Southern California Agricultural Water Team - Water Quality Regulation

Water Quality Regulation:

Rainbow Creek Nutrient TMDL

Location of Rainbow Creek Watershed
(Source: San Diego Regional Water Quality Control Board’s Presentation)

In 1996, Rainbow Creek was placed on the Clean Water Act Section 303(d) list of “impaired water quality” for excessive nutrient levels (total nitrogen and total phosphorus). The nutrient concentrations in the Rainbow Creek were exceeding the numeric goals specified in the San Diego Regional Water Quality Control Board’s “Basin Plan” (also known as Water Quality Control Plan). Elevated nutrient concentrations were causing excessive algal growth in some portions of the creek.

On February 9, 2005, the San Diego Regional Water Quality Control Board, finally, adopted Total Maximum Daily Loads (TMDLs) for Total Nitrogen and Total Phosphorus to address water quality impairments in Rainbow Creek. The TMDL was adopted by the State Water Resources Control Board on February 1, 2006 and the final approval granted by the USEPA on March 22, 2006.

Water Quality Objectives
The TMDL numeric targets for nitrate, total nitrogen, and total phosphorus set by the Regional Board are equal to those established in the Basin Plan. The numeric targets for nitrate (as nitrogen) is 10 mg/L, total nitrogen is 1.0 mg/L, and total phosphorus is 0.1 mg/L.

Compliance Schedule
Nutrient load reductions are required to reach the numeric target in a 16-year phased compliance schedule period. The TMDL requires all growers who irrigate in the Rainbow Creek watershed to reduce their contribution of nutrients by 20% each four-year period (beginning with 2005-2009). The goal of the first four-year phase is to attain the nitrate water quality objective and reduced phosphorus concentrations in Rainbow Creek.

Allocations and Reductions
By 2021, at the end of the 16-year TMDL implementation period, the cumulative reduction is to have reached 74% for total nitrogen and 85% for total phosphorus. Each point and nonpoint source has been assigned different percent reduction allocations (see Table below). Agricultural fields and orchards are required to reduce their loading of total nitrogen by 77% and total phosphorus by 90% in this 16-year compliance period. Although these compliance dates may seem far in the future, the San Diego Regional Water Board will likely soon implement a monitoring program that will require grower participation and be required through a conditional waiver. Data collected through the monitoring program could be used by the Regional Water Board to justify more stringent regulation or enforcement action.

Percent Reduction of Total Nitrogen and Total Phosphorus by Source

Percent Reduction by Source	Total Nitrogen	Total Phosphorus
Point Source
Caltrans	68%	64%
Nonpoint Source
Commercial Nurseries	77%	90%
Agricultural Fields	77%	90%
Orchards	77%	90%
Park	50%	50%
Residential Areas	77%	90%
Urban Areas	50%	50%
Septic Tank Disposal Systems	77%	Not Applicable

This Rainbow Creek TMDL is significant for the agricultural community in Northern San Diego County and will have long-term consequences as growers move to comply with the requirements. This TMDL will likely be copied in other regions with potentially far-reaching impacts (e.g., plans to reduce sources of nutrients and schedules for reduction).

Adoption of the TMDL also sets the stage for the San Diego Water Board to begin a process to develop a revised conditional AG waiver program. Waivers requiring registration are in place in the Central Valley and Central Coast Regions, and in Los Angeles and Ventura Counties.
For more information on the Rainbow Creek nutrient TMDL, please visit the following links.

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