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 remain assessed (monitored) as "partially supported" due to violations of state water quality criteria for dissolved oxygen. The assessment of support of the Class B(LR) uses is based on results of water quality monitoring conducted from 2000 through 2002 on Honey Creek near Melrose (station RA-40) by Iowa State University under contract with the U.S. Army Corps of Engineers-Kansas City District as part of the Rathbun Water Quality Project.
EXPLANATION: Although none of the 24 samples violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 1.0 mg/l) or for pH, monitoring results for dissolved oxygen suggest a potentially significant water quality problem. Three of the 24 samples (12.5%) collected at station RA-40 from 2000 through 2002 violated the Class B(LR) water quality criterion for dissolved oxygen; all of these violations occurred in the relatively dry year of 2000 and are as follows: June 13: 2.9 mg/l; June 27: 1.9 mg/l at RA-40; August 15: 4.7 mg/l. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), if criteria for conventional parameters (e.g., pH, dissolved oxygen and temperature) are exceeded in from 11% to 25% of the samples "partial support" of beneficial uses is indicated. Thus, the violation frequency of the Class B(LR) criterion for dissolved oxygen (12.5%) suggests "partial support" of the Class B(LR) aquatic life uses.
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 Honey Creek and other streams in the Rathbun watershed are relatively low for Iowa, levels of total phosphorus tend to be relatively high. The 2000-2002 mean and median levels for total phosphorus from Station RA-40 (N=24) were 0.23 and 0.22 mg/l, respectively; the corresponding mean and median levels of total nitrogen for this period (N=24) were 1.5 mg/l and 1.0 mg/l, respectively. 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, 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, Honey Creek does flow into Rathbun Reservoir which is designated for Class C uses. Thus, the seasonal elevation of pesticides levels in this stream and other tributaries of Rathbun Reservoir presents a continuing threat to full support of the Class C (drinking water) uses designated for Rathbun Reservoir. In contrast to mean levels of atrazine above the MCL in other tributaries of Rathbun Reservoir, the mean level of atrazine in the 24 samples collected at station RA-40 from 2000 through 2002 (1.7 ug/l) was below the atrazine MCL of 3 ug/l.
