When aquatic life can’t breathe in the San Joaquin River, nothing else matters.

Dissolved oxygen concentrations in the San Joaquin River near Stockton declined from 8 mg/L on January 1, 2003 to 0 mg/L in early February. This reach remained anoxic throughout most of February. This was the most severe dissolved oxygen deficit of record for the San Joaquin River. The environmental engineering research group at Pacific investigated potential causes for this phenomenon. A field and laboratory investigation was initiated to quantitatively determine the causes and mechanisms responsible for this extreme event. Kinetic rate studies of biochemical oxygen demand (BOD) and ammonia vitrification where performed in the laboratory and compared with modeled analyses of field observations. High ammonia concentrations were measured in the San Joaquin River at concentrations that exceeded levels toxic to sensitive aquatic life. Nitrifying bacteria readily convert ammonia to nitrate and in the process these microorganisms consume four times as much dissolved oxygen as ammonia on a mass basis. The ammonia load to the River originated from the discharge of treated wastewater effluent from the City of Stockton Wastewater Treatment Facility. Near zero net flows in the San Joaquin River during February exacerbated the problem by limiting dilution of the treated effluent and dramatically increasing the residence time of the water in the critical reach near Stockton. The long residence time of the ammonia in the River also permitted acclimation of nitrifying bacteria to levels critical for vitrification rates to exceed reaeration rates. This work indicates that hypoxia in the San Joaquin River could be eliminated by additional treatment of the City of Stockton effluent, and better management of water diversions and resources in the Sacramento-San Joaquin River Delta