Assessment Comments

Assessment is based on results of IDNR/UHL ambient monthly monitoring during the 2004-2006 assessment period at STORET station 10910003 (formerly station 426046) at the Highway 92 bridge near Ackworth.

Basis for Assessment

[Note 1:  Prior to the current (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 (2008) 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 2004-2006 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).   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 24 samples collected (181 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Six of the 24 samples (25%) 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 3-35of U.S.  EPA 1997b).  

The Class B(WW2) aquatic life uses are assessed (monitored) as "fully supported."  None of the 36 samples collected during the 2004-2006 assessment period at the IDNR/UHL monthly station violated Class B(WW2) water quality criteria for pH, dissolved oxygen, or ammonia-nitrogen; no violations occurred in the approximately 10 samples analyzed for toxic metals or in the seven samples analyzed for pesticides.   These results suggest full support of the Class B(WW2) aquatic life uses.