Assessment Comments
Assessment is based on results of ambient water quality monitoring conducted at station 15340005 at County Road T38 from April 2008 through November 2010 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, 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 2008-10. 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 April 2008 through November 2010 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 means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at CR-MC site 15340005 were as follows: the 2008 geometric mean was 624 orgs/100 ml, the 2009 geometric mean was 206 orgs/100 ml and the 2010 geometric mean was 497 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Thirteen of the 22 samples (59%) exceeded the Class A1 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 a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported". Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
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 2008-2010 assessment period showed no violations of Class B(WW) water quality criteria for dissolved oxygen, pH, or temperature in the approximately 19 samples collected from April 2008 through November 2010. Levels of ammonia were extremely low in this stream during the 2008-10 period with five of the six 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 was well below the pH-dependent Class B(WW1) chronic criterion of 4.7 mg/l.
Fish consumption uses remain not assessed due to the lack of fish contaminant monitoring in this assessment segment.
