Assessment Comments
Assessment remains based on IDNR/UHL ambient water quality monitoring conducted in 2002 and 2003 in support of TMDL development.
Basis for Assessment
[Note: 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 levels of indicator bacteria that exceed Iowa water quality criteria. The Class B(WW2) aquatic life uses remain assessed (monitored) as "fully supported." This assessment remains based on results of monitoring conducted in support of TMDL development during the 2002-2004 assessment period. The data used for this assessment were generated from the IDNR/UHL station near Chatsworth (station 9) monitored during the 2002-2004 assessment period in support of TMDL development (STORET station 11750001).
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 23 samples collected at the IDNR/UHL TMDL monitoring near Chatsworth during summer recreational seasons of 2002-2003 (564 orgs/100ml) far exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml. Fourteen of the 24 samples (61%) 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).
Regarding support of the Class B(WW2) aquatic life uses, results of ambient monitoring at the IDNR/UHL station from March to December 2003 showed no violations of Class B(WW2) water quality criteria for ammonia-nitrogen, dissolved oxygen, or pH in the 22 samples collected. Based on these data, the Class B(WW2) aquatic life uses remain assessed (monitored) as “fully supported.”
