Shell Rock River IA 02-SHL-783

Assessment Comments

Assessment is based on results of monthly (plus) ambient water quality monitoring from January 2006 through September 2008 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 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 "fully supported" based on results of monitoring for bacteria.   This assessment represents a de-listing of the bacterial impairment previously indentified for this assessment segment.   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 2006-2008 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 2006 through September 2008 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 (monitored) as "fully supported" based on results of ambient monitoring for indicator bacteria (E.  coli) at IDNR/UHL station 10120001.   The geometric mean level of indicator bacteria (E.  coli) in the 23 samples collected during the recreational seasons of 2006 through 2008 (120 orgs/100ml) is slightly below the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Five of the 23 samples (22%) 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 less than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   Although not designated for Class A1 uses until the 2008 assessment cycle, a review of historical data suggests that levels of E.  coli are relatively low in this assessment segment and typically meet the Class A1 geometric mean criterion:  2004 cycle:  GM=86 orgs/100ml (N=27); 2006 cycle:  GM=108 orgs/100 ml (N=31); 2008 cycle:  GM=142 orgs/100 ml (N=27).   If the geometric mean for the 2010 cycle (120 orgs/100 ml) is included, recreation season geometric means for three of the last four 305(b)/303(d) assessment/listing cycles (covering nine years of ambient monitoring) have met the Class A1 geometric mean criterion of 126 orgs/100 ml.   This analysis suggests that this segment of the Shell Rock River fully supports its designated Class A1 primary contact recreation uses.   Note:  the geometric mean reported for the 2008 cycle (261 orgs/100 ml; N=37) was based on daily duplicate samples and other serially correlated samples collected on consecutive days which, when combined with single-sample monthly monitoring data, bias the resulting three-season geometric mean.   If average values for duplicate samples and for serially correlated samples are used, the three-season geometric mean is 142 orgs/100 ml (N=27).

Results of chemical/physical monitoring at station 10120001 continue to suggest “full support” of the Class B(WW1) aquatic life uses.   Monitoring at this station from 2006 through 2008 showed no violations of Class B(WW1) water quality criteria ammonia, dissolved oxygen, or pH in the 34 samples analyzed or in the five samples analyzed for toxic metals.   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 Section 305(b) assessments.  

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