Assessment Comments
Assessment is based on an IDNR investigation of a fish kill near Washington, IA, in September 2002.
Basis for Assessment
SUMMARY: The Class B(WW) aquatic life uses are assessed (evaluated) as "partially supported" due to occurrence of a fish kill in September 2002. The cause of the kill remains “unknown.”
EXPLANATION: The fish kill occurred on September 19, 2002 downstream from the Washington, Iowa, wastewater treatment plant (WWTP). The investigation showed that the stream was covered with algae for 1-2 miles downstream from the facility. Dead fish were first observed from 0.5 to 0.75 miles downstream from the facility; the kill covered approximately 1.5 miles of this stream, and an estimated 12,500 fish were killed. No cause of the kill was identified. The Washington WWTP had been meeting all NPDES permit limits, and no odors, discharge pipes, or other potential causes were observed during the investigation. The leading hypothesis is that the die-off of the large amount of algae reduced oxygen levels in this stream such that aquatic life was affected. According to IDNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill within the most recent three-year period indicates impairment of the aquatic life uses.
