Assessment Comments
Assessment is based on results of IDNR fish kill investigations in July 2005 and August 2006.
Basis for Assessment
[Note: Prior to the current (2008) Section 305(b) cycle, this stream segment was classified only for Class B(LR). Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is now presumptively designated for Class A1 (primary contact recreation) uses and for Class B(WW2) aquatic life uses. Thus, for the current (2008) assessmentthe available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are "not assessed" due to a lack of water quality information upon which to base an assessment. The Class B(WW2) aquatic life uses of this waterbody are assessed (monitored) as “partially supported” due to fish kills in July 2005 and August 2006.
EXPLANATION: The most recent kill occurred on or before August 20, 2006 and was attributed to manure runoff. The kill affected approximately 2.52 miles of Lake Creek on the northeast edge of Rockwell City. An estimated 6555 fish were killed, although live fish were observed in the stream. The estimated value of the fish was $651.37. At the time of this assessment there are no records that IDNR has sought/received restitution for this fish kill.
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 (2005-2007) 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. Thus, this assessment segment will be placed in Category 5b of Iowa’s 2008 Integrated Report.
The second kill on this segment occurred on or before July 18, 2005 and was attributed to natural causes (water temperature). Approximately 3 miles of stream were affected. Results of the IDNR investigation show that levels of dissolved oxygen were high and levels of ammonia were very low. The kill was attributed to water temperatures that remained in the upper 70s (F) despite a recent rainfall event. The kill affected non-game species; no estimate of the number of fish killed was provided. The observation of live fish suggested that this was only a partial kill. Although believed unrelated to the kill, the Rockwell City area received from 1 to 2-inches of rainfall the evening prior to the kill.
