Assessment Comments
Assessment is based on results of (1) water quality monitoring conducted by the U.S. Army Corps of Engineers from 2000-02 as part of the Rathbun Water Quality Project, (2) the statewide survey of Iowa lakes conducted from 2000-02 by Iowa State University, (2) information on plankton communities at Iowa lakes in 2000, (3) the 2000, 2001, and 2002 results from the "Iowa Voluntary Atrazine Monitoring Program" sponsored by Syngenta Crop Protection Inc.
Basis for Assessment
For the 2004 report: SUMMARY: Class A (primary contact recreation) uses are assessed (monitored) as "fully supported / threatened" due to levels of non-algal (inorganic) turbidity that reduce water transparency. The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported / threatened" due to the occurrence (although infrequent) of low dissolved oxygen at up-lake monitoring stations. The Class C drinking water uses are assessed (monitored) as "fully supported / threatened" due to occasional violations of the atrazine MCL at up-lake monitoring stations and the frequent violations of this MCL in lake tributaries. Fish consumption uses are "not assessed" due to the lack of recent fish contaminant monitoring at this lake. The assessments of support of the Class A, 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), 2001 (3 events), and 2002 (6 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 2000, 2001, and 2002. These assessments were also based on results from the statewide survey of Iowa lakes conducted from 2000 through 2002 by Iowa State University (ISU) as well as information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002). The assessment of support of Class C uses was also based on the 2000, 2001, and -2002 results from the "Iowa Voluntary Atrazine Monitoring Program" sponsored by Syngenta Crop Protection Inc (formerly Novartis Crop Protection, Inc.).
EXPLANATION: The Class A (primary contact recreation) uses are assessed as "fully supported / threatened" due to levels of non-algal (inorganic) turbidity that reduce water transparency. Results from the ISU statewide survey of Iowa lakes from 2000-02 suggest a threat to full support of the Class A uses of Rathbun Reservoir. Data from this study collected near the dam of Rathbun Lake suggest that moderately high levels of non-algal turbidity may affect the Class A uses of this lake. Using the median values from this survey from 2000 through 2002 (approximately nine samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 65, 44, and 61, respectively. According to Carlson (1977), the index value for total phosphorus places this reservoir in the middle range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is in the middle range of mesotrophic lakes, and the value for secchi depth is in the lower range between eutrophic and hyper-eutrophic lakes. These index values suggest the following: (1) only moderately high levels of phosphorus in the water column, (2) extremely low (and much less than expected) production of suspended algae, and (3) marginally good water transparency. The TSI value for total phosphorus at Rathbun Lake (65) is much lower than for Iowa’s other flood control reservoirs (Red Rock Lake (78), Saylorville Lake (76), and especially Coralville Lake (82)), thus suggesting that Rathbun Lake likely has fewer nutrient-related water quality problems (e.g., excess algal production and low dissolved oxygen) than do these other lakes. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth, such as occur at Rathbun Lake, indicate that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The ISU lake data suggest that algal production at Rathbun Lake is most likely limited by moderately high levels of inorganic suspended solids; neither nitrogen limitation nor zooplankton grazing appear to influence algal production. Based on median values from ISU sampling from 2000 through 2002, the ratio of total nitrogen to total phosphorus for Rathbun Lake is 18. This TN:TP ratio suggests that algal production at this lake is likely limited by phosphorus versus nitrogen availability. In addition, data from Downing et al. (2002) show relatively small populations of zooplankton species (Cladocerans) at this lake that graze on algae. The summer 2000 average mass of these zooplankton grazers (2.4 mg/l) was the 20th lowest of the 131 lakes sampled. The levels of inorganic suspended solids at this lake, however, are moderately high and suggest the possibility that inorganic turbidity (1) attenuates light and limits production of suspended algae and (2) contributes to reduced water transparency that threatens full support of the Class A uses of this lake. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2002 was 4.8 mg/l; the median level at Rathbun Lake was 4.0 mg/l; this is the 51st highest ISS median of the 131 lakes sampled.
Results from ACOE monitoring near the dam of Rathbun Lake (Station RA3) were also used to calculate TSI values for the 2000-02 period. Using the median values from this survey from 2000 through 2002 (approximately 18 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 63, 42, and 66, respectively. Similar to the TSI values based on ISU monitoring, the index value for total phosphorus places this reservoir in the middle range between eutrophic and hyper-eutrophic lakes, and the index value for chlorophyll-a is in the lower range of mesotrophic lakes. The TSI value for secchi depth based on ACOE monitoring, however, is higher (66) than that based on ISU monitoring (61) and suggests a potential impairment of Class A uses due to moderately poor water transparency. ACOE monitoring did not include analysis for inorganic suspended solids; however, because the ACOE monitoring showed similarly extremely low levels of chlorophyll-a, the reductions in water transparency at this lake are likely due to inorganic turbidity.
Blooms of bluegreen algae do not appear to be a problem at Rathbun Lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise a very small portion of the summertime phytoplankton community of this lake. The average mass of bluegreen algae in summer 2000 at this lake (0.97 mg/l) was the 12th lowest of the 131 lakes sampled. These conditions do not suggest any threats or impairments due to presence of nuisance aquatic species (i.e., bluegreen algae).
Results of beach monitoring initiated during the 1999 sampling year ( Results of monitoring of indicator bacteria 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. These results suggest “full support” of the Class A uses. No additional beach monitoring has been conducted at this lake.
The Class B(WW) aquatic life uses were assessed as "fully supported / threatened" based on results of ACOE monitoring and ISU monitoring from 2000 through 2002 that showed no violations of Iowa Class B(WW) water quality criteria for pH and dissolved oxygen at the ISU monitoring station near the dam or at the ACOE 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 12 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" due to violations of the MCL for atrazine. Results of pesticide monitoring by ISU/ACOE from 2000, 2001, and 2002 near the dam (station RA-3) showed no violations of the MCL for atrazine in the 18 samples collected. The mean, minimum, and maximum atrazine levels at station RA-3 during the 2000-2002 period were 1.1 ug/l, 0.38 ug/l, and 3.4 ug/l, respectively. [Note: Rounding is required when comparing levels of atrazine to the MCL. Because the MCL, as published by U.S. EPA, is 0.003 mg/l, any values compared to the MCL must be in the proper MCL units (mg/l) and compared at the same number of significant figures. Thus, the maximum atrazine concentration of 3.4 ug/l at Station RA-3 rounds to 0.003 mg/l and thus, according to U.S. EPA protocol, does not exceed the MCL of 3 ug/l.] These results are similar to those for the 1998-1999 and 2000-2001 periods (see assessments for the 2000 and 2002 reports). One of 18 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 1.15 ug/l, 0.52 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 1.8/12.4 ug/l at RA-7 (Chariton arm) and 3.2/19.5 ug/l at RA-8 (South Fork arm).
Results from Syngenta's "Iowa Voluntary Atrazine Monitoring Program" also 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 2000, 2001, and 2002 were far below the MCL of 3.0 ug/l. The time-weighted mean of atrazine in 2000 (N=30) was 0.89 ug/l, in 2001 (N=30) was 0.96 ug/l, and in 2002 (N=16) was 1.1 ug/l. None of the combined 46 samples collected in 2000 and 2002 exceeded the MCL of 3.0 ug/l; one of the 30 samples collected in 2001, however, exceeded the MCL for atrazine (maximum value = 5.0 ug/l). Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, but a level in one or more samples is above the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported / threatened." None of the 46 moving annual averages for atrazine for the years 2000, 2001, and 2002 at Rathbun Lake exceeded the MCL (maximum average = 1.2 ug/l). The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. The results of the ISU survey from 2000-02 show that nitrate levels are very low at this lake (maximum value = 1.7 mg/l; median = 0.5 mg/l), Although average levels of atrazine as reported in the ACOE and Syngenta monitoring networks 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 ACOE’s 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). These data are now more than 10 years old and are thus not appropriate for characterizing current conditions. Fish contaminant monitoring is, however, planned for this lake in 2004 as part of the U.S. EPA/IDNR fish contaminant (RAFT) monitoring program.
