Assessment Comments
Assessment is based on results of monthly monitoring from 2010 through 2012 at the IDNR ambient city monitoring station located upstream from Ottumwa at the County Road T67 bridge at Chillicothe (STORET station 10900003) and on IDNR/SHL biological sampling conducted in 2012 near Ottumwa.
Basis for Assessment
SUMMARY: Despite low levels of indicator bacteria in this assessment segment, the Class A1 (primary contact recreation) uses remain assessed (monitored) as “not supported” (IR 5a) due to historic high levels of indicator bacteria. This assessment was based on results of IDNR/SHL ambient water quality monitoring upstream from Ottumwa from 2010-2012. The Class B(WW1) aquatic life uses are assessed (evaluated) as "not supported” (IR 3b-u) based on the 2012 biological sampling data. The Class C (drinking water) uses are assessed (monitored) as "fully supported" (IR 2a) based on results of the IDNR/SHL ambient monitoring. Fish consumption uses remain “not assessed” (IR 3a) due to the lack of fish contaminant monitoring in this river reach. The sources of data for these assessments are the results of monthly monitoring from 2010 through 2012 at the IDNR ambient city monitoring station located upstream from Ottumwa at the County Road T67 bridge at Chillicothe (STORET station 10900003) and 2012 IDNR/SHL biological sampling.
EXPLANATION: Despite very low levels of indicator bacteria in this assessment segment during the 2010-2012 period, the Class A1 uses remain assessed (monitored) as "not supported" based on past results of monitoring for indicator bacteria (E. coli) at the IDNR/SHL ambient monitoring station on the Des Moines River upstream from Ottumwa. The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10900003 upstream from Ottumwa were as follows: the 2010 geometric mean was 97 orgs/100 ml, the 2011 geometric mean was 24 orgs/100 ml, and the 2012 geometric mean was 86 orgs/100 ml. All three geometric means meet the Class A1 criterion of 126 orgs/100 ml. Only one of the 24 samples (4%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. Both the geometric means and the percentage of samples exceeding the single-sample maximum criterion suggest “full support” of the Class A1 uses. Because, however, the IDNR assessment/listing methodology requires that, before a bacterial impairment can be de-listed, geometric mean levels of E. coli must all be less than the applicable state water quality criteria for two consecutive listing cycles (i.e., five consecutive years), the impairment of the Class A1 uses will remain in effect. Of the monitoring results from this station over the last five years, only the 2008 and 2009 recreation season geometric mean (132 and 474 orgs/100 ml, respectively) exceeded the Class A1 criterion. If sufficient additional data are collected during the 2013-2014 period that show a continued decline in bacteria levels and compliance with the Class A1 water quality criterion, this impairment may be removed.
Based on results of monitoring from the IDNR/SHL ambient station at Chillicothe from 2010 through 2012, the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported". Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, temperature, ammonia-nitrogen, chloride, or sulfate in the 36 samples analyzed.
In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting". This evaluated biological assessment was based on data collected in 2012 as part of the IDNR/SHL large river sampling project. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach. The biological metrics were combined a benthic macroinvertebrate index (BMIBI). The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2012 BMIBI score was 15 (poor). The aquatic life use support was assessed (evaluated) as not supporting (=NS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008. The BMIBI BIC for this ecoregion is 51. This assessment is considered evaluated because the drainage area (13,257 mi2) above this sampling site was far greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site failed the BMIBI BIC (0/1), it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size. According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).
The Class C (drinking water) uses are assessed (monitored) as "fully supported.” None of 13 samples analyzed for alachlor from June to December 2012 violated the MCL of 2 ug/l (maximum level of 0.1 ug/l). None of the 7 samples analyzed for atrazine (maximum = 0.4 ug/l), carbofuran (maximum = 0.1 ug/l), and cyanazine (maximum = 0.1 ug/l) from June to December 2012 violated their respective Class C criteria. None of 36 samples exceeded the nitrate MCL of 10 mg/l (maximum: 7.3 mg/l). The 2010-12 average levels of both atrazine (0.18 ug/l) and nitrate (4.0 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.
