A Watershed Event for the Middle Santa Ana River Watershed: Quantitatively Parsing Fecal Bacteria Loads Between MS4 and Non-MS4 Sources
The Middle Santa Ana River Bacterial Indicator TMDL (MSAR TMDL, adopted by the Santa Ana Water Board in August 2005, became effective in May 2007 following USEPA approval. Since then, revisions to the Santa Ana Region Basin Plan (2012) and the Inland Surface Waters, Enclosed Bays & Estuaries (2018) have changed the underlying regulatory basis for protection of the REC-1 beneficial use. These changes include modifications to bacteria water quality objectives (WQO), a natural sources exclusion provision and a high flow suspension (HFS) of the REC-1 use when water conditions are unsafe. Given these regulatory changes and other changes in the watershed, stakeholders have initiated a process to revise the 2005-adopted TMDL. To support this effort, a MSAR Bacteria Synoptic Study (MBSS) was completed in late summer 2019 to gather data needed to support the TMDL revision process.
Stakeholders designed the MBSS to provide data to establish an updated loading analysis for indicator bacteria and HF183 human Bacteroides marker in the watershed during dry weather conditions. Of particular interest was the collection of a comprehensive set of samples that not capture inflows not only from the MS4 but also from POTWs that are the primary source of dry weather flows (DWF) in the watershed. Key MBSS findings include:
(a) MS4 Programs have hydrologically-disconnected the majority (66%) of the upper MSAR watershed during dry weather conditions through infiltration in unlined flood control channels, retention basins, and flow diversion projects; these disconnected areas no longer cause or contribute to exceedances of the WQOs in impaired water during dry weather conditions.
(b) Where hydrologic disconnection has not occurred, DWF rates continue a long-term downward trend as a result of outdoor water conservation measures implemented in the region over the past decade.
(c) Currently, nonpoint sources upstream of any MS4 discharge account for the majority (77%) of the total bacteria load in the Santa Ana River measured at downstream compliance monitoring sites. Moreover, based on source analyses completed in years 2007, 2012, and now 2019, the river would be in compliance with the TMDL targets and the state's new bacteria WQOs were it not for the excessive loads from these unknown nonpoint sources which are not conveyed through the MS4.
(d) Frequency and magnitude of human source bacteria at MS4 outfalls has declined, indicating that efforts to regulate septic systems and better maintain sewer collection systems have been effective.
(e) Lack of a relationship between E. coli and presence of human HF183 marker within receiving waters strongly suggests that the E. coli observed in impaired waters is more likely coming from natural or uncontrollable sources (bottom sediment, biofilms, wildlife) than controllable sources.
(f) Efforts to mitigate sources of E. coli bacteria within MS4s alone will not be enough to attain the E. coli WQOs in Santa Ana River Reach 3.
The findings from the MBSS were used to update bacteria mitigation priorities for the MS4s to support continued implementation of the existing TMDL. Meanwhile stakeholders have begun the process to revise the MSAR TMDL. The outcome of this process will drive future stormwater management in this watershed. Innovation will be needed to determine how to incorporate best new regulatory provisions including addressing uncontrollable and/or natural sources of bacteria and applying the HFS during wet weather conditions. The manner in which these elements are considered during the TMDL revision process can provide guidance for others working on bacteria TMDLs. The presentation will not only provide initial thoughts on how these elements may be included in the revised TMDL but will also seek input from conference participants on the approach.