Assessment Comments
Assessment is based on results of an IDNR investigation of a fish kill in September 2003.
Basis for Assessment
SUMMARY: The general uses of this stream are assessed (monitored) as “partially supported” due to a fish kill in September 2003. The kill was attributed to ammonia from an unspecified source. The party responsible for the kill was not identified. This impairment is appropriate for Category 5b of Iowa’s 2006 Integrated Report (=Section 303(d) list).
EXPLANATION: This kill occurred on or before September 16, 2003; the kill was attributed to ammonia from an unspecified/unknown source. An estimated 75 fish (unspecified species) were killed; no estimate of the value of the fish killed was provided. No responsible party was identified. The kill occurred near the intersection of highways 4 and 141; the location point for the kill is Section 17 of T81N, R30W, Guthrie County. According to the IDNR investigation, staff have been unable to locate a definite source of the ammonia contributions to Mosquito Creek in Guthrie County. IDNR staff reported the following levels of ammonia during the investigation: approximately 10 ppm at a location 3 to 4 miles southeast of Bagley; approximately 15-20 ppm 1 mile southeast of Bagley; 7.5 ppm at the outfall of the Bagley wastewater treatment plan. IDNR staff found very low concentrations of ammonia upstream from Bagley but felt that the lateness of the investigation may have allowed previously-high concentrations to decrease.
According to IDNR’s assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2002-2005) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired”. If a cause of the kill is identified, and the cause is either known, or suspected, to be a “pollutant”, the assessment type is considered “monitored” and the affected waterbody is a candidate for Section 303(d) listing. Fish kills attributed to a pollutant, but where a source of the pollutant was not identified and/or where enforcement actions were not taken against the responsible party, will be placed into Integrated Report subcategory 5b. The intent of placing these waterbodies into Category 5 is not to necessarily require a TMDL but to keep the impairment highlighted due to the potential for similar future kills from the unaddressed causes and/or sources.
