Thompson River IA 05-GRA-1351

Assessment Comments

Assessment is based on results of (1) IDNR monthly monitoring during 2000 & 2001 near Davis City and (2) fish tissue (RAFT) monitoring in 1997, and (3) biocriteria monitoring in 2001.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed as "fully supported / threatened."  Fish consumption uses remain assessed as "fully supported."  This assessment is based on results of (1) IDNR monthly ambient monitoring conducted during the 2000-2001 biennial period at Davis City (station 10270001 (formerly station 784089)), (2) IDNR/UHL biocriteria monitoring in 2001, and (3) results of U.S.  EPA/IDNR fish tissue monitoring in 1997.   EXPLANATION:  Monitoring at the IDNR station at Davis City showed no violations of Class B(WW) water quality criteria during the 2000-2001 biennial period for dissolved oxygen (minimum value = 5.8 mg/l), pH (range = 7.3 to 8.6 units), or ammonia-nitrogen (maximum value = 0.90) in the 24 samples analyzed.   In addition, levels of toxic metals in the four samples analyzed, and levels of pesticides in the six samples analyzed, were all below the analytical level of detection.   These results suggest "full support" of the Class B(WW) aquatic life uses.   Results of IDNR/UHL biocriteria monitoring, however, suggest that the Class B(WW) uses are "fully supported / threatened."  Data were collected in 2001 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 2001 Fish IBI score was 32 (fair) and the BM-IBI score was 42 (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.   Biological monitoring is better able to reflect cumulative impacts of water quality over time and thus is believed to more accurately represent water quality conditions of this segment of the Thompson River than do results from IDNR ambient monthly monitoring.   Thus, despite the results of IDNR ambient water quality monitoring that suggest "full support" of the Class B(WW) uses, these uses are assessed as "fully supported / threatened" based on results of biological monitoring in 2001.   Fish consumption uses remain "fully supported" based on results of EPA/DNR fish tissue (RAFT) monitoring in 1997 (see assessment for the 1998 report above).