Assessment Comments
Assessment is based on results of monitoring at STORET stations 15450017 (Trib to Silver Creek DS of WWTP, Site 2 and STORET station 15450018 (Trib to Silver Creek US of WWTP, Site 1) conducted from April 2011 through July 2012.
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 are assessed as “fully supporting (IR 2a) based on results of chemical/physical monitoring. The source of data for this assessment is the results of monitoring at STORET stations 15450017 (Trib to Silver Creek DS of WWTP, Site 2, [NULL 069]) and STORET station 15450018 (Trib to Silver Creek US of WWTP, Site 1 [NULL 070]) conducted as part of a Clean Water Act Section 319 water quality project from April 2011 through July 2012.
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 means of indicator bacteria (E. coli) in the 22 samples collected at Site 2 during the recreation seasons of 2011 and 2012 were as follows: the 2011 geometric mean was 272 orgs/100 ml and the 2012 geometric mean was 2,186 orgs/100 ml. Fifteen of the 22 samples (68%) exceeded the Class A1 single sample maximum criterion of 235 orgs/100 ml. Similarly, the geometric means of indicator bacteria (E. coli) in the 22 samples collected at Site 1 during the recreation seasons of 2011 and 2012 were as follows: the 2011 geometric mean was 315 orgs/100 ml and the 2012 geometric mean was 557 orgs/100 ml. Sixteen of the 22 samples (73%) 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 chemcial/physical monitoring at Sites 1 and 2 during the 2011-12 period suggest "full support" of the presumptive Class B(WW1) aquatic life uses. None of the approximately 15 samples analyzed from May to November 2011 at Sites 1 and 2 violated the Class B(WW1) criteria for ammonia, dissolved oxygen, pH, or temperature. Based on IDNR’s assessment methodology, these results suggest that the presumptive Class B(WW1) uses should be assessed as "fully supporting."
