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 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 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 (1) results of water quality monitoring conducted from 2002 through 2004 on Jordan Creek near Bethlehem (station RA-37) 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: Although none of the 27 samples collected from monitoring station RA-37 during the 2002-04 assessment period violated Class B(LR) criteria for ammonia-nitrogen, monitoring results for dissolved oxygen and pH show minor violations of Class B(LR) criteria. One of 30 samples collected (3%) at station RA-37 from 2002 through 2004 violated the Class B(LR) water quality criterion for dissolved oxygen: the sample collected on August 13, 2002 contained 4.5 mg/l); this violation occurred just before the stream was reported as “dry” in August 2002. Similarly, one of 30 samples violated the Class B(LR) criterion for low pH: the sample collected on October 19, 2004 had a pH level of 6.3 pH units, and this violates the Class B(LR) criterion of 6.5 pH unites. These isolated violations of the criteria for dissolved oxygen and pH do not suggest an impairment: according to U.S. EPA assessment guidelines, if less than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen and pH, the aquatic life uses should be assessed as "fully supported" (see pgs 3-17 of U.S. EPA 1997b). Although this stream segment has a history of problems with low levels of dissolved oxygen and high levels of ammonia, levels of dissolved oxygen and ammonia over the last six years of monitoring appear to have improved. The violations of Class B(LR) that led to the assessments of impairment occurred in 2000. Since that time, violations of Class B(LR) criteria have been rare. Based on results of recent monitoring, this stream segment does not currently show impairments of the Class B(LR) uses related to either dissolved oxygen or ammonia.
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 Jordan 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-37 were 0.19 mg/l and 0.15 mg/l, respectively (N=27); the mean and median levels of total Kjeldahl nitrogen for this period were 1.1 mg/l and 0.9 mg/l, respectively (N=27). 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 is, 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, Jordan Creek 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 full support of the Class C (drinking water) uses designated for the Chariton River immediately downriver from Rathbun Reservoir. For example, the mean level of atrazine in the 20 samples collected at Station RA-37 from 2002 through 2004 (3.1 ug/l) approximates the atrazine MCL of 3.0 ug/l. The maximum level of atrazine during the 2002-04 period (36.8 ug/l) 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 stations. 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=3) was 29 (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.
Based on the results of biological monitoring by the IDNR Fisheries Bureau, the Class B(LR) aquatic life uses should remain assessed (evaluated) as impaired. Because the previous impairments due to violations of Class B(LR) criteria for dissolved oxygen and ammonia no longer exist, 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. 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 and will remain on Iowa’s Section 303(d) list.
