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 2006 through 2008 and (2) results of biological monitoring conducted in 1994 and 2000 as part of the IDNR/UHL stream biocriteria project.

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), 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 (2010) 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 1994 and 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 2006 through 2008 and (2) results of biological monitoring conducted in 1994 and 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).   The geometric mean level of indicator bacteria (E.  coli) in the 22 samples collected (1,159 orgs/100ml) far exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Nineteen of the 22 samples (86%) 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 2006 through 2008 showed no violations of Class B(WW2) water quality criteria in the approximately 33 samples analyzed for dissolved oxygen, pH, and ammonia.   In addition, neither of the two samples analyzed for toxic metals 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 1994 and 2000 as part of the IDNR/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 1994 FIBI score was 19 (poor) and the BMIBI score was 69 (good).   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.   This segment passed the FIBI BIC 0/2 times and passed the BMIBI BIC 2/2 times in the past 15 years.

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 between 2004 and 2008 to be considered “monitored”.   This segment had multiple samples collected in the previous 15 years (1994-2008); however, the multiple samples were not collected during 2004-2008.   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).   However, despite this change in assessment methodology and type, this waterbody remains in IR Category 5 and remains on Iowa’s 2010 Section 303(d) list of impaired waters.