Assessment Comments
Assessment is based on resutls of monitoring conducted in 2000 as part of the Rathbun Lake Water Quality Project. Biological assessment based on 1999 biocriteria data: Fish IBI= 33 (fair), BM-IBI= 41 (fair).
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses were assessed as "partially supported." EXPLANATION: The previous assessment of support of the Class B(LR) uses was based, in part, on results of DNR stream use assessments conducted in 1991. The results of this assessment are now considered too old (greater than five years) for characterizing current water quality conditions. The current assessment of support of the Class B(LR) uses is based on results of water quality monitoring conducted on Wolf Creek near Chariton (station RA-41) by the U.S. Geological Survey (1998) and by Iowa State University/Limnology (1999) under contract with the U.S. Army Corps of Engineers-Kansas City District 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 pH in the 10 samples analyzed in 1999 or for ammonia-nitrogen in the 21 samples analyzed during 1998 and 1999. (The sample collected on November 10, 1998, however, had a level of ammonia-nitrogen of 3.03 mg/l. Because data for temperature and pH are not available for this sample, values of 10C and 8.0 pH units were estimated from data collected at this station in November 1999. According to the Iowa Water Quality Standards, the chronic Class B(LR) aquatic life criterion at 10C and 8.0 pH units is 3.0 mg/l. Thus, the ammonia-nitrogen level on November 10, 1998 is a potential violation of state aquatic life criteria. According to U.S. EPA guidelines for Section 305(b) reporting (U.S. EPA 1997b, page 3-18), if data are collected monthly or more frequently, one violation of water quality criterion for a toxic contaminant does not suggest an impairment of the aquatic life uses. According to DNR's methodology for Section 305(b) reporting, the occurrence of this potential violation indicates that the Class B(WW) uses should be assessed as "fully supporting / threatened.") Although these results suggest "full support" of the Class B(LR) uses, two violations of the Class B(LR) criterion for dissolved oxygen (5 mg/l) in the nine samples (22%) analyzed in 1999 suggest a potential impairment of these uses. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), "partial support" of beneficial uses is indicated if criteria are exceeded in from 11% to 25% of the samples for conventional parameters (pH, temperature, and dissolved oxygen). Violations of the criterion for dissolved oxygen occurred in the samples collected on June 11, 1999 (4.7 mg/l) and on June 15, 1999 (4.7 mg/l). Both samples were collected relatively early in the morning (between 8:00 and 8:30 AM); however, these early collection times likely were not the primary factor in the low levels of dissolved oxygen. In addition, the levels of dissolved oxygen in the samples collected on August 18 and October 13, 1999, equaled the Class B(LR) aquatic life criterion of 5.0 mg/l. Additional monitoring is needed to better define this potential impairment of the aquatic life uses. 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-41 and at stations on other reservoir tributaries, with the highest levels often associated with storm runoff. Levels of total suspended solids 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, the stream does flow to 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 21 samples collected at Station RA-41 in 1998 and 1999, the five samples that exceeded the atrazine MCL of 3.0 ug/l were collected from late May to mid-July following application of herbicides to farm fields and during months with typically high levels of rainfall. The mean levels of atrazine at Station RA-41 were 4.44 ug/l in 1998 (N=11; maximum of 30.10 ug/l) and 6.47 ug/l in 1999 (N=10; maximum of 41.5 ug/l).
