[Note:  Prior to the 2008 Section 305(b) cycle, this river segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.  Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008 and the results of an Use Attainability Analysis, this segment is also now designated for Class A1 (primary contact recreation) uses.  This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).] 

The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supporting”.  The current (2016) IR cycle is the second consecutive cycle where results of ambient monitoring have shown levels of indicator bacteria (E. coli) that meet the Class A1 criteria.  During 2010-2012 monitoring period, the recreation season geometric means of indicator bacteria (E. coli) in the 24 samples collected at IDNR ambient station 10120001 at Shell Rock were as follows:  the 2010 geometric mean was 48 orgs/100 ml, the 2011 geometric mean was 50 orgs/100 ml and the 2012 geometric mean was 46 orgs/100 ml.  The geometric means for all three recreation seasons were far below the Class A1 criterion of 126 orgs/100 ml.  Only one of the 24 samples (4%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.  These results did not suggest impairment of the Class A1 uses:  all geometric means meet the Class A1 criterion, and, according to IDNR’s assessment/listing methodology, the percentage of samples exceed the single-sample maximum criterion is not significantly greater than 10%. 

Similarly, results from the current (2012-14) IR monitoring period also indicate “full support of the Class A1 uses.  The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2012 through 2014 at IDNR ambient station 10120001 at Shell Rock were as follows:  the 2012 geometric mean was 46 orgs/100 ml, the 2013 geometric mean was 118 orgs/100 ml, and the 2014 geometric mean was 64 orgs/100 ml.  The geometric means for all three recreation seasons are below the Class A1 criterion of 126 orgs/100 ml.  Four of the 24 samples (17%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.  These results do not suggest impairment of the Class A1 uses:  all geometric means meet the Class A1 criterion, and, according to IDNR’s assessment/listing methodology, the percentage of samples exceed the single-sample maximum criterion is not significantly greater than 10%.

The IDNR assessment/listing methodology requires that, before a bacterial impairment can be de-listed, geometric mean levels of E. coli must all be less than the applicable state water quality criteria for two consecutive listing cycles (i.e., five consecutive years).  Because results of ambient monitoring over the last two consecutive IR cycles, which cover the five years of monitoring from 2010-2014, have shown that levels of indicator bacteria meet Class A1 criteria, the existing IR 5a impairment is proposed for de-listing.  

The aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting".  This evaluated biological assessment was based on data collected in 2011-2014 as part of the IDNR/SHL stream nutrient sampling project and fish sampling data collected in 2007 and 2008 by the IDNR Fisheries Bureau. 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 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 2007 and 2008 FIBI scores were 62 (good) and 74 (excellent).  The 2011-2014 BMIBI scores were 51, 53 (both fair) and 59, 65, 65, 69, 70 (all good).  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 artificial substrate BMIBI BIC for this ecoregion is 52 and the natural substrate BMIBI BIC is 70.  This assessment is considered evaluated because the drainage area (1626, 1662 and 1730 mi2) above the sampling sites was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.  The riffle/non-riffle FIBI BICs for this ecoregion are 65/44 and the riffle status is unknown at the two sites where fish sampling was conducted.  This site passed the artificial substrate BMIBI BIC (52) in 2/3 samples and passed the natural substrate BMIBI BIC (70) in 1/4 samples.  Overall, this site passed the BMIBI BIC in 3/7 samples.  Even though this site passed the FIBI BIC and failed the BMIBI BIC (3/7), 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.  According to IDNR’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).  IDNR 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). 

Despite the results of biological monitoring that suggest impairment of the Class B(WW1) aquatic life uses, results of chemical/physical monitoring at station 10120001 continue to suggest “full support” of these uses.  Monitoring at this station from 2012 through 2014 showed no violations of Class B(WW1) water quality criteria for ammonia, dissolved oxygen, temperature chloride, or sulfate in the 36 samples analyzed.  One of the 35 samples collected (3%) exceeded the state standard for pH.  According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), however, a violation frequency of less than 10 % for conventional parameters such as pH dose not indicate impairment of aquatic life uses.  Thus, the percentage of violations of the pH criterion at this station (3%) does not suggest an impairment of aquatic life uses in this stream segment.  These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.” 

Fish consumption uses remain assessed (monitored) as “partially supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring in 2011.  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 Shell Rock River near Clarksville show that levels of mercury are sufficiently high for concern and thus justify issuance of a one meal/week fish consumption advisory that extends from the mouth of the Shell Rock River to its confluence with the Winnebago River near Rockford in Floyd County.  Results from the 2011 sampling showed that the level of mercury in the sample of walleye muscle tissue (0.75 ppm) far exceeded the 1 meal/week trigger level (0.30 ppm) as defined in Iowa’s revised (2007) fish consumption advisory protocol.  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.  Because, however, the level of mercury in the 2011 sample of walleye was well above the one meal/week advisory trigger, a one meal/week consumption advisory was issued.  Follow-up monitoring was conducted in 2012 to better define the level of mercury in Shell Rock River fish and to provide information for any changes needed to the consumption advisory.  Results showed that the average level of mercury in tissue samples form three Walleye was just below the consumption advisory threshold of 0.3 ppm.  Thus, the one-meal per week advisory that was issued in 2013 remained in effect.  Because a one meal per week fish consumption advisory covers this waterbody, the fish consumption uses are assessed as “partially supported.”