Assessment Comments

Assessment based on results of (1) IDNR/UHL biocriteria sampling in 2002, (2) IDNR/UHL ambient monthly monitoring near Oakland Mills from 2000-02, and (3) USGS/NAWQA fish contaminant monitoring in 1995.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life are assessed (evaluated) as "partially supported" based on results of biological monitoring conducted in 2002.   Fish consumption uses are assessed (evaluated) as “fully supported” based on results of fish contaminant monitoring in 1995.   The sources of data for this assessment include (1) the results of IDNR/UHL ambient monthly monitoring during the 2000-2002 assessment period at STORET station 10440001 approximately 3 miles southwest of Oakland Mills in Henry County (formerly station number 821004), (2) results of IDNR/UHL biological monitoring conducted in 2002 as part of the stream biocriteria project, and (3) results of fish contaminant monitoring conducted in 1995 as part of the USGS NAWQA program.  

EXPLANATION:  Results of ambient chemical/physical water quality monitoring suggest “full support” of the Class B(WW) aquatic life uses.   None of the 35 samples collected during the 2000-2002 assessment period at the IDNR/UHL monthly station violated Class B(WW) water quality criteria for pH, dissolved oxygen, or ammonia-nitrogen; no violations occurred in the four samples analyzed for toxic metals or in the nine samples analyzed for pesticides and other toxic organic compounds.   Although the Class B(WW) uses were assessed as "threatened" for the 1998-1999 biennial period due to a single violation of the chronic Class B(WW) criterion for chlorpyrifos (see assessment for the 2000 report), none of the nine samples analyzed during the 2000-2002 period contained a detectable level of chlorpyrifos.  

Despite results of ambient chemical/physical monitoring that suggest “full support” of aquatic life uses, the results of biological monitoring conducted in 2002 as part of the IDNR/UHL stream biocriteria project suggest “non-support” of these uses.   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 that were collected in the stream sampling reach.   The biological metrics were combined to 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 2002 BM-IBI score was 37 (Fair).  The aquatic life use support was assessed (evaluated) as Partially Supported (=PS), based on a comparison of the BM-IBI scores with biological assessment criteria established for previous Section 305(b) reports.  The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.

The assessment type is considered  “evaluated” (of lower confidence) because the size of the segment’s watershed exceeds the range of reference condition watershed sizes used to calibrate the benthic macroinvertebrate and fish bioassessment indexes.   For this assessment, the drainage area for the segment (approximately 520 mi2) exceeds the maximum drainage area cutoff (500 mi2) that IDNR has established for use of BMIBI and FIBI data.   Therefore, IDNR considers the aquatic life use impairments indicated by these data as “evaluated” assessments that are 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).

Fish consumption uses are assessed (evaluated) as "fully supported based on results of fish contaminant monitoring conducted as part of USGS/NAWQA fish tissue monitoring near Oakland Mills in September 1995 (see assessment for the 1998 report).   However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively lower confidence).