Iowa River IA 02-IOW-641

The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2014 through 2016 at DNR station 10640002 downstream from Marshalltown were as follows: the 2014 geometric mean was 209 orgs/100 ml, the 2015 geometric mean was 293 orgs/100 ml, and the 2016 geometric mean was 218 orgs/100 ml. All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml. Nine of the combined 24 samples (38%) exceeded Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and Iowa DNR’s assessment/listing methodology, if the geometric mean is greater than 126 orgs/100 ml., the primary 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 "partially supported."

This aquatic life assessment based on biological sampling suggests the Class B(WW1) aquatic life uses remain assessed as"partially supporting". This evaluated biological assessment was based on data collected in 2012-2015 as part of the DNR/SHL stream nutrient 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 2012-2015 BMIBI scores were 21, 29 (both poor), 36 (fair) and 57 (good). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI score 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 (1635 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 failed the BMIBI BIC (1/4), 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 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).

In contrast to the biological sampling data, results of chemical/physical water quality monitoring at DNR station 10640002 downstream from Marshalltown suggest “full support” of the aquatic life uses. Monitoring showed no violations of Class B(WW1) water quality criteria for 36 Ammonia samples (maximum = 1.3 mg/L), 36 Dissolved Oxygen samples (minimum = 6.8 mg/L), 36 pH samples (range = 7.5 to 8.6), 36 Temperature samples (maximum = 25.2°C), 36 Chloride samples (maximum = 46 mg/L), or 36 Sulfate samples (maximum = 64 mg/L) occurred during monitoring from January 2014 to December 2016. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated. Thus, these results thus suggest “full support” of the Class B(WW1) aquatic life uses.

Fish consumption/human health uses remain assessed (monitored) as “partially supported” based on results of U.S.EPA/DNR fish contaminant (RAFT) monitoring 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 Iowa DNR/Iowa Dept. of Public Health fish 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 an 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.”