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 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 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). 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) 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.”
