Assessment Comments

Assessment is based on the results of: (1) IDNR/SHL ambient monthly monitoring during the 2010-2012 assessment period at STORET station 10630002 (formerly station 500680) at the Highway 156 bridge approximately 1.5 miles northwest of Bussey and (2) 2012 IDNR/SHL large river benthic macroinvertebrate sampling conducted near Bussey.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008, this segment is also now presumptively designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

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(WW1) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-c) based on results of IDNR/SHL biological sampling.   Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this stream segment.   The sources of data for this assessment are the results of: (1) IDNR/SHL ambient monthly monitoring during the 2010-2012 assessment period at STORET station 10630002 (formerly station 500680) at the Highway 156 bridge approximately 1.5 miles northwest of Bussey and (2) 2012 IDNR/SHL large river benthic macroinvertebrate sampling conducted near Bussey.    

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 10630002 near Bussey were as follows:  the 2010 geometric mean was 235 orgs/100 ml, the 2011 geometric mean was 184 orgs/100 ml, and the 2012 geometric mean was 260 orgs/100 ml.   All three geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml.   Eleven of the 24 samples (46%) 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.”  

Based exclusively on the water quality data, the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported.”  None of the 36 samples collected during the 2010-2012 assessment period at the IDNR/UHL monthly station violated Class B(WW1) water quality criteria for pH or ammonia-nitrogen, temperature, chloride, or sulfate.   One of the 36 samples collected (3%) exceeded the Class B(WW1) criterion of 5.0 mg/l for dissolved oxygen (4.8 mg/l).   According to U.S.  EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   According to IDNR’s assessment/listing methodology, however, the results from IDNR station 10630002 do not indicate that greater than 10% of the samples exceed the Class B(WW1) criteria for dissolved oxygen.   Thus, these results do not suggest impairment of the Class B(WW1) aquatic life uses.   Contrary to the biological data (below), these results suggest that the Class B(WW1) aquatic life should be assessed as "fully supported."

In disagreement with the water quality data and assessment for this segment, the biological data suggest that the aquatic life uses in this segment are "not supporting."  This evaluated aquatic life assessment was based on data collected in 2012 as part of the IDNR/SHL large river sampling project.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach.   The biological metrics were combined to make a benthic macroinvertebrate index (BMIBI).   The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2012 BMIBI score was 12 (poor).  The aquatic life use support was assessed as not supporting (=NS), based on a comparison of the BMIBI score with biological impairment criteria (BIC) established for previous Section 305(b) reports.    The BMIBI BIC for this ecoregion is 41.   The biological impairment criteria were determined from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008.   This assessment is considered "evaluated" because there were not two or more samples collected in multiple years from 2004-2008.  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).

Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this stream segment.