Famosa Slough Nutrient TMDL

Introduction

Excessive loading of nutrients (total nitrogen and total phosphorus) into Famosa Slough promotes excessive algae growth and depletes dissolved oxygen levels. These changes limit the ability of this water body to support its beneficial uses designated in the Water Quality Control Plan for the San Diego Basin (9). Famosa Slough was placed on the Clean Water Act (CWA) section 303(d) list of impaired water bodies in 1990 due to eutrophic conditions identified by excessive algal growth. To address the impairment a draft Total Maximum Daily Load (TMDL) has been developed that assesses conditions and identifies a strategy to improve water quality.

Famosa Slough is an estuary located along the San Diego River in the City of San Diego and is one of the only remnants of a historically large wetland that once encompassed the San Diego River floodplain and Mission Bay. This 37-acre wetland, located within a highly urbanized area, provides habitat to shorebirds and wildlife and offers opportunities for environmental education and public recreation.

The San Diego Water Board accepted the City's plan to restore Famosa Slough on October 31, 2017. The TMDL investigation indicated that non-storm water discharges from the local watershed and the storm water conveyance system are the primary sources of nutrients. These discharges are regulated under the existing Regional MS4 Permit, and the necessary actions to reduce nutrient loading and restore beneficial uses can be tracked through the reporting requirements of the Regional MS4 Permit. Therefore, this alternative restoration plan using the existing permit was implemented instead of adopting the TMDL as a Basin Plan amendment.

Water Quality Restoration

The City of San Diego developed a draft TMDL report to address the eutrophic conditions at Famosa Slough.  The TMDL analysis identifies pollutant sources, pollutant loads, load reductions and necessary actions to restore water quality.  The report shows the primary sources of nutrients affecting Famosa Slough come from non-storm water discharges from the surrounding areas and the City of San Diego's storm water conveyance system. 
The plan for water quality restoration is for the City of San Diego to implement focused actions that include nutrient reductions from storm and non-storm water discharges from the surrounding areas into Famosa Slough and algae harvesting when necessary.  The San Diego Water Board's Regional MS4 Permit regulates storm and non-storm water discharges in the San Diego Region.  Development of Water Quality Improvement Plans is a requirement in the Regional MS4 Permit.  The implemented actions and monitoring results will be documented in future revisions to the Water Quality Improvement Plan for the San Diego River.   
The draft TMDL report is the basis for the actions that the City of San Diego plans to implement.  Relying on the existing Regional MS4 Permit will streamline the water quality restoration process without formally adopting a TMDL as a Basin Plan amendment.  This approach identifies Famosa Slough as a high priority waterbody, and ensures resources are dedicated to water quality restoration efforts in a timely manner.  The Famosa Slough and Channel status on the CWA section 303(d) list has been updated to 5-ALT to indicate that an alternative regulatory approach is being implemented to restore water quality rather than adopting a formal TMDL. 

CORRESPONDENCE AND REPORTS

• Water Quality Restoration Plan for Nutrient Reduction in Famosa Slough: Alternative Approach to a Nutrient Total Maximum Daily Load
• Famosa Slough Nutrients / Eutrophication Total Maximum Daily Loads Technical Report
• City of San Diego Letter dated August 31, 2017
• San Diego Water Board Letter dated October 31, 2017