Use of the Hazard Assessment Approach for Evaluating the Impact of Chlorine and Ammonia in Pueblo, Colorado, Domestic Wastewaters on Water Quality in the Arkansas River

A short-term, intensive study has been conducted on the potential impact of the chlorine, and to a lesser extent the ammonia and nitrite, associated with the domestic wastewater discharge from the city of Pueblo, Colo. on the water quality (beneficial uses) of the Arkansas River downstream from the discharge. The hazard assessment approach was used in the planning of the study and in data interpretation.

The wastewater treatment plant (WWTP) effluent mixed slowly with the river, forming a narrow effluent plume extending downstream for about 1000 m; in this “zone of mixing” contaminant concentrations were sufficient to kill caged fathead minnows in 4 days or less. A continuous “zone of passage” existed past the discharge in which no visually apparent adverse effects on caged fish were found in 4 days' exposure.

Chlorine concentration decreased with the distance downstream of the discharge, but, based on the concentrations found, there is a potential for chlorine impact on aquatic life for a distance of about 15 km. Outside the zone of physical mixing of the effluent and the river, nitrite and un-ionized ammonia concentrations were below the corresponding Colorado water quality guidelines; however, questions regarding their impact on the water quality within this zone were raised and need to be addressed in future studies.