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 Class B(LR) aquatic life 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. The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses). 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.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses remain assessed (monitored) as "not supported" due to high levels of indicator bacteria that routinely violate state water quality standards. The Class B(WW2) aquatic life uses remain “not assessed” due to the lack of information upon which to base an assessment. The source of data for this assessment is the results for IDNR/UHL TMDL-related monitoring conducted at one location in this assessment segment during 2008: DC-Site 16 (at the west edge of Davenport, west of I-280). A TMDL for the bacterial impairment 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 suggest that the presumptive Class A1 uses of this segment of Duck Creek should be assessed (monitored) as "not supported." A total of 29 samples were collected at station DC-16 during this period; these data allowed calculation of twenty-five, 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 235 and 6,794 orgs/100 ml, with 24 of 29 samples (83%) 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 that exceed Iowa’s water quality criterion of 126 E. coli organisms/100.
The Class B(WW2) aquatic life uses remain “not assessed” due to the lack of information upon which to base an assessment. Levels of dissolved oxygen, however, were measured during the TMDL-related sampling in 2008 with no violations of the Class B(WW2) criterion occurring in the 29 samplings conducted at station DC-16. The minimum level of dissolved oxygen was 5.1 mg/l. Although these results suggest “full support” of the presumptive aquatic life uses, the lack of additional data for ammonia, toxic metals, and/or pesticides, and the lack of biological data for this segment, prevents development of a complete assessment of the aquatic life uses. Thus, these uses are considered “not assessed” for the 2010 cycle.
