A Natural Treatment System Master Plan Approach to meeting TMDLs: Case Study of San Diego Creek, Orange County, California

A Natural Treatment System (NTS) Master Plan comprised of a network of constructed wetlands was evaluated for treatment effectiveness of dry weather base flows and runoff from smaller more frequent storms in a 120 square mile (311 km2), urban watershed. The goal of the ‘regional retrofit’ wetland network is to serve as an integral component in watershed-wide BMPs for compliance with recent and proposed pollutant loading limits (TMDLs) requiring discharge limits of sediments, nutrients, pathogen indicators, pesticides, toxic organics, heavy metals, and selenium. The NTS Plan was assessed with ‘planning-level’ water quality models that account for the integrated effects of the planned 44 NTS facilities. The NTS Plan is estimated to achieve total nitrogen (TN) TMDL for base flows, and in-stream TN concentrations would be reduced below current standards at most locations. Total phosphorous TMDL targets would be met in all but the wettest years. The fecal coliform TMDL would be met during the dry season, but not all wet season base flow conditions, and not under storm conditions. The NTS Plan is not designed to meet the sediment TMDL, but would capture on average about 1900 tons/yr (1724000 kg/yr) of sediment from urban areas. The wetlands are estimated to remove 11 percent of the total copper and lead, and 18 percent of the total zinc in storm runoff. The NTS Plan provides a cost-effective alternative to routing dry-weather flows to the sanitary treatment system.