Assessment Comments

Assessment is based on: (1) the results of IDNR/UHL monthly ambient water quality monitoring conducted on the South Skunk River from 2008 through 2010 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 1999 & 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 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 (2012) 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 in 1999 and 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 2008 through 2010 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 1999 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).   The geometric means of indicator bacteria (E.  coli) in the 21 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10850002 near Cambridge were as follows:  the 2008 geometric mean was 394 orgs/100 ml, the 2009 geometric mean was 364 orgs/100 ml and the 2010 geometric mean was 804 orgs/100 ml.   All three geometric means slightly to moderately exceed the Class A1 criterion of 126 orgs/100 ml.   Thirteen of the 21 samples (62%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”  

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 1999 and 2002 as part of the IDNR/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 1999 BMIBI score was 31 (fair) and the 2002 BMIBI score was 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 32 and the BMIBI BIC for this ecoregion is 62.   This segment passed the FIBI BIC 1/1 times and passed the BMIBI BIC 0/2 times in the last 12 years.   This assessment is considered evaluated because the drainage area (605 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.  

This aquatic life assessment is now considered "evaluated" based on a change in the 2010 IDNR assessment methodology.   IDNR now requires a segment have two or more biological samples collected from the segment in multiple years over a five-year period to be considered “monitored”.   This segment had multiple BMIBI samples collected in the previous 12 years (1999-2010); however, the samples were not collected during a recent five-year period.   Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).   According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus 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 2008-2010 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 = 0.73 mg/l) in the approximately 30 samples analyzed during the 2008-10 assessment period.