Assessment Comments
Assessment is based on results of ambient water quality monitoring conducted at station 15340005 at County Road T38 from May 2006 through November 2007 as part of the Cedar River/Mitchell County study.
Basis for Assessment
[Note: Prior to the current (2008) Section 305(b) cycle, this river segment was designated only for Class B(WW) aquatic life uses, including fish consumption 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 also now presumptively designated for Class A1 (primary contact recreation) uses. This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]
SUMMARY: The presumptive Class A1 uses are assessed (monitored) as “not supported” due to high levels of indicator bacteria. The Class B(WW1) aquatic life uses are “not assessed” due to the lack of water quality information upon which to base an assessment. Fish consumption uses are not assessed due to the lack of fish contaminant monitoring in this assessment segment. The source of data for this assessment is the results of ambient water quality monitoring conducted at station 15340005 at County Road T38 from May 2006 through November 2007 as part of the Cedar River/Mitchell County study.
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. [Note: monitoring data for the Mitchell County project station from 2007 were combined with data from 2006 for this assessment. Although the data cutoff period for the 2008 assessment/listing cycle is the end of calendar year 2006, the inclusion of the 2007 data is necessary to develop a higher confidence (“monitored”) assessment and to confirm the existence of a Section 303(d) impairment suggested by the relatively few data values available for 2006.]
Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed. Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A1 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 Section 305(b) assessments and Section 303(d) listings.
The geometric mean of E. coli in the 15 samples collected at the monitoring station on Rock Creek from June 2006 through November 2007 was 328 orgs/100 ml; this geometric mean exceeds Iowa’s Class A1 criterion of 126 orgs/100 ml. Eight of the 15 samples (53%) exceeded Iowa’s 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 Class B(WW1) aquatic life uses are “not assessed” due to the lack of chemical or biological water quality information upon which to base an assessment.
Fish consumption uses are not assessed due to the lack of fish contaminant monitoring in this assessment segment.
