Iowa River IA 02-IOW-647

The Class B(WW1) aquatic life uses remain assessed (evaluated) as “fully supporting” based on biological data collected in 2005 as part of the DNR/SHL stream REMAP project. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2005 FIBI scores were 63 (good), 71 (excellent) and the BMIBI scores were 68 (good), 71 (good). The aquatic life use support was assessed (evaluated) as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. This segment passed the FIBI BIC 2/2 times and passed the BMIBI BIC 2/2 times (from both the 47b and 47f ecoregions). This assessment is considered evaluated because the drainage area (748 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site passed both the FIBI and BMIBI BICs, 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. This aquatic life assessment is now also considered "evaluated" based on a change in the 2010 DNR assessment methodology. DNR now requires a segment have two or more biological samples collected from the segment in multiple years over a five-year period to be considered “monitored”. This segment had multiple samples collected in 2005; however, the samples were not collected in multiple years. In addition, these data are now greater than five years old.

Fish consumption/human health uses remain assessed (monitored) as “partially supported” based on results of U.S.EPA/DNR fish contaminant (RAFT) monitoring at Marshalltown in 2011 and 2012. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. The fish contaminant data generated from the 2011 RAFT sampling conducted on the Iowa River at Marshalltown showed that levels of mercury were sufficiently high for concern. The average level of mercury in the tissue plugs from four walleye was 0.323 mg/kg (SD=0.045), thus slightly exceeding the one meal per week advisory threshold of 0.3 mg/kg. According to the DNR/DPHfish advisory protocol, the single occurrence of contaminant above an advisory trigger level does not typically result in issuance of an advisory. Such an advisory is issued only after follow-up monitoring confirms that contaminant levels exceed the advisory trigger level. Results of follow-up monitoring in 2012 confirmed that levels of mercury in predator fish (walleye) were above the threshold for issuance of a one meal/week fish consumption advisory. The 2012 average level of mercury in the tissue plugs from three walleye was 0.308 mg/kg (SD=0.071), thus again slightly exceeding the one meal per week advisory threshold of 0.3 mg/kg. Because the levels of mercury in both the 2011 and 2012 samplings were above the one meal/week advisory trigger, a one meal/week consumption advisory was issued in 2013 that extends from the upper end of Coralville Reservoir to the dam at Iowa Falls (this advisory incorporates a preexisting advisory from the Highway 20 bridge upstream to the dam at Iowa Falls). The advisory recommends that no more than one meal per week of predator fish from this river segment be eaten. According to the DNR assessment/listing methodology, the existence of a one meal per week consumption advisory indicates that the fish consumption uses should be assessed as “partially supported.” Additional fish contaminant monitoring was conducted in this assessment segment in 2011 in the Iowa River at Pine Lake State Park. The average level of mercury in the tissue plugs from five smallmouth bass was 0.226 mg/kg (SD=0.076). This average level of mercury was below the one meal per week advisory threshold and thus did not trigger issuance of an advisory. However, the results of fish contaminant monitoring in 2011 and 2012 at Marshalltown that suggested the need for a one meal per week advisory, in addition to the existence of a one meal per week advisory from Iowa Falls downriver to the Highway 20 bridge, suggested that this assessment segment should be included in the consumption advisory issued in 2013.