Assessment Comments
Assessment is based on results of water quality monitoring conducted from 2008 through 2010 on Walker Branch near Confidence (station RA-38) by Iowa State University/Limnology under contract with the U.S. Army Corps of Engineers as part of the Rathbun Water Quality Project.
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 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 uses are assessed (monitored) as "fully supported." The results of chemical/physical water quality monitoring conducted from 2002 through 2010 suggest improving water quality conditions in Walker Branch. The assessment of these uses is based on results of water quality monitoring conducted from 2008 through 2010 on Walker Branch near Confidence (station RA-38) by Iowa State University/Limnology under contract with the U.S. Army Corps of Engineers as part of the Rathbun Water Quality Project.
EXPLANATION: The presumptive Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 20 samples collected during the recreational seasons of 2008 through 2010 at the Walker Branch (site RA-38) were as follows: the 2008 geometric mean was 341 orgs/100 ml, the 2009 geometric mean was 996 orgs/100 ml and the 2010 geometric mean was 679 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Sixteen of the 20 samples (80%) 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.”
Regarding support of the Class B(WW2) aquatic life uses, none of the approximately 21 samples collected at station RA-38 during the 2008-2010 assessment period violated Class B(WW2) criteria for dissolved oxygen (minimum = 5.7 mg/L), ammonia-nitrogen (maximum = 0.7 mg/L), or pH (range = 6.8-7.5). Previous assessment cycles had shown impairments in this waterbody related to low levels of dissolved oxygen (see assessments developed for the 2002-04 and 2004-06 assessment cycles). The 2006-08 and 2008-10 monitoring results, however, suggest improving water quality conditions in this segment of Walker Branch.
Although this stream is not designated for Class C drinking water uses, Walker Branch does flow into Rathbun Reservoir which is used as a source of drinking water for a public water supply. Thus, the seasonal elevation of pesticide levels in Walker Branch and other tributaries of Rathbun Reservoir presents a continuing concern for full support of the Class C (drinking water) uses designated for the Chariton River immediately downriver from Rathbun Reservoir. Fortunately, however, the levels of atrazine in Walker Branch during the 2004-2006 period do not suggest a serious threat to support of drinking water uses in Rathbun Reservoir. For example, the mean level of atrazine in the 10 samples collected in 2004 at Station RA-38 (1.5 ug/l) was below the atrazine MCL of 3.0 ug/l; the maximum level of atrazine during this three-year period (2.3 ug/l)—also below the MCL-- occurred in the sample collected on May 19, 2004. Atrazine was not monitored at Station RA-38 during the 2006-10 period.
