Assessment Comments
Assessment is based on results of ambient water quality monitoring conducted from 2002-2004 as part of the Rathbun Lake Water Quality Project and on results of biological monitoring conducted from 1999-2002 by the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses were assessed (monitored) as "partially supported" based on (1) results of biological monitoring that suggest impairment of these uses and (2) information in U.S. EPA's May 23, 2006 letter transmitting Iowa's final 2004 Section 303(d) list. The assessment of support of the Class B(LR) uses is based on results of water quality monitoring conducted in 2002, 2003, and 2004 on the South Fork Chariton River near Promise City (station RA-12) 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 is also based results of biological monitoring (fish surveys) conducted by the IDNR Fisheries Bureau from 1999-2002.
EXPLANATION: None of the approximately 34 samples violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 1.0 mg/l) or for pH during the 2002-04 assessment period. Monitoring results for dissolved oxygen showed that only one of 32 samples (3%) violated the Class B(LR) water quality criterion for dissolved oxygen; this violation (3.5 mg/l) occurred on October 16, 2003. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), if 10% or less of samples exceed criteria for conventional parameters (e.g., dissolved oxygen and pH), uses should be assessed as “fully supported”. Although this stream segment has a history of potential problems with low levels of dissolved oxygen, levels of dissolved oxygen over the last six years of monitoring appear to have improved. Based on results of recent monitoring, this stream segment does not currently show a dissolved oxygen-related impairment of its Class B(LR) aquatic life uses.
Similar to results of previous monitoring conducted at this station, results from the 2002-2004 period continued to show high levels of nutrient parameters. While levels of total nitrogen in the South Fork Chariton River and other streams in the Rathbun watershed are relatively low for Iowa, levels of total phosphorus tend to be relatively high. The 2002-2004 mean and median levels for total phosphorus at Station RA-12 were 0.24 mg/l and 0.16 mg/l, respectively (N=31); the mean and median levels of total Kjeldahl nitrogen for this period were 0.6 mg/l and 0.5 mg/l, respectively (N=31). 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 river reach is not designated for Class C drinking water uses, the South Fork Chariton River does flow into Rathbun Reservoir which is used as a source of drinking water for a public water supply. Thus, the seasonal elevation of pesticide levels in this stream and other tributaries of Rathbun Reservoir presents a continuing concern for the full support of the Class C (drinking water) uses designated for the Chariton River immediately downriver from Rathbun Reservoir. Fortunately, however, the levels of atrazine in the South Fork Chariton River during the 2002-04 period do not suggest a serious threat to support of drinking water uses in Rathbun Reservoir. The mean level of atrazine in the 23 samples collected at Station RA-12 during the 2002-04 period (1.4 ug/l) was approximately one-half of the MCL for atrazine of 3 ug/l. The maximum levels of 15.1 ug/l of atrazine during the 2002-2004 period occurred in the sample collected on May 14, 2002.
The assessment was also 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=13) was 25 (poor). The aquatic life use support was assessed (evaluated) as Not Supporting (=NS), 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.
Based on the results of biological monitoring by the IDNR Fisheries Bureau, the Class B(LR) aquatic life uses remain assessed (evaluated) as impaired. Because the previous impairment due to violations of Class B(LR) criteria for dissolved oxygen no longer exists, and because the current impairment of the Class B(LR) uses is based on an “evaluated” assessment, the IDNR assessment/listing methodology indicates that this assessment segment should be moved from IR Category 5a (pollutant impaired) and placed into IR Category 3b (potentially impaired) based on an “evaluated” assessment. In their May 23, 2006 transmittal letter for Iowa’s 2004 Section 303(d) list, however, U.S. EPA Region 7 informed IDNR that biological impairments based on bioassessment data from the IDNR Fisheries Bureau, even though IDNR/UHL bioassessment protocols were not strictly followed, should nonetheless be placed into IR Category 5. Thus, this waterbody will be placed into Category 5b of Iowa’s 2006 Integrated Report and will remain on Iowa’s Section 303(d) list.
