Assessment Comments

Assessment is based on results of monthly (plus) ambient water quality monitoring from January 2004 through December 2006 at the IDNR/UHL ambient station located at the County Road C45 bridge at Shell Rock, Butler Co. (STORET station 10120001).

Basis for Assessment

Note:  Prior to the current (2008) Section 305(b) cycle, this river 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 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf) and the results of an Use Attainability Analysis, this segment is also now 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 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(WW1) aquatic life uses were assessed (monitored) as "fully supported" based on results of IDNR/UHL ambient water quality monitoring conducted during the 2004-2006 assessment period.   Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring in this river segment.   The source of data for this assessment is the results of monthly (plus) ambient water quality monitoring from January 2004 through December 2006 at the IDNR/UHL ambient station located at the County Road C45 bridge at Shell Rock, Butler Co.  (STORET station 10120001).  

EXPLANATION:  The Class A1 uses are assessed 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 37 samples collected during the recreational seasons of 2004 through 2006 (261 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Nineteen of the 37 samples (51%) 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).  

Despite the impairment suggested by results of bacterial monitoring, results of chemical/physical monitoring continue to suggest “full support” of the Class B(WW1) aquatic life uses.   Monitoring at this station from 2004 through 2006 showed no violations of Class B(WW1) water quality criteria for the following parameters:  dissolved oxygen or pH in the 42 samples analyzed; ammonia in the 40 samples analyzed, pesticides in the seven samples analyzed; and toxic metals in the 10 samples analyzed.   These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”  This assessment of the aquatic life uses is consistent with previous (2004 and 2006) Section 305(b) assessments.  

Fish consumption uses remain “not assessed” due to lack of recent fish tissue monitoring in this river segment.