Assessment Comments
This segment was monitored in 2004 as part of a joint monitoring project conducted UHL and the Davenport Water Pollution Control Plant.
Basis for Assessment
[Note: Prior to the current (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 (2008) 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" due to high levels of indicator bacteria that routinely violated 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 a joint monitoring project conducted in summer 2004 by University of Iowa Hygienic Laboratory (UHL) and the Davenport Water Pollution Control Plant at the following sample sites in this assessment segment: Site DC-3 (near Hickory Grove Road in Davenport) and DC-Site 16 (at the west edge of Davenport; the most upstream sampling station).
EXPLANATION: Results of monitoring for indicator bacteria conducted in mid to late summer 2004 suggest that the preumptive Class A1 uses of Duck Creek are "not supported." Levels of indicator bacteria were monitored once per week at three stations in this assessment segment from mid-July through mid-October, 2004 as part of a joint monitoring project between the University of Iowa Hygienic Laboratory and the Davenport Water Pollution Control Plant. A total of 14 samples per station were collected during this period; this allows calculation of ten, 30-day/five-sample geometric means for each monitoring station.
According to IDNR’s assessment methodology, two conditions need to be met for results of monitoring for indicator bacteria to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) all five-sample, thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not significantly more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
Due to recent changes in Iowa’s Water Quality Standards, Iowa’s 2006 assessment methodology for indicator bacteria has changed. Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses: the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff." Due to a change in the Standards in July 2003, E. coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E. coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.” Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E. coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of the 2006 Section 305(b) assessments and Section 303(d) listings.
Results of monitoring in this segment of Duck Creek, whether based on data from UHL or from the Davenport Water Pollution Control Plant, suggest impairment of the Class A1 primary contact recreation uses. For the UHL data, 18 of the 20 geometric means from the two monitoring stations exceeded Iowa’s Class A1 criterion of 126 organisms / 100 ml. For the DWPC data, all 20 geometric means exceeded the Class A criterion. The following summary is based on results of UHL data. At Station 3, the minimum and maximum geometric means were 69 and 1,042, with 7 of 14 samples exceeding Iowa’s single-sample maximum value of 235 orgs/100 ml. At station 16, , minimum and maximum geometric means were 391 and 2,490, with all 14 samples exceeding Iowa’s single-sample maximum value. According to U.S. EPA guidelines and IDNR’s assessment/listing methodology, these results would suggest non-support of the Class A1 (primary contact recreation) uses due to thirty-day geometric means that exceed Iowa’s water quality criterion of 126 E. coli organisms/100.
The Class B(WW2) aquatic life uses were “not assessed” due to the lack of information upon which to base an assessment.
