Assessment Comments
Assessment is based on results of monitoring at STORET station 13480009 (Willow Creek at Benton-Iowa Road near T Ave, Site 7B [at Iowa/Benton county line, 2.5 miles S of Norway]) conducted from June through November 2011.
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 presumptively 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 are assessed (monitored) as “not supported” (IR 5p) due to violations of Iowa water quality criteria for indicator bacteria. This is a new impairment for this previously unassessed segment. The presumptive Class B(WW1) aquatic life uses remain "not assessed" (IR 3a) due to the lack of sufficient water quality information upon which to base an assessment. The source of data for this assessment is the results of monitoring at STORET station 13480009 (Willow Creek at Benton-Iowa Road near T Ave, Site 7B) conducted as part of a Clean Water Act Section 319 water quality project from June through November 2011 [NULL 120].
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as “not supported" (IR 5p) due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion. The geometric mean of indicator bacteria (E. coli) in the 11 samples collected at Site 7B during the recreation season of 2011 was 7,692 orgs/100 ml. All eleven samples (100%) exceeded the Class A1 single sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean is greater than the respective criterion (126 orgs/100 ml for Class A1, the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). In addition, the percentage of samples with levels of E. coli that exceeded Iowa single-sample maximum criteria indicates impairment of presumptive Class A1 uses at this monitoring site.
The presumptive Class B(WW1) aquatic life uses remain "not assessed". Other than indicator bacteria, the only other parameter monitored that is supported by Class B(WW1) crieria is temperature (there were no violations of the Class B(WW1) criterion for temperature in the eight samples collected). Due to in sufficient parameter coverage, the presumptive Class B(WW1) aquatic life uses remain "not assessed".
