Shell Rock River IA 02-SHL-787

Note: Monitoring for indicator bacteria has not been conducted by MPCA at this station since 2010. Although recreation season geometric means for 2009 and 2010 were very low and easily met the Class A1 geometric mean criterion, the Class A1 uses of this segment remain assessed as "not supported" based on results of ambient monitoring for indicator bacteria (E. coli) from 2006-2008. The geometric means of indicator bacteria (E. coli) in the 15 samples collected during the recreational seasons of 2009 and 2010 at MPCA station S000-084 near Gordonsville, MN, were as follows: the 2009 geometric mean was 78 orgs/100 ml and the 2010 geometric mean was 84 orgs/100 ml. Both geometric means were below and met the Class A1 criterion of 126 orgs/100 ml. Two of the 15 samples (13%) 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 DNR’s assessment/listing methodology, if a recreation season geometric means are below the water quality criterion, and the percentage of samples exceeding the state’s single-sample maximum criterion is not significantly greater than 10%, then the primary contact recreation uses should be assessed as “fully supporting.” According to Iowa’s assessment/listing methodology, the percentage of samples exceeding the single-sample maximum criterion is not significantly greater than 10%.

The geometric mean level of indicator bacteria (E. coli) in the 12 independent samples collected by MPCA during the 2006 recreation season (198 orgs/100ml), however, exceeded the Iowa Class A1 water quality criterion of 126 orgs/100ml. According to DNR’s methodology for de-listing impairments, geometric mean levels of bacteria must meet state standards over two consecutive assessment/listing cycles in order for the impairment to be removed. Although monitored for a number of parameters during the current (2012-14) period, MPCA did not sample for indicator bacteria in this assessment segment beyond June 2010. Thus, due to the lack of subsequent data for indicator bacteria in this assessment segment, the impairment based on bacteria monitoring from 2006-08 will remain. If future monitoring shows that geometric mean levels are below state standards, this impairment can be considered for de-listing.

Results of chemical/physical monitoring from MPCA monitoring at the Gordonsville, MN station suggest “full support” of the aquatic life uses. Monitoring showedno violations of Class B(WW1) water quality criteria for 62 Temperature samples (maximum = 26.3° c) occurred during monitoring from March 2014 to November 2016.9 of the 61 samples (15%) analyzed for Dissolved Oxygen (minimum = 0.1 mg/L) and 2 of the 61 samples (3%) analyzed for pH (range = 6.6 to 9.2) violated the Class B(WW1) criteria during the 2014-2016 monitoring period. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17) and Iowa DNR's assessment/listing methodology, a violation frequency of significantly greater than 10% for conventional parameters such as Dissolved Oxygen, or pH suggests impairment of aquatic life uses. Because the frequency of violations for these parameters are not significantly greater than 10 percent, these results do not suggest impairment of the Class B(WW1) aquatic life uses.According to DNR's methodology, a stream must meet water quality standards for two consecutive cycles to suggest de-listing. Therefore, Shell Rock River will remain listed for the 2018 cycle as "partially supported" its Class B(LW) uses due to dissolved oxygen.

This aquatic life assessment is also considered "evaluated" because there were not two or more samples collected from this segment in multiple years over a recent five-year period. Additionally, 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).

Fish consumption uses remain assessed (monitored) as "fully supported" based on the 2015 DNR fish tissue sampling on the Shell Rock River south of Northwood. The results from the 2015 sampling show low levels of the primary contaminants (chlordane, PCBs and mercury) in the composite sample of common carp fillets: mercury: 0.19 ppm; total PCBs: <0.06 ppm; and technical chlordane: <0.02 ppm.The 2015 sampling results also show low levels of the mercury in the tissue plug samples of walleye (n=5): ave mercury: 0.218 ppm (sd = 0.035, max = 0.240). 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.Because levels of all these contaminants from the 2015 fish tissue monitoring are below Iowa’s advisory trigger levels, the fish consumption uses in this segment of the Shell Rock River are assessed as “fully supported.”