Assessment Comments
Assessment is based on results of monitoring conducted from 2000-02 as part of the Rathbun Lake Water Quality Project.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses are assessed (monitored) as "fully supported / threatened" due to violations of state water quality criteria for dissolved oxygen and ammonia-nitrogen. The assessment of support of these uses is based on results of water quality monitoring conducted from 2000 through 2002 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. This assessment differs from that for the 2002 reporting cycle (“partial support”) due to lowered percentages of violations in the 2000-02 period versus the 2000-01 period. The very dry year of 2000—and the violations of state water quality standards that resulted from these dry conditions—remains the primary factor affecting the assessments of the tributaries of Rathbun reservoir monitoring as part of the Rathbun Water Quality Project.
EXPLANATION: One of the 25 samples collected during the 2000-2002 assessment period violated Class B(LR) criteria for ammonia-nitrogen and pH: the sample collected on June 13, 2000 had a pH of 9.1 and an ammonia concentration of 0.28 mg/l; this level exceeded the pH-dependent chronic criterion of 0.21 mg/l. Monitoring results for dissolved oxygen suggest a potential water quality problem but, at least for the 2000-2002 period, do not suggest impairment of the aquatic life uses. Two of the 25 samples collected (8%) at station RA-38 from 2000 through 2002 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 25 samples collected (5%) from 2000 through 2002 violated the Class B(LR) criterion for high pH (9.0 units); as stated above, this violation (9.1 units) occurred on June 13, 2000. According to U.S. EPA guidelines for Section 305(b) reporting (U.S. EPA 1997b, page 3-18), uses are “fully supported” if chronic criteria for a toxic pollutant (for example, ammonia) are exceeded in up to one sample from a dataset based on monthly monitoring. IDNR assessment guidelines, however, specify that one violation of a toxic parameter (e.g., ammonia) indicates a “threat” to full support of the designated uses. Also, 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 (4%) and dissolved oxygen (8%) do 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. Similar to results of monitoring conducted in 1998 and 1999, results from the 2000-2002 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 2000-2002 mean and median levels for total phosphorus at Station RA-38 were 0.15 mg/l and 0.11 mg/l, respectively (N=25); the mean and median levels of total nitrogen for this period were 1.7 mg/l and 1.2 mg/l, respectively (N=25). The Iowa Water Quality Standards do 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, however, continues to participate 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 25 samples collected at Station RA-38 from 2000 through 2002 (3.7 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.
