East Nishnabotna River IA 05-NSH-1816

The Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supporting” based on data collected in 2007 as part of the DNR/SHL large river benthic macroinvertebrate 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 to make a benthic macroinvertebrate index (BMIBI). The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2007 BMIBI scores were 42, 46, 50 (all 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 54. This segment passed the BMIBI BIC 0/3 times in 2007. This assessment is considered evaluated because the drainage areas (895 and 892 mi2) above the sampling sites were greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this segment failed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the sites used for the assessment don’t fall in the calibrated watershed size. Also, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). According to DNR’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). DNR 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 assessed as "fully supported" based on results of the DNR fish tissue (RAFT) monitoring near Red Oak in 2011 and 2012. The 2011 composite samples of fillets from channel catfish had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.039 ppm; total PCBs: <0.09 ppm; and technical chlordane: 0.037 ppm. The average level of mercury in the 2011 tissue plug samples from four freshwater drum was 0.295 ppm (SD=0.209) which is just below the threshold for issuance of a one meal per week advisory. Follow-up monitoring was conducted in 2012 to determine whether mercury levels were above the advisory threshold. The results of the follow-up monitoring showed that the average level of mercury in the tissue plugs from five freshwater drum was 0.216 ppm (SD=0.089), thus suggesting that average levels of mercury do not exceed the 0.3 ppm advisory threshold. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2011 and 2012 RAFT samplings conducted in this assessment segment indicate that the levels of contaminants do not exceed any of Iowa’s advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.