Assessment Comments
Assessment is based on results of monitoring conducted from 2002-04 as part of the Rathbun Lake Water Quality Project and on results of biological monitoring conducted by the IDNR Fisheries Bureau from 1999-2002.
Basis for Assessment
SUMMARY: The Class B(LR) aquatic life uses were assessed (monitored) as "partially supported” based on (1) results of biological monitoring by the IDNR Fisheries Bureau 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 (1) results of water quality monitoring conducted from 2002 through 2004 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 and (2) results of biological monitoring (fish surveys) conducted by the IDNR Fisheries Bureau from 1999-2002.
EXPLANATION: None of the 31 samples collected during the 2002-2004 period violated Class B(LR) criteria for ammonia-nitrogen (maximum level = 1.4 mg/l) or for dissolved oxygen (minimum level = 5.4 mg/l). Only one of the 34 samples analyzed for pH exceeded the low-end pH criterion of 6.5 pH units: the sample collected on October 19, 2004 had a pH of 6.0 pH units. 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 are “fully supported”.
Similar to results of previous monitoring, results from the 2002-2004 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 2002-2004 mean and median levels for total phosphorus at Station RA-39 were 0.34 mg/l and 0.26 mg/l, respectively (N=31); the mean and median levels of total Kjeldahl nitrogen for this period were 1.04 mg/l and 0.93 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 pesticide 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 26 samples collected at Station RA-39 from 2002 through 2004 (1.2 ug/l) was below the atrazine MCL of 3 ug/l. The maximum level of atrazine during the 2002-2004 period was 12.5 ug/l of atrazine in the sample collected on May 14, 2002.
The assessment was partially based on data collected in 1999-2002 as part of a DNR Fisheries stream sampling project: Chariton research station. Despite results of chemical water quality monitoring that show “full support” of the Class B(LR) uses, results of biological monitoring suggest only “partial support” of these uses. 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 (FIBI). 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 FIBI 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 FIBI score with biological impairment criteria (BIC) established for previous Section 305(b) reports. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The FIBI BIC for this ecoregion is 33.
Based on the results of biological monitoring by the IDNR Fisheries Bureau, the Class B(LR) aquatic life uses of this waterbody should be assessed (evaluated) as impaired. 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 placed into IR Category 3b (potentially impaired and in need of further investigation). That is, the biological data from this segment lack the necessary documentation or metadata to determine whether the data meet quality and completeness criteria required of “monitored” [high-confidence] assessments. Thus, FIBI scores calculated from IDNR Fisheries Bureau sampling data collected in this segment provide useful, but less than conclusive evidence of impairment. With respect to FIBI sample results, the following criteria are considered: sampling conditions; sampling gear; crew size; and length of stream sampled. One or more of these sample descriptors are not available for the Fisheries Bureau data from 1999-2002. Therefore, IDNR considers the aquatic life use impairments indicated by these data as “evaluated” assessments and thus as not appropriate for Section 303(d) listing (Category 5). 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.
