Assessment Comments

Assessment is based on results of IDNR/UHL ambient monthly monitoring during the 2008-2010 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, 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 (2012) 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 2008-2010 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 means of indicator bacteria (E.  coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10910003 near Ackworth were as follows:  the 2008 geometric mean was 898 orgs/100 ml, the 2009 geometric mean was 998 orgs/100 ml and the 2010 geometric mean was 844 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Sixteen of the 22 samples (73%) 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.”

The Class B(WW2) aquatic life uses are assessed (monitored) as "fully supported.”  None of the approximately 30 samples collected during the 2008-2010 assessment period at the IDNR/UHL monthly station violated Class B(WW2) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen.