Assessment Comments

Assessment is based on results of (1) IDNR/UHL ambient monthly water quality monitoring NNW of Cambridge downstream from outfall of Ames WWTF from 2000-02; (2) IDNR/UHL biological monitoring conducted from 1999-2002 and (3) 2002 Fisheries sampling: Fish IBI = 48 (fair).

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses are assessed (evaluated) as "partially supporting" based on results of biological monitoring from 1999-2002.   The sources of data for this assessment include (1) the results of IDNR/UHL monthly ambient water quality monitoring conducted on the South Skunk River from 2000 through 2002 at the Co.Rd.  E54 bridge approximately 4 miles upstream from Cambridge (STORET station 10850002; formerly station 390566); (2) results of IDNR/UHL biological monitoring conducted from 1999-2002 as part of the stream biocriteria project and (3) results of IDNR Fisheries sampling conducted in 2002.

EXPLANATION:  The results of monthly monitoring during the 2000-2002 assessment period at the Cambridge station (located approximately 1/3 mile downstream from the outfall of the Ames wastewater treatment plant) suggest relatively good water quality.   Results of this monitoring show no violations of Class B(WW) criteria for pH, dissolved oxygen, or ammonia nitrogen (maximum value = 0.05 mg/l) in the 36 samples analyzed during the assessment period and no violations of Class B(LR) chronic criteria for pesticides and other toxic organic compounds in the nine samples analyzed during the biennial period.   One of the 30 samples analyzed for toxic metals during the biennial period, however, exceeded the Class B(LR) chronic criterion for copper:  the sample collected on July 18, 2000 contained 160 ug/l, thus far exceeding the Class B(LR) criterion of 55 ug/l.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), the single violation of chronic criterion for copper does not suggest an impairment of aquatic life uses.   Based on DNR's assessment methodology for Section 305(b) reporting, however, this violation suggests that the Class B(LR) aquatic life uses should be assessed as "fully supported / threatened."

Despite the results of ambient chemical/physical monitoring that suggest full support of the Class B(LR) uses, results of biological monitoring conducted from 1999-2002 as part of the DNR/UHL stream biocriteria project and the 2002 IDNR Fisheries sampling suggest that these uses are “partially supported.”  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 2002 Fish IBI score was not available and the 1999-2002 BM-IBI scores were 31, 51, 42, 40 (all fair).   The 2002 Fisheries Fish IBI score was 48 (fair).   The aquatic life use support was assessed (evaluated) as partial supporting (=PS), based on a comparison of the IDNR-biocriteria BM-IBI score 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 600 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).