Assessment Comments

Assessment based on results of (1) IDNR/UHL ambient monitoring from 2000-02 and (2) IDNR/UHL biological monitoring in 2000.

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses remain assessed (monitored) as "fully supported / threatened" due to (1) a single violation of the Class B(LR) water quality criterion for the pesticide chlorpyrifos and (2) results of biological monitoring in 2000 that show somewhat lower biological integrity than expected.   The assessment of support of the Class B(LR) uses is based on results of IDNR/UHL monthly ambient water quality monitoring conducted on Old Mans Creek near Iowa City at the Co.Rd.  W62 bridge (STORET station 10520001) from May 2000 through December 2002.  

EXPLANATION:  Results of the IDNR monitoring showed no violations of Class B(LR) water quality criteria in the 117 samples analyzed for dissolved oxygen and temperature, or in the 27 samples analyzed for pH and ammonia.   With the exception of chlorpyrifos, no violations of Class B(LR) chronic criteria occurred in the 8 samples analyzed for pesticides.   The sample collected on December 3, 2001, contained 0.11 ppb of chlorpyrifos; this level exceeds the Class B(WW) chronic criterion of 0.041 ppb.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), this single violation of chronic criteria for chlorpyrifos does not suggest an impairment of aquatic life uses.   Based on DNR's assessment methodology for Section 305(b) reporting, however, these violations suggest that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened."  

The assessment based on results of biological monitoring (“fully supported / threatened”) is consistent with the assessment based on results of IDNR/UHL chemical/physical monitoring.   This assessment was based on data collected in 2000 as part of the DNR/UHL stream biocriteria project.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria 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 (F-IBI) and a benthic macroinvertebrate index (BM-IBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2000 Fish IBI score was 28 (fair) and the BM-IBI score was 54 (fair).  The aquatic life use support was assessed as fully supported / threatened (=FST), based on a comparison of the F-IBI and BM-IBI scores with biological assessment criteria established specifically for the 2002 Section 305(b) report.  The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.