Assessment Comments
Assessment is based on results of water quality monitoring conducted by the U.S. Army Corps of Engineers in 2000-01 as part of the Rathbun Water Quality Project.
Basis for Assessment
SUMMARY: Class A (primary contact recreation) uses remain assessed as "fully supported." The Class B(WW) aquatic life uses are assessed as "fully supported / threatened." The Class C drinking water uses were assessed as "fully supported / threatened." Fish consumption uses are "not assessed." The assessments of support of the Class B(WW) and Class C uses were based on results of approximately 10 water quality monitoring events conducted at Rathbun Lake in 2000 (7 events) and 2001 (3 events) by the U.S. Army Corps of Engineers at the following stations in Rathbun Lake: (1) downlake near dam (RA-3), (2) 10 miles uplake from dam in the Chariton arm (RA-7), (3) 10.4 miles uplake from dam in the South Fork arm (RA-8),and (4) Honey Creek arm (RA-25). The results of this monitoring are summarized in the "Rathbun Lake Water Quality Reports" for 1999, 2000, and 2001. In addition, the 2000-2001 results from the "Iowa Voluntary Atrazine Monitoring Program" sponsored by Syngenta Crop Protection Inc (formerly Novartis Crop Protection, Inc.) were used to assess support of the Class C uses. EXPLANATION: The Class A (primary contact recreation) uses remain assessed as "fully supported" based on results of beach monitoring initiated during the 1999 sampling year (see assessment for the 2000 report above). Results of monitoring at Buck Creek beach, Buck Creek Marina, Island View beach, and South Fork Marina on three dates in 1999 (June 14, July 12, and August 18) showed that levels of indicator bacteria (fecal coliforms) were all below the Iowa Class A water quality criterion of 200 orgs/100 ml. Thus, the Class A uses were assessed as "fully supported." The Class B(WW) aquatic life uses were assessed as "fully supported / threatened" based on results of ACOE monitoring in 2000 and 2001 that showed no violations of Iowa Class B(WW) water quality criteria for pH and dissolved oxygen at monitoring stations near the dam (RA-3), in the Chariton Arm (RA-7), and in the Honey Creek arm (RA-25). The occurrence, however, of violations of the Class B(WW) criterion for dissolved oxygen in two of 9 surface samples from the South Fork arm station (RA-8) on July 21, 2000 (4.9 mg/l) and August 16, 2000 (4.1 mg/l), however, suggests a threat to the full support of these uses. The Class C drinking water uses remain assessed as "fully supported / threatened." Results of pesticide monitoring by ISU/ACOE in 2000 and 2001 near the dam (station RA-3) showed no violations of the MCL for atrazine in the 12 samples collected. The mean, minimum, and maximum atrazine levels at station RA-3 during the 2000-2001 biennial period were 1.0 ug/l, 0.47 ug/l, and 1.8 ug/l, respectively. These results are similar to those for the 1998-1999 biennial period (see assessment for the 2000 report above). One of 12 samples collected at Station RA-28 (reservoir outlet) exceeded the atrazine MCL (3.9 ug/l on June 13, 2001. The mean, minimum, and maximum atrazine levels at the outlet area during the biennial period were 1.05 ug/l, 0.50 ug/l, and 3.9 ug/l. Levels of atrazine during the 2000-2001 period did occasionally exceed the atrazine MCL at the uplake stations (RA-7 and RA-8) with mean/maximum levels of 2.0/12.4 ug/l at RA-7 (Chariton arm) and 2.5/13.2 ug/l at RA-8 (South Fork arm). In addition, results from Syngenta's "Iowa Voluntary Atrazine Monitoring Program" show relatively low levels of atrazine in Rathbun Reservoir. This monitoring showed that the time-weighted mean levels of atrazine in samples collected from Rathbun Reservoir in calendar years 1999, 2000 and 2001 were far below the MCL of 3.0 ug/l. The time-weighted mean of atrazine in 1999 (N=30) was 0.05 ug/l, in 2000 (N=29) was 0.89 ug/l, and in 2001 (N=30) was 0.96 ug/l. None of the combined 59 samples collected in 1999 and 2000 exceeded the MCL of 3.0 ug/l with a maximum value of 1.2 ug/l; one of the 30 samples collected in 2001 exceeded the MCL for atrazine (maximum value = 5.0 ug/l). No samples collected by ISU/ACOE from in-lake stations during the 2000-2001 period contained levels of cyanazine above the 1.0 ug/l MCLG. Although levels of atrazine were well below the MCL at the stations near the dam and at the outlet area (the vicinity of the drinking water intake), the occasionally higher levels at the uplake stations, and the routinely higher levels in reservoir tributaries, suggests a continued threat to the full support of the drinking water uses of this reservoir. Fish consumption uses were considered "not assessed" due to the lack of recent fish tissue monitoring data for the reservoir. The most recent fish tissue monitoring was conducted in 1990 (see assessment for the 1998 and 2000 reports above).
