Water quality at a major ocean outfall on the San Pedro Shelf, California: summary of a nine-year study

Water quality data (temperature, salinity, dissolved oxygen, pH, light transmission, ammonium, and fecal coliform bacteria) were collected from July 1985 to June 1994 to evaluate the relationship between the ocean outfall and ambient water quality and assess compliance with California Ocean Plan criteria. Data were collected as part of a routine monitoring program of a major ocean outfall on the San Pedro Shelf, California for the Orange County Sanitation District. Very small differences were detected near the outfall between the wastewater discharge and natural seawater for temperature, dissolved oxygen, light transmission, and pH. In contrast, values above ambient were observed almost every month for ammonium, and below ambient for salinity in the nearfield (⩽0.5 kilometer [km]; 0.3 miles [mi]) of the outfall in the longshore direction). Occasionally, ammonium was measured above background concentrations in the farfield (0.5-3.0 km; 0.3-1.9 mi). The largest differences from background (three orders of magnitude) were observed for fecal coliform bacteria. Most of the time the higher concentrations were found at depths greater than 10 meters (m) (33 feet [ft]), but they did reach waters at 5 m (16 ft) from December to mid-February when minimal or no water column density stratification existed. While changes due to the discharge of highly treated wastewater have been observed, the data from this long-term monitoring program indicated that there are no environmentally significant water quality effects from the district´s discharge. In addition, compliance with state receiving water criteria was achieved well over 90% of the time and all observed values fell within the range of natural variability