Des Moines River IA 04-LDM-1011

The current (2012-2014) monitoring period is the second consecutive monitoring period where levels of indicator bacteria met both the Class A1 geometric mean criterion (126 orgs/100 ml) and the Class A1 single-sample maximum criterion of 235 orgs/100 ml.  In the previous (2010-2012) monitoring period, 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.

In the current (2012-2014) monitoring period, the geometric means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2012 through 2014 station 10900003 were as follows: the 2012 geometric mean was 86 orgs/100 ml, the 2013 geometric mean was 40 orgs/100 ml, and the 2014 geometric mean was 100 orgs/100 ml. All three geometric means meet the Class A1 criterion of 126 orgs/100 ml. Three of the 22 samples (14%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to the Iowa DNR assessment/listing methodology, the violation frequency of the single-sample maximum criterion is not significantly greater than 10%.  Thus, similar to the previous monitoring period, both the geometric means and the percentage of samples exceeding the single-sample maximum criterion suggest “full support” of the Class A1 uses.

The IDNR assessment/listing methodology requires that, before a Section 303(d) 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). Of the monitoring results from this station over the last five years 2010-2014), both the Class A1 geometric mean and single-sample criteria have been met.  Thus, this impairment is proposed for de-listing.

Based on results of monitoring from the IDNR/SHL ambient station at Chillicothe from 2012 through 2014, 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 33 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 "not 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 the site used for the assessment 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 32 samples analyzed for alachlor from June 2012 to July 2014 violated the MCL of 2 ug/l (maximum level of 0.1 ug/l). None of the 26 samples analyzed for atrazine (maximum = 2 ug/l), carbofuran (maximum = 0.1 ug/l), and cyanazine (maximum = 0.1 ug/l) from June 2012 to July 2014 violated their respective Class C criteria.  Two of the 33 samples (6%) analyzed for nitrate exceeded the Class C criterion (MCL) of 10 mg/l (maximum: 14 mg/l; average:  4.0 mg/l).  Based on IDNR’s assessment methodology, “full support” of the Class C drinking water uses is indicated if less than 10% of samples exceed the nitrate criterion of 10 mg/l.  Thus, the monitoring results for nitrate from 2012-2014 do not suggest impairment of the Class C drinking water uses.

Fish consumption uses remain not assessed due to lack of recent fish tissue monitoring in this river segment.