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 IDNR’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 IDNR’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 ambient chemical/physical monitoring from MPCA suggest that the Class B(WW1) aquatic life uses should be assessed (monitored) as “partially supported.”  Of the 44 samples analyzed for dissolved oxygen during the 2012-2014 period, 8 samples (18%) violated the Class B(WW1) aquatic life criterion of 5.0 mg/l.  Four of the violations were less than 4.0 mg/l of dissolved oxygen; the minimum level was 0.1 mg/l on March 16, 2014 at 10:00 AM.  Samples violating the Class B(WW1) criterion for dissolved oxygen were collected during daylight hours (8:30 AM to 5:30 PM).  Although four of the 48 samples analyzed for pH during the 2012-2014 period violated the Class B(WW1) criterion of 9.0 pH units, the percentage of samples violating this criterion (8%) is below the 10% threshold used to identify impairments for this parameter. 

According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed state criteria for conventional parameters such as pH, dissolved oxygen, or temperature, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S. EPA 1997b).  According to IDNR’s assessment/listing methodology, the results from the MPCA monitoring at the Gordonsville, MN station indicate that significantly greater than 10% of the samples exceed the Class B(WW1) criteria for dissolved oxygen.  Thus, the results of chemical/physical water quality monitoring from 2012 to 2014 in this segment suggest that the Class B(WW1) aquatic life uses should be assessed (monitored) as “impaired”.  Thus, based on these data, the aquatic life uses are assessed as “partially supporting” due to violations of the Class B(WW1) criterion for dissolved oxygen. 

Consistent with the results of dissolved oxygen monitoring from 2008-10, results of IDNR/SHL biological monitoring in 2002 suggest that the aquatic life uses should be assessed (evaluated) as “partially supported” (IR 3b-c).  The biological assessment was based on data collected in 2002 as part of the IDNR/SHL REMAP project.  A series of biological metrics which 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 that were 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 2002 FIBI score was not available and the BMIBI score was 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 for previous Section 305(b) reports.  The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008.  The natural substrate BMIBI BIC for this ecoregion is 70.  This assessment is considered evaluated because of recent high-water/flood event directly preceding the sampling and the sampling conditions at the time of the sampling were unfavorable. 

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 2005 EPA/IDNR fish tissue (RAFT) sampling on the Shell Rock River south of Northwood.  The composite sample of common carp fillets had low levels of the primary contaminants (mercury, total PCBs, and technical chlordane) that were all below levels of concern.  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 results from the 2005 sampling show low levels of the primary contaminants (chlordane, PCBs and mercury) in the composite sample of common carp fillets:  mercury:  0.181 ppm; total PCBs:  0.108 ppm; and technical chlordane:  <0.03 ppm.  Because levels of all these contaminants from the 2005 RAFT 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.”