Assessment Comments
Assessment is based on results TMDL-related monitoring by IDNR/UHL from April through October, 2008.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was designated 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, 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" due to high levels of indicator bacteria that routinely violate state water quality standards. The presumptive Class B(WW1) aquatic life uses are assessed (evaluated) as “partially supported” due to the violations of Class B(WW1) criterion for dissolved oxygen. The source of data for this assessment is the results for IDNR/UHL TMDL-related monitoring conducted station CLC1 (STORET station 11820006) from April through October 2008. A TMDL for the bacterial impairment for streams in the Duck Creek basin was prepared by Iowa DNR in March 2010 (see http://www.iowadnr.gov/water/watershed/tmdl/files/final/duck10tmdl.pdf); this TMDL has not yet been approved by U.S. EPA.
EXPLANATION: Results of monitoring for indicator bacteria conducted weekly from April through October 2008 at Station CLC1 suggest that the presumptive Class A1 uses of this segment should be assessed (monitored) as "not supported." A total of 28 samples were collected at station CLC1 during this period; these data allowed calculation of twenty-four, 30-day/five-sample geometric means. All of the 30-day geometric means exceeded Iowa’s Class A1 criterion of 126 organisms / 100 ml. The minimum and maximum geometric means were 947 and 6,919 orgs/100 ml, with 27 of the 28 samples (96%) exceeding Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines and IDNR’s assessment/listing methodology, these results suggest non-support of the presumptive Class A1 (primary contact recreation) uses due to geometric means for E. coli that exceed Iowa’s water quality criterion of 126 E. coli organisms/100.
The presumptive Class B(WW1) aquatic life uses are assessed (evaluated) as “partially supported” due to violations of criteria for dissolved oxygen. Four of the 29 samples (14%) collected in 2008 at station CLC1 violated the Class B(WW1) criterion of 5 mg/l. The minimum level of dissolved oxygen was 3.3 mg/l. According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S. EPA 1997b). According to IDNR’s assessment/listing methodology, however, the results from station CLC1, however, do not indicate that significantly greater than 10% of the samples exceed the Class B(WW1) criterion for dissolved oxygen. Thus, these results suggest that the presumptive Class B(WW1) uses should be assessed (monitored) as “fully supported.” Despite this indication of full support, the occurrence of four violations of the dissolved oxygen criterion in a single year is unusual and suggests a potential adverse water quality impact. Thus, this assessment segment will be added to Iowa’s list of waters in need of further investigation. Additional monitoring is needed to better determine any potential water quality impacts on the aquatic life of this stream.
