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 are assessed as "partially supported." The assessment of support of these uses is based on results of water quality monitoring conducted in 2000 and 2001 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 results of this monitoring are summarized in the "Rathbun Lake Water Quality Reports" for 2000 and 2001. EXPLANATION: Although none of the 19 samples violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 3.1 mg/l), monitoring results for dissolved oxygen suggest a potential water quality problem. Two of the 19 samples collected (10.5%) at station RA-38 in 2000 and 2001 violated the Class B(LR) water quality criterion for dissolved oxygen; both violations occurred in the relatively dry year of 2000 and are as follows: 3.1 mg/l on June 13 and 2.1 mg/l on June 27. In addition, one of the 19 samples collected (5%) in 2000 and 2001 violated the Class B(LR) criterion for high pH (9.0 units); this violation (9.1 units) occurred on June 13, 2000. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), the percentage of violations for pH at this station (5%) does not suggest a water quality impairment (the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated). According to these same guidelines, however, "partial support" of beneficial uses is indicated if criteria are exceeded in from 11% to 25% of the samples for conventional parameters (e.g., pH, temperature, or dissolved oxygen). Because the violation frequency of the Class B(LR) criterion for dissolved oxygen (10.5%) slightly exceeds this 10% guideline, "partial support" of the Class B(LR) aquatic life uses in indicated. Similar to results of monitoring conducted in 1998 and 1999, results from the 2000-2001 period continued to show high levels of nutrient parameters. While levels of total nitrogen in Walker Branch and other streams in the Rathbun watershed are relatively low for Iowa, levels of phosphorus tend to be relatively high. The 2000-2001 mean and median levels for total phosphorus at Station RA-38 were 0.18 mg/l and 0.14 mg/l, respectively (N=19); the mean and median levels of total nitrogen for this period were 2.0 mg/l and 1.4 mg/l, respectively (N=19). 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). The Iowa DNR is, however, participating in a nationwide program, led by U.S. EPA, for states to develop and adopt surface water criteria for total phosphorus, total nitrogen, chlorophyll, and some measure of turbidity. Although this stream is not designated for Class C drinking water uses, Walker Branch does flow into Rathbun Reservoir which is designated for Class C uses. Thus, the seasonal elevation of pesticides levels in Walker Branch and other tributaries of Rathbun Reservoir presents a continuing threat to full support of the Class C (drinking water) uses designated for Rathbun Reservoir. For example, the mean level of atrazine in the 20 samples collected at Station RA-38 in 2000 and 2001 (3.5 ug/l) exceeded the atrazine MCL of 3.0 ug/l; the maximum levels of 32.3 ug/l of atrazine occurred in the sample collected on June 13, 2000.
