Rock Creek IA 01-MAQ-4

Assessment Comments

Assessment remains based on results of ambient water quality monitoring conducted in 2001 by IDNR & UHL in support of TMDL development.

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, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

SUMMARY:  The assessment of the presumptive Class A1 (primary contact recreation) uses remains “not assessed.”  The assessment of support of the presumptive Class B(WW1) aquatic life uses developed for the previous assessment/listing cycles (“fully supported-evaluated”; IR 2a) is changed to "not assessed" (IR 3a) for the current cycle due to the age of the data (> 10 years) upon which the previous assessment was based.   remains valid for the current (2012) cycle; no additional data have been generated for this assessment segment.   Data used for previous assessments were collected in 2001 at the TMDL monitoring station at Highway 30 (STORET station 11230009).   This monitoring was part of IDNR/UHL monitoring conducted in support of the TMDL for lower Rock Creek.  

EXPLANATION:  The presumptive Class A1 (primary contact recreation) uses remain “not assessed” due to lack of water quality data upon which to base an assessment:  the TMDL monitoring conducted in 2001 did not include analysis for indicator bacteria.   Results of monitoring at the IDNR/UHL station at Highway 30 showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the nine samples collected from March to November 2001.   These results suggested water quality adequate to fully support the presumptive Class B(WW1) aquatic life uses of this stream segment.   Because the data upon which this assessment was based are now well over ten years old, the assessment type is changed from “evaluated” (a lower confidence assessment) to “not assessed.”  As water quality data age, they are less able to represent current water quality conditions.   As data age beyond ten years, their ability to represent current water quality conditions is increasing suspect.