Assessment Comments
Assessment is based on results of (1) IDNR/UHL biological (biocriteria) monitoring in 2000 (Fish IBI= 2(poor),10(poor); BM-IBI= 58(good), 40(fair)) and (2) IDNR/UHL ambient water quality monitoring conducted in 2002 in support of TMDL development.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses are assessed (monitored) as “not supported” due to (1) results of biological monitoring for fish and aquatic macroinvertebrates and (2) results of ambient chemical/physical water quality monitoring that show levels of ammonia-nitrogen violate state water quality standards. The sources of data for this assessment include (1) results of IDNR/UHL biological monitoring conducted in 2000 as part of the stream biocriteria project and (2) results of IDNR/UHL ambient water quality monitoring conducted from March to November 2002 as part of TMDL monitoring (station 8; STORET station 11840003).
EXPLANATION: Results of biological monitoring conducted in 2000 as part of the DNR/UHL stream biocriteria project suggest impairment of the Class B(LR) aquatic life 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 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 scores were 2 (poor) and 10 (poor); the BM-IBI scores were 58 (good) and 44 (fair). The aquatic life use support was assessed as not supporting (=NS), 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.
Results of IDNR/UHL ambient water quality monitoring conducted in 2002 in support of TMDL development also suggest “non-support” of the Class B(LR) aquatic life uses. Levels of ammonia-nitrogen in two of the 11 samples collected violated state chronic water quality standards; these samples were collected on June 9, 2002 (2.6 mg/l) and on June 12, 2002 (1.2 mg/l). According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-18), more than one violation of a water quality criterion for a toxic pollutant in an abundant data set (at least 10 samples over at three-year period) indicate an impairment of aquatic life uses. Thus, the Class B(LR) uses are assessed as “not supported.” No other violations of Class B(LR) criteria occurred at this station during the 2000-2002 assessment period.
A previous (2002) assessment of the aquatic life uses ("partially supported") was based on the occurrence of a fish kill in June 1998 (see assessment developed for the 2000 reporting cycle). According to IDNR's assessment methodology, the absence of a fish kill in the three-year period following a fish kill suggest that impacts from the kill have dissipated. Thus, due to the lack of a subsequent kill in this stream reach, the 1998 fish kill is no longer considered in determining support of the Class B(LR) aquatic life uses.
