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Since 1998, the County of Orange, California, has been implementing and managing a Nutrient TMDL for the San Diego Creek and Newport Bay watershed. The TMDL establishes final targets of a 50% reduction in annual loads of both total nitrogen and total phosphorus. Over the past several years, monitoring data have shown that total nitrogen concentrations and total nitrogen loads to Newport Bay have been steadily decreasing. The first interim TMDL target of 2002 (requiring a 30% reduction), as well as the second interim target of 2007 (requiring a 50% reduction), was significantly met for summer loading of total nitrogen and yearly loading of total phosphorus by December 2002. In addition, algal biomass in Newport Bay has been considerably reduced and the green algae species Ulva (indicative of nutrient pollution) has been mostly replaced by a group of non-nuisance species.

The decreased loadings and algal biomass are the result of control measures implemented by the local nurseries, the restoration of a marsh that serves as a treatment wetland, and a decrease in flow rates attributable to water conservation efforts and below average rainfall that has reduced the impact of groundwater seepage into surface waters. The next phase of the TMDL will depend upon the analysis of beneficial use impairment and the interpretation of the narrative water quality objectives. Due to lingering algal blooms in a small portion of Upper Newport Bay, the Santa Ana Regional Water Quality Control Board is considering further reductions to both the TMDL loading targets and numeric water quality objectives. However, many stakeholders in the watershed consider this TMDL to be a great success, evidenced by the significant reduction in nutrient concentrations and loadings and the resultant reduction in beneficial use impairments of Newport Bay.
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Document Type: Research Article

Publication date: 2005-01-01

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