Assessment Comments

Assessment is based on results of IDNR/UHL ambient monthly monitoring during the 2006-2008 assessment period at STORET station 10910003 (formerly station 426046) at the Highway 92 bridge near Ackworth.

Basis for Assessment

[Note 1:  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 are assessed (monitored) as "fully supported” based on results of physical/chemical monitoring.   The source of data for this assessment is the results of IDNR/UHL ambient monthly monitoring during the 2006-2008 assessment period at STORET station 10910003 (formerly station 426046) at the Highway 92 bridge near Ackworth.  

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 21 samples collected (392 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Eight of the 21 samples (38%) exceed 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 3-35 of U.S.  EPA 1997b).  

The Class B(WW2) aquatic life uses are assessed (monitored) as "fully supported."  None of the 34 samples collected during the 2006-2008 assessment period at the IDNR/UHL monthly station violated Class B(WW2) water quality criteria for ammonia-nitrogen; no violations occurred in the two samples analyzed for toxic metals.   One of the 34 samples analyzed for dissolved oxygen violated the Class B(WW1) criterion of 5.0 mg/l (this sample was collected on February 2, 2007 and contained a dissolved concentration of 4.0 mg/l).   According to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), however, a violation frequency of less than 10% for conventional parameters such as dissolved oxygen nonetheless suggest "full support" of aquatic life uses.   Thus, the percentage of violations of the dissolved oxygen criterion at this station (3%) does not suggest an impairment of aquatic life uses in this stream segment.   Similarly, one of the 34 samples analyzed for pH violated the Class B(WW1) criterion of 6.5 pH units (this sample was collected on March 7, 2007 and contained pH of 6.4 units).   According to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), however, a violation frequency of less than 10% for a conventional parameters such as pH nonetheless suggest "full support" of aquatic life uses.   Thus, the percentage of violations of the pH criterion at this station (3%) does not suggest an impairment of aquatic life uses in this stream segment.   These results suggest full support of the Class B(WW2) aquatic life uses.