Unnamed Tributary to Twelvemile Creek IA 05-GRA-6576

Assessment Comments

Assessment is based on results of monitoring at STORET station 13880003 (Site 12m-3) conducted as part of a Clean Water Act Section 319 water quality project from May to June 2012.

Basis for Assessment

[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was classified only for general 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 and for Class B(WW1) aquatic life uses.  According to the Iowa Water Quality Standards, all perennial rivers and streams and all intermittent streams with perennial pools that are not specifically listed in the Iowa surface water classification are presumptively designated as Class A1 and Class B(WW1) waters.  Thus, for the current assessment, perennial flow is presumed, and the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW1) water quality criteria.]

SUMMARY: The presumptive Class A1 (primary contact recreation) uses remain “not assessed” (IR 3a) due to lack of information upon which to base an assessment.   The presumptive Class B(WW1) aquatic life uses are assessed (evaluated) as “fully supporting” (IR 2a) based on results of chemical/physical monitoring in 2012.   The source of data for this assessment is the results of monitoring at STORET station 13880003 (Site 12m-3) conducted as part of a Clean Water Act Section 319 water quality project from May to June 2012.  

EXPLANATION: The presumptive Class A1 uses remain “not assessed” due to lack of monitoring for indicator bacteria (E.  coli) in this assessment segment.

The presumptive Class B(WW1) aquatic life uses are assessed (evaluated) as “fully supporting” based on results of water quality monitoring during 2012.   None of the five samples collected violated Class B(WW1) criteria for ammonia or pH.   One of the five samples (20%) violated the Class B(WW1) criterion for dissolved oxygen of 5 mg/l.   This assessment is considered evaluated (of lower confidence)for two reasons:  (1) the Iowa DNR assessment/listing methodology requires that at least 10 samples be collected over the three-year assessment period (in this case, from 2010 through 2012) in order to develop a “monitored” (i.e., higher confidence) assessment that is appropriate for Section 303(d) listing.