Assessment Comments
Assessment is based on results of chemical/physical/bacterial water quality monitoring conducted on Dry Run Creek from March 2006 through June 2008 at three stations sampled as part of the Dry Run Creek Watershed Project: Site DRC5 at University Avenue (STORET station 1507003), at Site DRC8 at Greenhill Road (STORET station 15070005), and Site DRC10 at Viking Road (STORET station 11070007).
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 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), 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 (2010) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria. This waterbody has not been assessed for Section 305(b)/303(d) purposes Prior to the 2008 assessment/listing cycle.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported” due to levels of indicator bacteria that exceed state water quality criteria. The presumptive Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of chemical/physical monitoring in 2005 and 2006. The sources of data for this assessment include the results of chemical/physical/bacterial water quality monitoring conducted on Dry Run Creek from March 2006 through June 2008 at three stations sampled as part of the Dry Run Creek Watershed Project: Site DRC5 at University Avenue (STORET station 15070003), at Site DRC8 at Greenhill Road (STORET station 15070005), and Site DRC10 at Viking Road (STORET station 11070007).
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 overall (2006-2008) geometric means of E. coli in the samples collected at all three monitoring stations in this assessment segment exceeded Iowa’s Class A1 criterion of 126 orgs/100 ml. These geometric means and the number of samples collected from March 2006 to June 2008 are as follows: 575 orgs/100 ml at Site DRC5 (41 samples); 184 orgs/100 ml at Site DRC8 (41 samples); and 260 orgs/100 ml at Site DRC10 (40 samples). The percentages of samples at these stations that exceeded Iowa’s single-sample maximum criterion are as follows: Site DRC5: 28 of 41 samples (68% violated); Site DRC8: 21 of 41 samples (51% violated), and Site DRC10: 22 of 40 samples (55% violated). 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” based on results of chemical/physical monitoring conducted for the Dry Run Creek Watershed project from March 2006 through June 2008. No violations of Class B(WW1) criterion for dissolved oxygen, temperature, or pH occurred in the approximately 40 samples analyzed at each station over this period. One of 43 samples (2%) analyzed during the 2006-08 period contained a level of ammonia that exceeded the Class B(WW1) chronic criteria to protect aquatic life uses. The sample collected on March 11, 2008 contained 1.6 mg/l of ammonia, thus exceeding the temperature/pH dependent chronic criterion of 1.09 mg/l. According to the IDNR assessment /listing methodology, aquatic life impairments for toxic parameters are identified when significantly more than 10% of the samples exceeded a chronic criterion. Thus, the single violation of the chronic criterion for ammonia does not suggest impairment of the presumptive Class B(WW1) uses.
