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Total Maximum Daily Load (TMDLs)

A TMDL (Total Maximum Daily Load) is defined in federal regulations as “the sum of the individual waste load allocations for point sources and load allocations for nonpoint sources and natural background such that the capacity of the waterbody to assimilate pollutants (the Loading Capacity) is not exceeded.” 

In other words, a TMDL sets a limit for the total amount of a particular pollutant that can be discharged to a waterbody. This limit ensures that pollutant loads from all sources will not impair the designated beneficial uses of the waterbody. The timeframe for compliance with TMDL targets varies but may take many years. TMDLs will often include a compliance schedule, identifying interim and final targets. 

A TMDL is developed when a waterbody has been identified as impaired. Section 303(d) of the federal Clean Water Act requires states to establish a listing of all impaired waterbodies and to rank those waterbodies according to priority for TMDL development. This list, called the 303(d) List is updated every two years and is developed by the Regional and State Water Quality Control Boards and approved by EPA. 

The following TMDLs have been established or are being developed for Orange County waterbodies:  

  • Coyote Creek: Metals (copper, lead, zinc) 
  • San Diego Creek/Newport Bay: 
    • Sediment 
    • Nutrient   
    • Toxics  
    • Fecal Coliform (Note: The Fecal Coliform TMDL applies only to Newport Bay) 

The Santa Ana Regional Water Quality Control Board's  website contains additional information on TMDLs in the Santa Ana Region. 

Coyote Creek – Metals and Bacteria TMDLs


Coyote Creek drains a watershed of 165 square miles, 85.5 square miles of which lie in north Orange County, with the remainder in Los Angeles County. The upper watershed contains some open space, most of which is used for oil production. However, most of the watershed is highly urbanized with a mixture of residential, commercial, and industrial development. Coyote Creek flows along the border between Orange and Los Angeles counties and finally flows into the San Gabriel River just above its tidal prism. For additional information about Coyote Creek and San Gabriel watersheds, see the San Gabriel River-Coyote Creek Watershed page.  Monitoring results from 2000-03 in lower Coyote Creek have shown dissolved metals are a potential concern to wildlife. 

Regulatory History

The Los Angeles Regional Water Quality Control Board (LARWQCB) found that metals loads in the San Gabriel River watershed, including Coyote Creek, may be harmful to aquatic life and also may impair the water supply. The LARWQCB Staff Report did not indicate any dry weather impairments in Coyote Creek. However, analysis of storm samples collected by the Los Angeles County Department of Public Works indicated too many exceedances of the copper, lead, and zinc chronic criteria from the California Toxics Rule. 

Consequently, the LARWQCB adopted the San Gabriel River and Impaired Tributaries Metals and Selenium TMDL on July 13, 2006. These include wet weather TMDLs for copper, lead, and zinc in Coyote Creek. No dry weather TMDLs were adopted for the creek. 

However, LARWQCB has no jurisdiction over the Orange County portion of the Coyote Creek watershed. TMDL provisions in north Orange County watersheds are enforced by the Orange County Stormwater National Pollutant Discharge Elimination System Permit  issued by the Santa Ana Regional Water Quality Control Board (SARWQCB). Although the SARWQCB has not adopted TMDLs for Coyote Creek, it deferred to the LARWQCB and incorporated requirements in the Stormwater Permit to comply with TMDLs established for Coyote Creek by LARWQCB, including development of a source control plan and monitoring program (SCP) for copper, lead, and zinc. 

Later the LARWQCB adopted Resolution No. R15-005 to incorporate the TMDL for Indicator Bacteria in the San Gabriel River, Estuary and Tributaries that also applies to Coyote Creek. 


Legislation to phase out copper in brake pads in California (SB 346) is expected to help watershed stormwater copper concentrations comply with TMDL requirements. Scientific studies have shown that brake pad dust is by far the greatest source of copper in urban watersheds. 

Data Collection and Reporting 

The County of Orange and watershed cities formed a workgroup to develop an SCP, which was formally approved by all parties in 2010. Monitoring under the SCP began in July 2010 and is ongoing, including dry weather and storm monitoring elements. 

Monitoring results to date have shown that dry weather metals loads have been very low and fully comply with the TMDL. Stormwater lead and zinc concentrations also comply with TMDL limits. Section 12 of the Unified Annual Progress Report includes annual updates on regional watershed efforts for these TMDLs. 

Newport Bay Sediment TMDL


In March 1999 the Santa Ana Regional Water Quality Control Board approved a sediment total maximum daily load (TMDL) for the Newport Bay watershed to address water quality impairment due to excessive sedimentation. The TMDL for sediment requires the implementation and maintenance of sediment control measures aimed at ensuring that existing habitat acreages of Upper Newport Bay are not significantly changed and sediment discharges in the watershed are reduced by 50% over a multi-year period. The long-term goal of the sediment TMDL is to reduce the frequency of dredging Upper Newport Bay to once every 20 to 30 years. 

Quantifiable targets of the TMDL are to: 

  • Reduce the annual average sediment load from a total of 250,000 tons per year to 125,000 tons per year, thereby reducing the sediment load to Newport Bay to 62,500 tons per year and limiting sediment deposition in the drainages to 62,500 tons per year. 
  • Maintain the existing acreages of aquatic, wildlife, and rare and endangered species habitat in the Bay 
  • Maintain a minimum depth of 7 feet below mean sea level in Units I and II of the Bay 
  • Maintain 50% available storage capacity levels of the in-channel and foothill basins. 

In November 1999 the Santa Ana Regional Water Quality Control Board subsequently adopted Monitoring and Reporting Program No. 99-74 which requires monitoring, surveys, and reporting in accordance with the requirements of the sediment TMDL. The sediment monitoring and maintenance program consists of two study area elements: (1) the Upstream Monitoring Element which includes those activities performed in the San Diego Creek watershed upstream of Jamboree Road Bridge and in the Santa Ana-Delhi Channel, and (2) the Newport Bay Monitoring Element which includes those activities performed in Upper Newport Bay. 

Monitoring Programs 

The Upstream Monitoring Element consists of monitoring and maintaining the available capacities of three in- channel basins along San Diego Creek, seven foothill basins, and the collection of suspended sediment samples during dry flow and storm flow conditions and streamflow data from eight monitoring stations. 

The Newport Bay Monitoring Element consists of conducting vegetative and bathymetric surveys as needed, removal of sediment from the in-bay basins as needed, and fluvial sediment and flow monitoring during storm events. 

An annual report is submitted to the Santa Ana Regional Water Quality Control Board by November 15 of each year verifying that the in-channel and foothill basins have at least 50% design capacity available for the upcoming storm season. The TMDL Annual Report (a compilation of sediment monitoring data and TMDL compliance analysis) is to be submitted by February 27 of each year. 

In general, the available data suggests that sediment loads in the Newport Bay/San Diego Creek watershed have been reduced significantly from rates recorded in the pre-TMDL period and that the target reduction is being attained.  

Sediment TMDL Annual Report

The Newport Bay Sediment TMDL data report is linked here:     

Sediment Links

Newport Bay/San Diego Creek Nutrient TMDL


Excess nutrients flowing into the Newport Bay , primarily from San Diego Creek, have resulted in seasonal algae blooms that have impaired the Bay’s environment and our enjoyment of this important natural resource. The nutrients of concern are nitrogen and phosphorous, which are both essential for plant growth and necessary for healthy ecological functions in the Bay. However, high nutrient concentrations can cause excessive plant growth, including algae. This can lead to a problem called eutrophication, a condition in which excessive plant growth from nutrient enrichment impairs the capacity of a waterbody to sustain healthy ecological functions. This can lead to aesthetic problems, habitat loss, and poor biological diversity, among other adverse effects. Both nitrogen and phosphorous occur in nature, but eutrophication usually results from human activities that promote the input of these nutrients into our waterbodies. Some sources of these nutrients include agriculture, excessive garden or lawn fertilization, and pet waste. 

Regulatory History

In 1998, the Santa Ana Regional Water Quality Control Board  adopted a Total Maximum Daily Load for nutrients in the Newport Bay watershed (TMDL) to decrease the mass of nutrients flowing into the Bay, thus restoring and protecting its beneficial uses. Beneficial uses are assigned to every water body in the United States as a means of systematically assessing the quality of the nation’s surface waters, as required by the federal Clean Water Act of 1972. If a water body’s beneficial uses are impaired, then the water body is deemed impaired and requires restoration measures. The TMDL establishes maximum nutrient loads (targets) at levels similar to those observed in the 1970s, prior to observations of eutrophic conditions. Thus, if these targets are met, signs of eutrophication in the Bay should diminish. 

The State Water Board is proposing to adopt a statewide water quality objective for biostimulatory substances along with a program of implementation as an amendment to the Water Quality Control Plan for Inland Surface Water, Enclosed Bays, and Estuaries of California. 

Data Collection and Reporting 

In February 2000, a Regional Nutrient Monitoring Program (RMP) for the Newport Bay watershed was initiated by the County of Orange on behalf of the watershed cities (Costa Mesa, Irvine, Laguna Hills, Laguna Woods, Lake Forest, Newport Beach, Orange, Santa Ana, and Tustin), as mandated by the TMDL to assess compliance with the required nutrient reductions. The RMP is composed of routine and special monitoring components. The RMP has been revised since 2000, including most recently in 2014, and monitoring and reporting efforts are currently being implemented. 

The Newport Bay Nutrient TMDL data report is linked here.

Newport Bay Toxics TMDL


Toxic pollutants are different from conventional pollutants such as sediment and bacteria in that they can cause biological impairment at low concentrations due to their high toxicity. Many toxic pollutants tend to bioaccumulate, in other words, their concentrations will increase along the food chain. Many are persistent and tend to attach onto suspended and bedded sediments, therefore the period of impact can greatly exceed the period of discharge. 

Regulatory History

In 2002, the U.S. Environmental Protection Agency (EPA) established Total Maximum Daily Loads for Toxic Pollutants, San Diego Creek and Newport Bay, California. Referred to as the Toxics TMDLs, it covers 14 toxic pollutants for the San Diego Creek/Newport Bay watershed as listed below. 

Waterbody Trace Elements / Metal Organic Compounds 
San Diego Creek (freshwater) Cd, Cu, Pb, Se, Zn chlorpyrifos, diazinon, chlordane, dieldrin, DDT, PCBs, toxaphene 
Upper Newport Bay (saltwater) Cd, Cu, Pb, Se, Zn chlorpyrifos, chlordane, DDT, PCBs, toxaphene 
Lower Newport Bay (saltwater) Cu, Pb, Se, Zn chlordane, dieldrin, DDT, PCBs 
Rhine Channel (saltwater) Cd, Cu, Pb, Se, Zn, Cr, Hg chlordane, dieldrin, DDT, PCBs 

The Santa Ana Regional Water Quality Control Board is currently dividing the Toxics TMDL into five separate TMDLs based primarily on chemical class, and developing individual implementation plans for each TMDL. These TMDLs include: 

On June 20, 2019, a revised Selenium TMDL was approved by the U.S. EPA. A watershed-wide collaborative effort to address nitrogen and selenium issues supported the development of the Selenium TMDL and continues to support the development of selenium site-specific objectives (SSOs).  

Newport Bay Fecal Coliform TMDL

Background Information

Fecal coliforms are a type of fecal indicator bacteria (FIB) found in the digestive systems of humans and animals that are monitored worldwide to evaluate water quality. The presence of fecal coliforms in surface waters is used as an indicator of human pathogens, which can cause illness in swimmers and recreators. FIBs have historically been used as indicators of human pathogens because they are easier and less costly to measure than pathogens themselves. Recently, the choice of FIBs have shifted to E. coli for freshwater and Enterococci for marine waters. Therefore, the Newport Bay Fecal Coliform TMDL is being revised to reflect the latest science and to be consistent with state and federal guidance. 

Regulatory History

In 1999 the Santa Ana Regional Water Quality Control Board adopted a Total Maximum Daily Load (TMDL) for fecal coliforms in the Newport Bay. Given the complexity of the problem, the paucity of relevant data on bacteria sources and fate, the expected difficulties in identifying and implementing appropriate control measures and uncertainty regarding the nature and attainability of the shellfishing beneficial use in the Bay, a prioritized, phased approach to the control of bacterial quality was developed. The phased approach is intended to allow for additional monitoring and assessment to address areas of uncertainty and for future revision and refinement of the TMDL. 

A public stakeholder process was implemented beginning in 2017 to address bacterial water quality regulations in the Newport Bay with respect to Recreational and Shellfish Harvesting Beneficial Uses. ( ).  Following the efforts of this stakeholder group, a Time Schedule Order R8-2019-0050 was adopted on December 6, 2019 for additional studies related to the Fecal Coliform TMDL.   

Data Collection and Reporting

Per California Water Code (Section 13267), and Time Schedule Order R8-2019-0050), the County of Orange and the Cities of Costa Mesa, Irvine, Lake Forest, Newport Beach, Orange, Santa Ana, and Tustin, the Irvine Ranch Water District (IRWD) and the Irvine Company are currently supporting studies and monitoring in the Bay towards developing a TMDL implementation plan to achieve the TMDL targets. 

Monitoring and study efforts have included the development and implementation of a routine monitoring program, development of a water quality model for bacterial indicators, recreational and shellfish harvesting beneficial use assessments, a human health risk assessment, evaluation of the vessel waste program, and source identification and characterization of the Dunes Resort, agricultural, urban and natural runoff sources. The County of Orange, on behalf of watershed stakeholders, implements a routine monitoring program to determine compliance with bacterial water quality objectives in the Bay. Orange County Health Care Agency collects at least five samples per 30-day period at 35 stations throughout Newport Bay and publishes this data on-line

The Newport Bay Fecal Coliform Annual Data and Progress Report reports data from the water quality monitoring program, along with analysis on attainment of the water quality objectives, and information on additional implementation tasks.