Assessment Comments
Assessment is based on results of monthly monitoring from 2008 through 2010 at the IDNR ambient city monitoring station located upstream from Ottumwa at the County Road T67 bridge at Chillicothe (STORET station 10900003).
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) 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.” Both assessments are based on results of IDNR/UHL ambient water quality monitoring upstream from Ottumwa. The Class C (drinking water) uses are assessed (monitored) as "fully supported" based on results of the IDNR/UHL ambient monitoring. Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring in this river reach. The source of data for this assessment is the results of monthly monitoring from 2008 through 2010 at the IDNR ambient city monitoring station located upstream from Ottumwa at the County Road T67 bridge at Chillicothe (STORET station 10900003).
EXPLANATION: The Class A1 uses are assessed (monitored) as "not supported" based on results of monitoring for indicator bacteria (E. coli) at the IDNR/UHL ambient monitoring station on the Des Moines River upstream from Ottumwa. The geometric means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10900003 upstream from Ottumwa were as follows: the 2008 geometric mean was 132 orgs/100 ml, the 2009 geometric mean was 474 orgs/100 ml and the 2010 geometric mean was 97 orgs/100 ml. The 2008 and 2009 geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml; the 2010 geometric mean is below (meets) the Class A1 criterion. Eight of the 22 samples (36%) 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" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). 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 (monitored) as "fully supported" based on results of monitoring from the IDNR/UHL ambient station at Chillicothe from 2008 through 2010. Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the approximately 30 samples analyzed.
The Class C (drinking water) uses are assessed (monitored) as "fully supported.” During the 2006-2008 assessment period, none of 11 samples collected violated the atrazine MCL of 3 ug/l (3.5 ug/l) (this station was not monitored for atrazine during the current (2008-2010) assessment period). None of 30 samples exceeded the nitrate MCL of 10 mg/l (maximum: 7.3 mg/l). The average levels of both atrazine (0.11 ug/l) and nitrate (4.6 mg/l) are well below their respective MCLs. Based on IDNR’s assessment methodology, if the average level of nitrate is less than the respective MCLs, the Class C drinking water should be assessed as “fully supported.” Similarly, if the average level of a pesticide is less than the respective MCL, the Class C drinking water should be assessed as “fully supported.”
Fish consumption uses remain not assessed due to lack of recent fish tissue monitoring in this river segment.
