Assessment Comments

Assessment based on: (1) results of IDNR/SHL monthly ambient water quality monitoring conducted on Old Mans Creek near Iowa City at the County Road W62 bridge (STORET station 10520001) from 2010 through 2012 and (2) results of biological sampling conducted in 2000 and 2010 as part of the IDNR/SHL 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, 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 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" (IR 5p) 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” (IR 5b-t) based on results of biological sampling in 2000 and 2010.   Sources of data used for this assessment include (1) results of IDNR/SHL monthly ambient water quality monitoring conducted on Old Mans Creek near Iowa City at the County Road W62 bridge (STORET station 10520001) from 2010 through 2012 and (2) results of biological sampling conducted in 2000 and 2010 as part of the IDNR/SHL 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 means of indicator bacteria (E.  coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10520001 near Iowa City were as follows:  the 2010 geometric mean was 1,625 orgs/100 ml, the 2011 geometric mean was 521 orgs/100 ml, and the 2012 geometric mean was 926 orgs/100 ml.   All three geometric means far exceed the Class A1 criterion of 126 orgs/100 ml.   Twenty of the 24 samples (83%) 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.”  Levels of indicator bacteria in this assessment segment are high relative to other streams/rivers in Iowa, thus suggesting sources of bacteria beyond that contributing to background levels.  

Results of the IDNR/UHL ambient water quality monitoring from 2010 through 2012 showed no violations of Class B(WW2) water quality criteria in the approximately 36 samples analyzed for dissolved oxygen, pH, ammonia, temperature, chloride and sulfate.   These results suggest “full support” of the Class B(WW2) aquatic life uses.  

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 and 2010 as part of the IDNR/SHL 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 2010 FIBI score was 24 (fair) and the BMIBI score was 66 (good).   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 1994-2008.   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 13 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 over a five-year period to be considered “monitored”.   This segment had multiple samples collected in the previous 11 years (2000-2012); however, the multiple samples were not collected during a five-year period.   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 5b-t and remains on Iowa’s Section 303(d) list of impaired waters.