Walker Branch IA 05-CHA-1329
mouth (S36 T70N R20W Wayne Co.) to confluence with S. Fk. Walker Branch in SE 1/4 S26 T70N R20W Wayne Co.
- Cycle
- 2018
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trend
- Stable
- Created
- 7/10/2019 8:46:24 AM
- Updated
- 7/30/2019 2:40:45 PM
The presumptive Class A1 (primary contact recreation) uses are assessed as "partially supported" due to levels of indicator bacteria that violate state water quality criteria based on samples collected during the 2010-2012 sampling seasons. The Class B(WW2) aquatic life uses are assessed as "fully supported" based on results of chemical/physical monitoring from 2012-14. The results of chemical/physical water quality monitoring conducted from 2012 through 2014 suggest improving water quality conditions in Walker Branch. The assessment of these uses is based on (1) results of water quality monitoring conducted from 2012 through 2014 on Walker Branch near Confidence (station RA-38) by the State Hygienic Lab under contract with the U.S. Army Corps of Engineers as part of the Rathbun Water Quality Project and (2) the indicator bacteria (E. coli) results from 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.
The presumptive Class A1 (primary contact recreation) are assessed (evaluated) as "partially 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 DNR’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). In the 2014 assessment/listing cycle, the presumptive Class A1 uses should have been listed as (monitored)“partially supported" due to levels of indicator bacteria that exceeded state water quality criteria. Therefore, due to the lack of new data collected during the 2012 -2014 sampling seasons, the Class A1 uses will remain be listed as "partially supported" for the current assessment/listing cycle. Results of chemical/physical water quality monitoring at the Walker Branch (site RA-38) suggest “full support” of the aquatic life uses. Monitoring showed no violations of Class B(WW2) water quality criteria for 20 Ammonia samples (maximum = 0.7 mg/L), 34 pH samples (range = 6.8 to 7.9), 34 Temperature samples (maximum = 27.8° c), or 13 Chloride samples (maximum = 32 mg/L) occurred during monitoring from March 2012 to October 2014.Three of the 34 samples (9%) analyzed for Dissolved Oxygen (minimum = 2.3 mg/L) violated the Class B(WW2) criteria during the 2012-2014 monitoring period. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as Ammonia, pH, Temperature, Chloride, or Sulfate suggests impairment of aquatic life uses. Because the frequency of violations for this parameters is not greater than 10 percent, these results do not suggest impairment of the Class B(WW2) aquatic life uses. 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 2012-2014 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 18 samples collected during the 2012-14 period at Station RA-38 (2.8 ug/l) was below the atrazine MCL of 3.0 ug/l; the maximum level of atrazine during this three-year period was 25.0 ug/l.