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 2002-04; (2) IDNR/UHL biological monitoring conducted in 2000 & 2002 and (3) biological monitoring by the IDNR Fisheries Bureau in 2002.

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 2002 through 2004 at the County Road E54 bridge approximately 4 miles upstream from Cambridge (STORET station 10850002; formerly station 390566); (2) results of IDNR/UHL biological monitoring conducted in 2000 and 2002 as part of the stream biocriteria project and (3) results of biological monitoring conducted by the IDNR Fisheries Bureau in 2002.

EXPLANATION:  The results of routine monthly ambient water quality monitoring during the 2002-2004 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 in this segment of the South Skunk River.   Results of this monitoring show no violations of Class B(LR) criteria for pH, dissolved oxygen, or ammonia nitrogen (maximum value = 0.35 mg/l) in the 36 samples analyzed during the assessment period.   Also, no violations of Class B(LR) chronic criteria occurred in the approximately 10 samples analyzed for pesticides or in the approximately 30 samples analyzed for toxic metals during the 2002-04 period.  

Despite the results of ambient chemical/physical monitoring that suggest full support of the Class B(LR) uses, results of biological monitoring conducted in 2000 and 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 (FIBI) and a benthic macroinvertebrate index (BMIBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2002 FIBI score was not available and the 2000 and 2002 BMIBI scores were 49 and 40 (fair).   The 2002 Fisheries FIBI score was 48 (fair).   The aquatic life use support was assessed (evaluated) as partial supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.   The FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51.   This assessment is considered evaluated because the drainage area (585 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   Even though this site passed the FIBI BIC and failed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size.

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).