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TMDL Information

Section 303(d) of the Clean Water Act requires states, territories and authorized tribes to submit a list of impaired waters to the US Environmental Protection Agency every two years. Impaired waters are defined as waters where required pollution controls are not sufficient to attain or maintain applicable water quality standards. For water bodies on the 303(d) list, states are required to develop a Total Maximum Daily Load (TMDL). A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can receive and still safely meet water quality standards. TMDLs allocate pollution control responsibilities among different pollution sources in a watershed. The establishment of a TMDL for a waterbody is the basis for taking the actions needed to restore the waterbody for beneficial uses.

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Photo by Mission Resource Conservation District

In 1996, Rainbow Creek was placed on the 303(d) list of "water quality limited" waterbodies because data indicated that beneficial uses in the creek were impaired due to excessive nutrient concentrations. Impaired beneficial uses include: municipal water supply, warm freshwater habitat, cold freshwater habitat, contact water recreation and non-contact water recreation. These beneficial uses were found threatened due to excessive levels of nutrients in Rainbow Creek.

Water quality sampling by the San Diego Regional Water Quality Control Board, in the year 2000, documented nutrient concentrations as high as 21 mg/L of nitrate as nitrogen, 23 mg/L of total nitrogen and 1.7 mg/L of total phosphorus. Excessive nutrient concentrations can lead to problems ranging from annoyance due to an overabundance of algae and vegetation to human health problems and adverse ecological effects. Elevated nutrient concentrations have caused excessive algal growth in portions of the creek. This algal growth impairs aesthetic and recreational uses while creating harmful conditions for aquatic life and degrades water quality.

Nutrient concentrations in Rainbow Creek did not meet the objective for nitrates in a municipal supply or the numeric goals for biostimulatory substances (total nitrogen and total phosphorus) contained in the Water Quality Control Plan for the San Diego Basin (Basin Plan). The Basin Plan established that total nitrogen and total phosphorus levels for beneficial uses should not exceed 1.0 mg/L and 0.1 mg/L, respectively, more than 10% of the time. The maximum contaminant level for nitrate as nitrogen set forth in the Basin Plan for waters designated for use for municipal supply is 10 mg/L.

The San Diego Regional Board has adopted TMDLs for total nitrogen and total phosphorus to address the water quality impairments in Rainbow Creek. On February 9, 2005, the Regional Board conducted a public hearing and adopted the amendment to the Basin Plan to include the Total Nitrogen and Total Phosphorus TMDLs and an implementation plan. The State Water Resources Control Board, on November 16, 2005, and Office of Administrative Law, on February 1, 2006, subsequently approved the Basin Plan amendment. The U.S. Environmental Protection Agency granted final approval of the TMDLs on March 22, 2006. The TMDLs set the numeric targets for nitrate (as nitrogen) at 10 mg/L, total nitrogen at 1.0 mg/L and total phosphorus at 0.1 mg/L. The TMDLs for total nitrogen and total phosphorus discharges into Rainbow Creek were calculated to be 1,658 kilograms of nitrogen per year (kg N/yr) and 165 kilograms of phosphorus per year (kg P/yr).The targeted levels of total nitrogen and total phosphorus in the TMDLs were based on the calculated loading capacity of Rainbow Creek. Loading capacity is the maximum amount of total nitrogen and total phosphorus that Rainbow Creek can receive and still attain water quality objectives and protection of designated beneficial uses.

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In order to meet the established TMDLs of 1,658 kg N/yr and 165 kg P/yr, a 74% overall reduction of total nitrogen loading and an 85% reduction of total phosphorus loading to Rainbow Creek are required. CalTrans, the County of San Diego, commercial nurseries, agricultural fields, orchards, parks, septic tank disposal systems, and residential and urban areas were all identified by the Regional board as causing or permitting nutrient discharges into Rainbow Creek. The table to the right outlines the necessary nutrient loading reductions needed to be achieved by each identified discharger.


Between 2002 and 2005, MRCD conducted a citizen water quality monitoring program, called the Home2Ocean Program, which sampled water quality in Rainbow Creek, Stone Creek (another tributary of the Santa Margarita River) and the mainstem of the Santa Margarita River. Water samples were taken on a bi-monthly schedule and the Final Data Report was finalized at the end of the program.

TMDL Resources