Rathbun Reservoir - Main Basin IA 05-CHA-1309

Assessment Comments

Assessment is based on results of water quality monitoring conducted by the U.S. Army Corps of Engineers.

Basis for Assessment

Summary:  Class A (primary contact recreation) uses were assessed as "fully supported."  The Class B(WW) aquatic life uses were assessed as "fully supported / threatened."  The Class C drinking water uses were assessed as "fully supported / threatened."  Fish consumption uses were "not assessed."  EXPLANATION:  The assessments of support of the Class A, Class B(WW), and Class C uses were based on results of approximately 18 water quality monitoring events conducted at Rathbun Lake in 1998 (11 events) and 1999 (7 events) by the U.S.  Army Corps of Engineers at the following stations in Rathbun Lake:  (1) outlet (RA-28), (2) Honey Creek arm (RA-25), (3) downlake near dam (RA-3), (4) 10 miles uplake from dam in the Chariton arm (RA-7), and (5) 10.4 miles uplake from dam in the South Fork arm (RA-8).   The results of this monitoring are summarized in the "Rathbun Lake Water Quality Reports" for 1998 and 1999 (see ACOE 1999 and Kirsh and Leonard 2000).   The Class A (primary contact recreation) uses were assessed as "fully supported" based on results of beach monitoring initiated during the 1999 sampling year.   Results of monitoring at Buck Creek beach, Buck Creek Marina, Island View beach, and South Fork Marina on three dates (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 that showed no violations of Iowa Class B(WW) water quality criteria for pH and dissolved oxygen.   The occurrence of a violation of the Class B(WW) criterion for dissolved oxygen at the South Fork arm station (RA-8) on June 15, 1999 (3.6 mg/l in surface samples), 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 in 1998 and 1999 near the dam (station RA-3) and at the lake outlet area (RA-28) showed no violations of the MCL for atrazine.   The mean, minimum, and maximum atrazine levels at station RA-3 were 1.2 ug/l, 0.73 ug/l, and 1.71 ug/l in 1998 and 0.86 ug/l, 0.52 ug/l, and 1.31 ug/l in 1999.   The mean, minimum, and maximum atrazine levels at the outlet area (station RA-28) were 1.15 ug/l, 0.90 ug/l, and 1.72 ug/l in 1998 and 0.85 ug/l, 0.51 ug/l, and 1.50 ug/l in 1999.   Levels of atrazine did exceed the MCL at the uplake stations (RA-7 and RA-8) in May and June 1998, with mean/maximum levels of 2.14/12.60 ug/l at RA-7 (Chariton arm) and 3.54/16.7 ug/l at RA-8 (South Fork arm).   All samples were below the MCL at these station in 1999 (maximum levels:  RA-7:  1.00 ug/l; RA-8:  1.91 ug/l).   Levels of cyanazine at the up-lake stations exhibited a similar pattern to atrazine, with maximum levels above the 1.0 ug/l MCLG in 1998 but not in 1999.   The mean/minimum/maximum levels of cyanazine at station RA-7 were 0.27/0.05/1.05 ug/ in 1998 and 0.07/0.04/0.18 ug/l in 1999.   The mean/minimum/maximum levels of cyanazine at station RA-8 were 0.30/0.05/1.30 ug/ in 1998 and 0.06/0.04/0.20 ug/l in 1999.   No samples exceeded the cyanazine MCLG at the station near the dam (RA-3) or at the outflow area (RA-28) during either 1998 or 1999.  Although levels of atrazine were 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 (at times, over 50 ug/l-in reservoir tributaries), suggest a continued threat to the full support of the drinking water uses of this reservoir.   In addition to the Army Corps of Engineer monitoring for pesticides, data on atrazine were collected in 1998 as part of the Novartis "Iowa Voluntary Atrazine Monitoring Program."  This monitoring showed that the time-weighted mean level of atrazine in samples collected from the Rathbun raw water source from January to December 1998 (0.9 ug/l, N=31, maximum=1.6 ug/l) was well below the MCL of 3.0 ug/l.   Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, and levels in all samples are below the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported."  Due, however, to the relatively high levels of atrazine in tributaries to Rathbun Reservoir, the Class C (drinking water) uses were assessed as "fully supported / threatened."  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 report above).