Assessment Comments
Assessment remains based on results of a fish kill investigation in May 2009. Due to a review of this assessment and the results of IDNR's 2009 fish kill investigation, this assessment segment was moved from Category 3b of the 2010 Integrated Report (potentially impaired) to IR Category 5b (Section 303(d) impaired) for the 2012 assessment/listing cycle.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was classified only for general uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is now presumptively designated for Class A1 (primary contact recreation) uses and for Class B(WW1) aquatic life uses. According to the Iowa Water Quality Standards, all perennial rivers and streams and all intermittent streams with perennial pools that are not specifically listed in the Iowa surface water classification are designated as Class A1 and Class B(WW1) waters. Thus, for the current assessment, perennial flow is presumed, and the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW1) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses remain "not assessed" (IR 3a) due to a lack of information upon which to base an assessment. The presumptive Class B(WW-1) aquatic life uses are assessed (evaluated) as "partially supported" (IR 5b) based on results of a fish kill investigation in May 2009.
EXPLANATION: A fish kill occurred on or before May 24, 2009. The kill affected hundreds of fish along approximately a mile-and-a-half long stretch of this tributary of Ralston Creek. The creek was traced back to a tile line in a rural area, but there was no indication of manure or fertilizer run-off based on field testing. Analyses of water samples were inconclusive, and no cause could be determined.
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 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 was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). ). Due, however, to the large number of fish killed ("hundreds"), the length of the kill (1.5 miles), and the lack of environmental extremes during spring 2009 that would have contributed to this kill (e.g., low stream flow and/or high water temperatures), some type of pollutant cause is suspected. Thus, this assessment was moved from Category 3b of Iowa's 2010 Integrated Report (potentially impaired) to IR Category 5b (impaired) of the 2012 IR. This impairment remains in IR Category 5b.
