Assessment Comments
Assessment is based on results of water quality monitoring from April to October 2011 at Site 10 at Centennial Road (STORET station 15960039).
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(WW1) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” (IR 4a) due to levels of indicator bacteria that exceed state water quality criteria. This is a new impairment for this previously unassesed segment. The presumptive Class B(WW1) aquatic life uses are assessed (monitored) as “fully supported” (IR 2a) based on results of chemical/physical water quality monitoring in 2011. The source of data for this assessment is the results of water quality monitoring from April to October 2011 at Site 10 at Centennial Road (STORET station 15960039).
Note: A TMDL for the bacterial impairment for streams in the Yellow River basin was prepared by Iowa DNR in 2012 and approved by U.S. EPA in 2013 (see http://www.iowadnr.gov/Environment/WaterQuality/WatershedImprovement/WatershedResearchData/WaterImprovementPlans/PublicMeetingsPlans.aspx). Due to completion of the TMDL, the impairment of the Class A1 recreational uses is appropriate for Category 4a (impaired; TMDL approved) of Iowa's Integrated Report.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to violations of Iowa’s water quality criteria for indicator bacteria. The geometric mean of E. coli in the14 samples collected during the recreation season of 2011 at station 15960039 was 1,265 orgs/100 ml, thus far exceeding the Class A1 criterion of 126 orgs/100 ml. Eleven of the 14 samples (79%) 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 the geometric mean of E. coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
The presumptive Class B(WW1) aquatic life uses are assessed (monitored) as “fully supported”. None of the 14 samples analyzed in 2011 for dissolved oxygen or the eight samples analyzed for ammonia violated the respective Class B(WW1) criteria. Three of the 14 samples collected in 2011 at station 15960039 violated the Class B(WW2) criterion for pH. The samples collected on June 2, August 2, and August 9, 2011, contained a pH levels of 9.2, 9.2, and 9.7 units, respectively, thus exceeding the Class B(WW1) criterion of 9.0 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 and dissolved oxygen suggests impairment of aquatic life uses. Based on IDNR’s assessment methodology, however, these results suggest that the frequency of violations is not significantly greater than10 percent; thus, these results do not suggest impairment of the presumptive Class B(WW1) aquatic life uses. Thus, these aquatic life uses are assessed as “fully supported”.
