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." The assessment of support of the Class B(LR) uses is based on (1) results of water quality monitoring conducted in 2000 and 2001 on Wolf Creek near Chariton (station RA-41) 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 and (2) results of IDNR/UHL biocriteria monitoring in 1999. The results of the ISU/ACOE water quality monitoring are summarized in the "Rathbun Lake Water Quality Reports" for 2000 and 2001. EXPLANATION: Although none of the 20 samples violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 0.48 mg/l), monitoring results for other paramters suggest some potentially significant water quality problems. Five of the 20 samples collected (25%) at station RA-41 in 2000 and 2001 violated the Class B(LR) water quality criterion for dissolved oxygen; all of these violations occurred in the relativey dry year of 2000 and are as follows: 4.5 mg/l on May 16, 2.6 mg/l on June 13, 2.0 mg/l on June 27, 4.8 mg/l on August 15, and 4.4 mg/l on September 12. In addition, seven of the 20 samples collected (35%) in 2000 and 2001 violated the Class B(LR) criterion for low pH (6.5 units); all of these violations occurred from November 2000 to November 2001; violations ranged from 6.3 to 5.9 pH units. 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 (e.g., pH, temperature, or dissolved oxygen), and "nonsupport" is indicated if criteria for conventional parameters are exceeded in more than 25% of the samples. While the violation frequency of Class B(LR) criterion for dissolved oxygen (25%) is on the boundary between "partial support" and "nonsupport," the violation frequency of the criterion for pH (35%) suggests "nonsupport" of the aquatic life uses.
Results of IDNR/UHL biocriteria monitoring, however, suggest that the Class B(LR) uses are "fully supported / threatened." This biological assessment was based on data collected in 1999 as part of the DNR/UHL stream biocriteria project. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (F-IBI) and a benthic macroinvertebrate index (BM-IBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 1999 Fish IBI score was 33 (fair) and the BM-IBI score was 41(fair). The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001. Biological monitoring is better able to reflect cumulative impacts of water quality over time and thus is believed to more accurately represent water quality conditions of this segment of Wolf Creek than do results of chemcial water quality monitoring in 2000 and 2001. Thus, accounting for the assessment based on chemical/physical water quality monitoring ("not supported") and the results of biological montioring ("fully supported / threatened"), and accounting for the sampling dates of the two monitoring efforts (chemcial/physical: 2000-2001; biological: 1999), the Class B(LR) aquatic life uses for Wolf Creek are assessed as "partially supported."
Although results of chemical/physical monitoring during the 1998-1999 period suggested problems with low dissolved oxygen, this monitoring did not suggest problems with low pH (see assessment for the 2000 report above). 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 Wolf Creek 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 of total phosphorus at Station RA-41 were 0.23 mg/l and 0.19 mg/l, respectively (N=20); the mean and median levels of total nitrogen for this period were 1.9 mg/l and 1.2 mg/l. 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 phosporus, total nitrogen, chlorophyll, and some measure of turbidity.
Although this river reach is not designated for Class C drinking water uses, Wolf Creek does flow into Rathbun Reservoir which is designated for Class C uses. The seasonal elevation of pesticides levels in the Chariton River 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-41 in 2000 and 2001 (3.7 ug/l) exceeded the atrazine MCL of 3.0 ug/l; the maximum levels of 37.4 ug/l of atrazine occurred in the sample collected on June 1, 2001.
