Assessment Comments
Assessment is based on results of (1) water quality monitoring conducted from 2000-02 as part of the Rathbun Lake Water Quality Project; (2) IDNR/UHL biological (biocriteria) monitoring conducted in 1999, and (3) 1999-2002 Fisheries data: Fish IBI average (n=10) = 26 (fair).
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 and pH. The assessment of support of the Class B(LR) uses is based on (1) results of water quality monitoring conducted from 2000 through 2002 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, (2) results of IDNR/UHL biocriteria monitoring in 1999, and (3) results of IDNR Fisheries fish sampling conducted from 1999-2002.
EXPLANATION: Although none of the 29 samples collected during the 2000-2002 period violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 0.48 mg/l), monitoring results for other parameters suggest some potentially significant water quality problems. Seven of the 29 samples collected (24%) at station RA-41 from 2000 through 2002 violated the Class B(LR) water quality criterion for dissolved oxygen; these violations occurred in the relatively dry years of 2000 and 2002 and are as follows: 4.5 mg/l on May 16, 2000; 2.6 mg/l on June 13, 2000; 2.0 mg/l on June 27, 2000; 4.8 mg/l on August 15, 2000; 4.4 mg/l on September 12, 2000; 4.5 mg/l on July 7, 2002; and 1.9 mg/l on December 11, 2002. In addition, seven of the 29 samples collected (24%) from 2000 through 2002 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 and IDNR Fisheries 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 and on data collected in 1999-2002 as part of a DNR Fisheries stream sampling project: Chariton research station. 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 1999-2002 evaluated Fish IBI average score (n=10) was 26 (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 chemical water quality monitoring from 2000 through 2002. Thus, accounting for the assessment based on chemical/physical water quality monitoring ("not supported") and the results of biological monitoring ("fully supported / threatened"), and accounting for the sampling dates of the two monitoring efforts (chemical/physical: 2000-2002; 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 2000-2002 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 0.25 mg/l and 0.16 mg/l, respectively (N=29); the mean and median levels of total nitrogen for this period were 1.7 mg/l and 1.1 mg/l. 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 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 29 samples collected at Station RA-41 from 2000 through 2002 (4.2 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.
