS4 systems, with the goal of achieving compliance with permit limitations across each watershed.

Revisions to the CIMP in response to Regional Board comments were submitted on April 15, 2024, June 16, 2024, and January 27, 2025. The CIMP was approved on May 12, 2025. A request to add a missing footnote for the monitoring tables was approved on December 23, 2025, and a revised CIMP incorporating this change was submitted in January 2026. Monitoring transitioned from 2010 Permit requirements to the CIMP during the 2025-2026 monitoring year.

The Regional Permit and its attachments are included below.