Shell Rock River IA 02-SHL-786

[Note:  A TMDL for indicator bacteria in this segment of the Shell Rock River was prepared by and approved by EPA in February 2010.  Because all Section 303(d) impairments identified for the 2008 assessment/listing cycle (indicator bacteria) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved) of Iowa’s Integrated Report.]

The Class A1 uses remain assessed (monitored) as "partially supported” due to levels of indicator bacteria (E. coli) that exceed state criteria.  This assessment remains based on results of IDNR/SHL ambient water quality monitoring for E. coli.  The geometric mean level of indicator bacteria (E. coli) in the 23 samples collected during the recreational seasons of 2002 through 2004 (72 orgs/100ml) was well-below the Iowa Class A1 water quality criterion of 126 orgs/100ml and indicated relatively low levels of bacteria in this river reach (note:  results of ambient (IDNR/SHL) monitoring downriver in Shell Rock River segment IA 02-SHL-0010_2 also suggest low levels of E. coli).  Five of the 23 samples, however, exceeded Iowa’s 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 the geometric mean of E. coli is less than the applicable state criteria, the contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).  Thus, these data suggest that the Class A1 uses should be assessed as “fully supported.”  These EPA guidelines, however, also suggest that if more than 10% of samples exceed the single-sample maximum criterion for indicator bacteria, the primary contact recreation uses should be assessed as "partially supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).  According to IDNR’s assessment/listing methodology, the monitoring data from 2002-04 suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum criterion, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired.” 

The water quality sampling data suggests the Class B(WW1) uses should remain assessed (evaluated) as "fully supported" based on results of (1) IDNR/SHL and (2) USGS ambient water quality monitoring conducted during the 2002-2004 period.  Results of IDNR/SHL monitoring showed no violations of state water quality criteria for conventional parameters (dissolved oxygen, pH, or ammonia) in the 31 monthly samples collected between January 2002 and December 2004 (these samples were not analyzed for toxic metals or pesticides).  Similarly, results of USGS monitoring showed no violations of Class B(WW1) criteria for ammonia, dissolved oxygen, or pH in the nine samples collected from April to December 2004.

Contrary to the water quality aquatic life assessment, the evaluated assessment of the aquatic life uses based on data collected in 2012-2014 as part of an IDNR Fisheries stream sampling project: Manchester research station suggest the aquatic life uses are "partially supporting".  A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the Fisheries sampling data.  The biological metrics are based on the numbers and types of 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).  The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).  The 2012-2014 evaluated FIBI scores (n=9) ranged from 10 (poor) to 79 (excellent).  The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the FIBI scores with biological assessment criteria established for previous Section 305(b) reports.  The non-riffle habitat FIBI BIC for this ecoregion is 44.  This assessment is considered evaluated because the drainage area (517-531 mi2) above the sampling sites 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 FIBI BIC (4/9), 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).

This segment of the Shell Rock River was sampled as part of the 1998-99 statewide study of freshwater mussels in Iowa streams and rivers (Arbuckle et al. 2000).  As part of this study, sampling results from 1998 and 1999 (Arbuckle et al.  2000) were compared to results from stream sites surveyed in 1984 and 1985 by Frest (1987).  On a statewide basis, this comparison showed sharp declines in the numbers of mussel species ("species richness") in Iowa streams and rivers from the mid-1980s to the late 1990s.  For stream segments having four or more species reported for the 1984-95 survey, results of this comparison were used by staff of the Iowa DNR to assess the degree to which the aquatic life uses of the sampled stream segments are supported.  The results of this sampling on this segment of the Shell Rock River, however, do not meet IDNR guidelines for developing an assessment of support for the aquatic life uses.  Species richness of freshwater mussels at the three sample sites in this segment was 0, 2, and 3 in the 1984-85 period and was 0, 0, and 0 in the 1998-99 period, respectively, for an average 1984-85 species richness of approximately 2 and an average percent change of minus 67%.  Based on these results, the aquatic life uses are considered “not assessed” due (1) to IDNR’s assessment methodology in which assessment decisions are developed only for those stream segments having an average of four or more species reported in the 1984-85 (Frest) survey and (2) the difficulty of interpreting status of mussel communities showing relatively low species richness during the both the historical (1984-85) and current (1998-99) surveys. 

Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.