Assessment Comments

Assessment is based on: (1) results of IDNR/UHL monthly ambient water quality monitoring conducted on Old Mans Creek near Iowa City at the County Road W62 bridge (STORET station 10520001) from 2004 through 2006 and (2) results of biological monitoring conducted in 2000 as part of the IDNR/UHL stream biocriteria project.

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), this segment is now presumptively 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 presumptive 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 are assessed (evaluated) as "partially supported” based on results of biological monitoring in 2000.   The assessment of support of the Class B(WW2) uses is based on (1) results of IDNR/UHL monthly ambient water quality monitoring conducted on Old Mans Creek near Iowa City at the County Road W62 bridge (STORET station 10520001) from 2004 through 2006 and (2) results of biological monitoring conducted in 2000 as part of the IDNR/UHL stream biocriteria project.  

EXPLANATION:  The presumptive 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 (529 orgs/100ml) far exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Twenty of the 24 samples (83%) exceeded Iowa’s single-sample maximum criterion 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).  

Results of the IDNR/UHL ambient water quality monitoring from 2004 through 2006 showed no violations of Class B(WW2) water quality criteria in the approximately 35 samples analyzed for dissolved oxygen, pH, and ammonia.   In addition, none of the approximately 10 samples analyzed for toxic metals, and none of the seven samples analyzed for pesticides, exceeded the respective Class B(WW2) chronic criteria.  

Despite the results of chemical/physical monitoring that suggest “full support” of the Class B(WW2) uses, the results of biological monitoring suggest that these uses should be assessed as “partially supported.”  This assessment is based on biological data collected in 2000 as part of the DNR/UHL stream biocriteria project.   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 2000 FIBI score was 27 (fair) and the BMIBI score was 55 (fair).  The aquatic life use support was assessed (evaluated) as Partially Supported (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports.  The biological impairment criteria were determined 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.   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).   Despite this change in assessment category, the impairment indicated by these data remains appropriate for IR Category 5 (i.e., Section 303(d) list) until more recent data suggest a good cause for de-listing.