Assessment Comments
Assessment is based on results of ambient water quality monitoring conducted at station 15340005 at County Road T38 from May 2006 through August 2008 as part of the Cedar River/Mitchell County study.
Basis for Assessment
[Note: Prior to the 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 assessed (monitored) as “fully supported” based on results of chemical/physical water quality monitoring from 2006-08. Fish consumption uses remain 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 August 2008 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. The geometric mean of E. coli in the 21 samples collected during summer recreation seasons at the monitoring station on Rock Creek from May 2006 through August 2008 was 394 orgs/100 ml; this geometric mean exceeds Iowa’s Class A1 criterion of 126 orgs/100 ml. Twelve of the 21 samples (57%) 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 assessed as “fully supported” based on results of chemical/physical monitoring at Cedar River/Mitchell County station 15340005. Results of ambient water quality monitoring at this station during the 2006-2008 assessment period showed no violations of Class B(WW) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the approximately 20 samples collected from May 2006 through August 2008. Levels of ammonia were extremely low in this stream during the 2006-08 period with 16 of the 19 samples analyzed having less than the detectable level of ammonia (< 0.05 mg/l); the maximum ammonia level of 0.24 mg/l on April 1, 2008.
Fish consumption uses are not assessed due to the lack of fish contaminant monitoring in this assessment segment.
