Assessment Comments
Assessment is based on results of IDNR/SHL ambient monthly monitoring during the 2010-2012 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 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" (IR 5p) due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW2) aquatic life are assessed (monitored) as "fully supported” (IR 2a) based on results of physical/chemical monitoring. The source of data for this assessment is the results of IDNR/SHL ambient monthly monitoring during the 2010-2012 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 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10910003 near Ackworth were as follows: the 2010 geometric mean was 844 orgs/100 ml, the 2011 geometric mean was 206 orgs/100 ml, and the 2012 geometric mean was 440 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Thirteen of the 24 samples (54%) 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 35 samples collected during the 2010-2012 assessment period at the IDNR/SHL monthly station violated Class B(WW2) water quality criteria for dissolved oxygen, pH, temperature, ammonia-nitrogen, chloride, or sulfate.