Assessment Comments
Assessment is based on results of bacterial monitoring in 2009 and 2010 at three sites in the Black Hawk Creek Section 319 project: BHC-11, BHC-8, and BHC-9.
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” due to violations of Iowa water quality criteria for indicator bacteria. This is a new impairment for this previously unassessed segment. The Class B(WW1) aquatic life uses remain “not assessed” due to the lack of water quality information upon which to base an assessment. The source of data for this assessment is the results of monitoring at three sites (BHC-11, BHC-8, and BHC-9) conducted as part of a Clean Water Act Section 319 water quality project from June 2009 through November 2010.
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as “not supported due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion. The geometric means (GM) of indicator bacteria (E. coli) in the 22 samples collected at these three stations during the recreational seasons of 2009 and 2010 were as follows:
N 2009 GM 2010 GM No. & percent of samples > single sample max criterion
BHC-11 22 1,840 637 20; 91% violation
BHC-8 23 733 205 16; 70% violation
BHC-9 22 1,400 669 20; 91% violation
The geometric means at all three stations exceed Iowa’s Class A1 geometric mean criterion of 126 E. coli orgs/100 ml, thus indicating nonsupport of the Class A1 primary contact recreation uses. From 70% to 90% of the samples exceeded Iowa's 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 additional, the percentage of samples with levels of E. coli that exceeded Iowa single-sample maximum criteria indicate impairment of Class A1 uses at this monitoring site.
