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 DNR/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. 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 DNR’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 3-35 of 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 DNR’s assessment/listing methodology, the monitoring data from 2002-04 suggest that significantly greater than 10% of the samples exceed DNR’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) DNR/SHL and (2) USGS ambient water quality monitoring conducted during the 2002-2004 period. Results of DNR/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 2012-2015 as part of an DNR 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 2012-2015 evaluated FIBI scores (n=12) 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 (7/12), 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 DNR’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). DNR 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).