Assessment Comments
Assessment is based on results of water quality monitoring conducted from 2010 through 2012 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 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 uses are assessed (monitored) as "fully supported" (IR 2) based on results of chemical/physical monitoring from 2010-12. The results of chemical/physical water quality monitoring conducted from 2002 through 2012 suggest improving water quality conditions in Walker Branch. The assessment of these uses is based on results of water quality monitoring conducted from 2010 through 2012 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 11 samples collected during the recreational seasons of 2010 through 2012 at the Walker Branch (site RA-38) were as follows: the 2010 geometric mean was 679 orgs/100 ml, the 2011 geometric mean was 26 orgs/100 ml and the 2012 geometric mean was 45 orgs/100 ml. The 2010 geometric mean exceeded the Class A1 criterion of 126 orgs/100 ml. Four of the 11 samples (36%) 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 28 samples collected at station RA-38 during the 2010-2012 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 = 6.8-8.0). 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). Monitoring results from 2006 through 2012, 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. The levels of atrazine in Walker Branch during the 2010-2012 period, however, do not suggest a serious threat to support of drinking water uses in Rathbun Reservoir. For example, the mean level of atrazine in the 11 samples collected during the 2010-12 period at Station RA-38 (2.5 ug/l) was below the atrazine MCL of 3.0 ug/l; the maximum level of atrazine during this three-year period was 13.0 ug/l.
