Assessment Comments
Assessment is based on results of monitoring conducted from 2000-02 as part of the Rathbun Lake Water Quality Project and on 1999-2002 Fisheries data: Fish IBI average (n=7) = 28 (fair).
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses were assessed (monitored) as "fully supported / threatened" due to several 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 Jackson Creek near Bridgeport (station RA-39) 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. This assessment differs from that for the 2002 reporting cycle (“partial support”) due to the lowered percentage of violation of the dissolved oxygen criterion 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—is the primary factor affecting the assessments of the tributaries of Rathbun reservoir monitoring as part of the Rathbun Water Quality Project.
EXPLANATION: Although none of the approximately 32 samples violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 3.1 mg/l) or for pH, monitoring results for dissolved oxygen suggest a threat to full support of the Class B(LR) uses. Three of the 30 samples collected (10%) at station RA-39 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: 4.4 mg/l on May 16, 2.8 mg/l on June 13, and 2.0 mg/l on June 27. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), if 10% of less of samples exceed criteria for conventional parameters (e.g., dissolved oxygen and pH), uses are “fully supported.” Although the violation frequency of the Class B(LR) criterion for dissolved oxygen (10%) suggests "full support" of the Class B(LR) aquatic life uses, the history of episodes of low dissolved oxygen in this stream suggests that these uses should be assessed as “threatened.” 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 Jackson 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 at Station RA-39 were 0.33 mg/l and 0.32 mg/l, respectively (N=32); the mean and median levels of total nitrogen for this period were 1.6 mg/l and 1.3 mg/l, respectively (N=32). 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, Jackson 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 32 samples collected at Station RA-39 from 2000 through 2002 (2.0 ug/l) was below the atrazine MCL of 3.0 ug/l. The maximum levels of 17.6 ug/l of atrazine occurred in the sample collected on June 1, 2001.
The assessment was partially based 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 Fisheries sampling data. The biological metrics are based on the numbers and types of 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). The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 1999-2002 evaluated Fish IBI average score (n=7) was 28 (fair). The aquatic life use support was assessed (evaluated) as Partially Supporting (=PS), based on a comparison of the F-IBI score with biological assessment criteria established for previous Section 305(b) reports. The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.
