Des Moines River IA 04-LDM-1020

Note: A TMDL for bacteria was prepared by IDNR and approved by U.S. EPA in March 2010. Because this TMDL covers all the identified impairments for the assessment segment, this segment was moved to IR Category 4a (TMDL approved).

The Class A1 primary contact recreation uses are assessed (monitored) as "partially supported" based on results of monitoring for indicator bacteria (E. coli) at both the IDNR ambient station and the ISU/ACOE station near Runnells.  The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2012 through 2014 at IDNR station 10770003 downstream from Des Moines were as follows: the 2012 geometric mean was 139 orgs/100 ml, the 2013 geometric mean was 123 orgs/100 ml, and the 2014 geometric mean was 587 orgs/100 ml. The 2012 and 2014 geometric means slightly exceeded the Class A1 criterion of 126 orgs/100 ml; the 2013 geometric mean was below (met) the Class A1 criterion. Nine of the 24 samples (39%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.

The geometric means of indicator bacteria (E. coli) in the 36 samples collected during the recreational seasons of 2012 through 2014 at ISU/ACOE station 7 were as follows: the 2012 geometric mean was 71 orgs/100 ml, the 2013 geometric mean was 142 orgs/100 ml, and the 2014 geometric mean was 284 orgs/100 ml. The 2013 and 2014 geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml. Twelve of the 36 samples (33%) 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 should be assessed as “impaired” (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.” Regardless of the impairment, levels of indicator bacteria in this river segment were relatively low during the 2012-2014 assessment period and in previous assessment periods.

Results from the both the ISU/ACOE and IDNR/SHL monitoring stations at Runnells suggest that the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported.” No violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, temperature, or ammonia-nitrogen, chloride or sulfate occurred in the 33 samples collected at the IDNR/SHL station during the 2012-2014 assessment period. At the ISU/ACOE station, no violations of Class B(WW1) water quality criteria for dissolved oxygen, ammonia, temperature, or pH occurred in the 66 samples collected during the 2012-2014 assessment period.

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 and 2013 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 13 (poor) and the 2013 BMIBI score was 34 (fair). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), 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 (11,655 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 (0/2), 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).

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this assessment segment.  Results of monitoring for toxic metals, however, showed no violations of the respective Class HH (Human Health) criteria for cadmium, copper, or mercury in the nine samples analyzed at ISU/ACOE Station 7.  Despite the lack of fish contaminant monitoring in this assessment segment, these results suggest “full support” of the Class HH (Human Health) uses.