Tracking sources of bacterial contamination in stormwater discharges to Mission Bay, California

Sources of the indicator bacteria total coliform, fecal coliform, and enter ococcus were investigated,in stormwater flows discharging to Mission Bay, a heavily used aquatic park in San Diego, California. Stormwater flows were targeted because long-term receiving water monitoring of the bay. indicated that wet weather discharges were the predominant source of bacterial conta mination. Exceedences in water quality objectives for body contact recreati on established by the State of California most often occurred in the east b ay, where the least amount of circulation and largest quantities of stormwa ter discharges occur. Unlike the wet weather results, almost all of the 89 storm drains that discharge to the bay either did not have flowing fresh wa ter or did not contain exceedingly high bacteria densities during dry weath er. Upstream tracking during multiple storm events on two of the largest wa tersheds draining to the bay showed that sources of indicator bacteria were diffuse and widespread. Densities were as high at the head of each watersh ed as they were at the mouth, where both discharged to the bay. Every reach in each creek exceeded State of California water quality objectives and ha d densities similar to surface flows measured before they entered the separ ate municipal storm sewer system from urban land uses, such as residential, commercial, and industrial, as well as open lands.