Assessment Comments
Assessment is based on results of monitoring conducted in 1998 and 1999 as part of the Rathbun Lake Water Quality Project.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses were assessed as "fully supported." EXPLANATION: The assessment of support of the Class B(LR) uses is based on results of water quality monitoring conducted on Walker Branch near Confidence (station RA-38) by the U.S. Geological Survey (1998) and by Iowa State University/Limnology (1999) under contract with the U.S. Army Corps of Engineers as part of the Rathbun Water Quality Project. The results of this monitoring are summarized in the "Rathbun Lake Water Quality Reports" for 1998 and 1999 (see ACOE 1999 and Kirsh and Leonard 2000). Results of this monitoring show no violations of Iowa Class B(LR) water quality criteria for dissolved oxygen in the 6 samples analyzed in 1999, for pH in the 7 samples analyzed in 1999, or for ammonia-nitrogen in the 19 samples analyzed during 1998 and 1999. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), these results suggest "full support" of the Class B(LR) uses. Additional monitoring is needed to better define the water quality conditions in this stream reach. This additional monitoring is being conducted in 2000 as part of the ongoing Rathbun Water Quality Project. Monitoring conducted during 1998 and 1999 showed high levels of both nutrient parameters (total nitrogen and total phosphorus) and pesticides at station RA-38 and at stations on other reservoir tributaries, with the highest levels often associated with storm runoff. The Iowa Water Quality Standards does not currently have aquatic life criteria for these nutrient parameters or for other nonpoint source-related parameters (for example, total suspended solids). Although this stream reach is not designated for Class C drinking water uses, Walker Branch is a tributary of Rathbun Reservoir which is designated for Class C uses. The seasonal elevation of pesticides levels in the tributaries of the Rathbun watershed presents a continuing threat to full support of the Class C (drinking water) uses designated for Rathbun Reservoir. For example, of the 19 samples collected at Station RA-38 in 1998 and 1999, four of the five samples that exceeded the atrazine MCL of 3.0 ug/l were collected in late May or June following application of herbicides to farm fields and during months with typically high levels of rainfall. The mean levels of atrazine at Station RA-38 were 3.62 ug/l in 1998 (N=10; maximum of 20.40 ug/l) and 5.11 ug/l in 1999 (N=7; maximum of 27.4 ug/l).
