Assessment Comments
Assessment remains based on results of monitoring of USGS water quality monitoring June 2008 to August 2010 near Marengo and is based on result of IDNR/EPA fish contaminant monitoring in 2011 and 2012.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) remain “not assessed” (IR 3a) due to the lack of recent information upon which to base an assessment. The Class B(WW1) uses remain assessed (evaluated) as "fully supported" (IR 2a) based on results of ambient chemical/physical monitoring from 2008-10. Fish consumption uses are assessed as “partially supporting” (IR 5a) due to issuance of a one meal per week fish consumption advisory in 2013. This is a new impairment for this assessment segment. The sources of data for this assessment are (1) the results of ambient water quality monitoring conducted by USGS at station 05453100 at Marengo from June 2008 through August 2010 and (2) results of IDNR/EPA fish contaminant monitoring at Marshalltown in 2011 and 2012. Note: no additional water quality monitoring was conducted at USGS station 05453100 between August 2010 and December 2012.
EXPLANATION: The Class A1 (primary contact recreation) uses remain “not assessed”. Only one sample was analyzed for indicator bacteria (E. coli) during the 2008-10 assessment period. According to IDNR’s assessment/listing methodology, this amount of data is not sufficient for developing an assessment of the degree to which the Class A1 uses are supported.
Regarding support of the Class B(WW1) aquatic life uses, results of USGS monitoring at Marengo showed no violations of Class B(WW1) criteria for ammonia, pH, or dissolved oxygen in the approximately 12 samples collected during the 2008-10 period. These results suggest that the Class B(WW1) aquatic life uses should be assessed (monitored) as “fully supporting”.
Fish consumption uses are assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR 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 new advisory incorporates an existing 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 Iowa 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.”
