Assessment Comments
Assessment remains based on results of chemical/physical/bacterial water quality monitoring conducted on Dry Run Creek from January 2008 through November 2010 at three stations sampled as part of the Dry Run Creek Watershed Project. Assessment is also based on results of bacteria monitoring at these three sites from June 2010 to November 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 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(WW2) water quality criteria.
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported” (IR 5p) 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" (IR 2a) based on results of chemical/physical monitoring in 2008 and 2009. The sources of data for this assessment include (1) the results of chemical and physical water quality monitoring conducted on Dry Run Creek from January 2008 through August 2009 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 15070007) and (2) the results of Section 319 bacterial monitoring from June 2010 through November 2012 at these three stations: DRC-5, DRC-10, and DRC-8.
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 means of indicator bacteria (E. coli) in the approximately 24 samples collected at each station (total N=73 samples) during the recreational seasons of 2010, 2011, and 2012, respectively, at each of the three Section 319 monitoring stations in this assessment segment were as follows:
1. DRC-10 (15070007): geometric means were 509, 456, and 606 orgs/100 ml;
2. DRC-5 (15070003): geometric means were 833, 731, and 665 orgs/100 ml;
3. DRC-8 (15070005): geometric means were 544, 475, 62 orgs/100 ml.
From 75 to 92% of the samples at each of the Section 319 sites 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 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 January 2008 through August 2009. No violations of Class B(WW1) criterion for dissolved oxygen, temperature, or pH occurred in the approximately 15 samples analyzed at each station over this period. One of 27 samples (4%) analyzed during the 2008-09 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 one sample exceeded an acute or chronic criterion during the assessment period. Thus, the single violation of the chronic criterion for ammonia does not suggest impairment of the presumptive Class B(WW1) uses.
