Assessment Comments
Assessment is based on results of monitoring at STORET station 15960038 (Unnamed Creek at 130th Ave., Site 6) conducted from April through October 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 and for pH. The presumptive Class B(WW1) aquatic life uses are assessed as “partially supporting (IR 5p) due to violations of water quality criteria for pH. These are new impairments for this previously unassessed segment. The source of data for this assessment is the results of monitoring at STORET station 15960038 (Unnamed Creek at 130th Ave., Site 6) conducted as part of a Clean Water Act Section 319 water quality project from April through October 2011 [NULL 095].
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 14 samples collected at Site 6 during the recreation season of 2011 was 698 orgs/100 ml. Ten of the 14 samples (71%) 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.
Results of chemical/physical monitoring at Site 6 during 2011 suggest impairment of the presumptive Class A1 and the presumptive Class B(WW1) aquatic life uses. At Site 6, none of the 14 samples analyzed from April to October 2011 violated the Class B(WW1) criteria for dissolved oxygen or temperature, and none of the eight samples collected violated the Class B(WW1) criteria for ammonia. Five of the 14 samples collected (36%), however, violated the Class A1 & B(WW1) criterion for pH (all violations were greater than 9 pH units). According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as pH suggests impairment of aquatic life uses. Based on IDNR’s assessment methodology, these results suggest that the frequency of violations is significantly greater than 10 percent; thus, these results suggest impairment of the Class B(WW1) aquatic life uses.
