Assessment Comments

Assessment is based on: (1) the results of IDNR/UHL monthly ambient water quality monitoring conducted on the South Skunk River from 2004 through 2006 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.

Basis for Assessment

[Note:  Prior to the current (2008) Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf) and due to results of an Use Attainability Analysis, this segment is now designated for Class A1 (primary contact recreation) uses.   The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses).   Thus, for the current (2008) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW2) aquatic life uses remain 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 2004 through 2006 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 Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.  coli).   Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed.   Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses:  the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff."  Due to a change in the Standards in July 2003, E.  coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E.  coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.”  Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E.  coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.  

The geometric mean level of indicator bacteria (E.  coli) in the 24 samples collected (411 orgs/100ml) far exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Sixteen of the 24 samples (67%) exceed Iowa’s single-sample maximum value of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if the geometric mean level of E.  coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).  

The Class B(WW2) aquatic life uses remain assessed (evaluated) as “partially supported” based on results of biological monitoring.   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).

Despite the results of biological monitoring that suggest impairment of the Class B(WW2) uses, results of ambient chemical/physical monitoring suggest full support of these uses.   The results of routine monthly ambient water quality monitoring during the 2004-2006 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(WW2) criteria for pH, dissolved oxygen, or ammonia nitrogen (maximum value = 0.32 mg/l) in the 36 samples analyzed during the assessment period.   Also, no violations of Class B(WW2) chronic criteria occurred in the approximately 10 samples analyzed for pesticides or in the 14 samples analyzed for toxic metals during the 2004-06 period.