Assessment Comments
Assessment is based on results of (1) results of water quality monitoring conducted at Rathbun Reservoir from 2010-2012 by the U.S. Army Corps of Engineers (ACOE) at the downlake station (RA-3) near the dam of Rathbun Reservoir, (2) results from the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (3) results of the ambient lake monitoring program conducted by the University Hygienic Laboratory (UHL) in 2008, and (4) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Rathbun Reservoir in 2004, (5) an IDNR Fisheries analysis of the influence of water quality conditions on sport fish production at Rathbun Reservoir, and (6) results from the Syngenta voluntary atrazine monitoring program conducted in 2010-2012.
Basis for Assessment
[Note: For the 2006, 2008, 2010, 2012, and 2014 305(b)/303(d) cycle, Rathbun Reservoir was subdivided into the following segments for purposes of developing water quality assessments: main lake basin, South Fork Arm, Chariton Arm, Honey Creek Arm, and Buck Creek Arm. These segments were created to better use the water quality data collected as part of the Rathbun Water Quality Project from monitoring stations located in the main lake basin and in the several arms of the reservoir. Water quality data from the arms of Rathbun Reservoir suggest differing water quality conditions than do data collected from the main lake basin. Thus, developing separate assessments for the main lake basin as well as for the reservoir’s arms represents a more accurate characterization of water quality conditions of the reservoir than did past assessments in which only one assessment was developed for the entire reservoir.]
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported” due to high levels of non-algal turbidity that create aesthetically objectionable conditions. The Class B(WW1) aquatic life uses are assessed as "fully supporting" based on 2008-2012 results of ISU, UHL, Army Corp water quality monitoring and based on an IDNR Fisheries assessment. Fish consumption uses remain assessed as “fully supported” based on results of U.S. EPA/INDR fish contaminant monitoring in 2004. The sources of data for this assessment include (1) results of water quality monitoring conducted at Rathbun Reservoir from 2010 through 2012 by the U.S. Army Corps of Engineers (ACOE) at the downlake station (RA-3) near the dam of Rathbun Reservoir, (2) results from the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (3) results of the ambient lake monitoring program conducted by the University Hygienic Laboratory (UHL) in 2008, (4) results of monitoring from the "Iowa Voluntary Atrazine Monitoring Program" sponsored by Syngenta Crop Protection Inc. in 2010-2012, (5) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Rathbun Reservoir in 2004, and (6) an IDNR Fisheries analysis of the influence of water quality conditions on sport fish production at Rathbun Reservoir.
EXPLANATION: The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" due to high levels of turbidity that violate the narrative water quality standard protecting against aesthetically objectionable conditions. Data collected by the U.S. Army Corps of Engineers at Station RA-3 also suggest that the Class A1 uses be assessed as "partially supported" due to poor water transparency caused by high levels of non-algal turbidity that creates aesthetically objectionable conditions. Using the median values from the approximately 38 samples collected as part of the ACOE, ISU, and UHL surveys from 2008 through 2012, Carlson’s (1977) trophic state indices were 65 for total phosphorus, 49 for chlorophyll-a and 65 for Secchi depth. According to Carlson (1977), the index values for total phosphorus and Secchi depth are in between the eutrophic and hypereutrophic categories, and the index value for chlorophyll-a is at the upper end of the mesotrophic range. These index values suggest very high levels of phosphorus in the water column, very poor water transparency, and extremely low levels of chlorophyll-a and suspended algae in the water column.
The 2008-2012 TSI value for total phosphorus at Rathbun Reservoir (65) is much lower than the corresponding TSIs for Iowa’s other flood control reservoirs (Red Rock Lake (91), Saylorville Lake (71), and Coralville Lake (80)), thus suggesting that Rathbun Reservoir likely has fewer water quality problems related to excess algal production than do these other lakes.
The levels of inorganic suspended solids (ISS) 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 the reduced water clarity. The median level of inorganic suspended solids based on data from the ISU and UHL lake surveys from 2008-2012 at Rathbun Reservoir was 6.7 mg/l; and ranked 93rd of the 134 lakes sampled.
The levels of nuisance (=noxious) algal species (i.e., cyanobacteria) at this lake do not suggest an impairment of Class A1 uses. Data from the ISU and UHL surveys from 2008-2012 suggest that cyanobacteria comprise only 54% of this lake’s summertime phytoplankton community, these data showed that the median cyanobacteria mass at Rathbun Reservoir was 8.7 mg/L, and ranked 18th of the 134 monitored lakes.
Rathbun Reservoir has not been recently sampled for indicator bacteria. Results of monitoring of indicator bacteria at Buck Creek beach, Buck Creek Marina, Island View beach, and South Fork Marina from three dates in 1999 (June 14, July 12, and August 18) showed that levels of indicator bacteria (fecal coliforms) were all below the former Iowa Class A1 water quality criterion of 200 orgs/100 ml. These results suggested “full support” of the Class A1 uses. No additional beach monitoring has been conducted at this lake since 1999.
The Class B(WW-1) (aquatic life) uses are assessed as “fully supported” based on information from the IDNR Fisheries Bureau, results from the ACOE monitoring at station RA-3, and results from the ISU and UHL lake surveys. The ISU, UHL, and ACOE lake surveys show that during 2008-2012 there were no violations of the Class B(WW-1) criterion for ammonia in 28 samples and no violations of the Class B(WW-1) criterion for pH in 28 samples. There were 2 violations of the Class B(WW-1) criterion for dissolved oxygen in 32 samples (6%). Based on IDNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(WW-1) aquatic life uses. These results suggest "full support" of the Class B(WW-1) uses of Rathbun Lake.
In addition, a recent comparison of water quality data to production of the primary sport fish at Rathbun Reservoir (crappie) suggest “full support” of the aquatic life uses of the main lake basin. In 2006, the IDNR Fisheries Bureau summarized approximately 35 years of data on abundance of crappies at Rathbun Reservoir. These data were combined with the available water quality data (707 Secchi depth readings) from that period to determine the spatial and temporal differences in crappie abundance and water quality conditions within Rathbun Reservoir. Results of this analysis showed the following. Compared to the near-dam and mid-lake areas, all tributary arms of the reservoir have relatively poor water quality (measured as water transparency) and relatively poor crappie production. The greatest transparency at all locations occurred recently, in the period from 2000-2006. These improvements in water transparency are attributed to implementation of best management practices in the Rathbun watershed, although the recent improvements may also be related to generally lower rainfall and runoff during recent years. Based on the Secchi depth readings and the analysis of crappie production for the period 1970-2006, the best water clarity and crappie production occurs near the dam and in central portion of the main lake basin where Secchi depths average approximately one meter.
Note: The Rathbun Lake watershed had several years of high rainfall between 2008-2012, which may contribute to the elevated levels of inorganic suspended solids and reduced water clarity observed in some recent years.
However, the somewhat elevated levels of inorganic suspended solids as measured by the ACOE, ISU, and UHL monitoring during the 2010-2012 period does suggest concern that turbidity levels are increasing in the lower reservoir near the dam and that this increase may adversely impact the reservoir's fisheries and other aquatic life.
Levels of atrazine in this segment of Rathbun Reservoir are low and do not suggest a threat to support of the drinking water (Class C) uses of the Chariton River downriver from Rathbun Dam. Results of pesticide monitoring by ACOE and Syngenta from 2010-2012 in the reservoir near the dam (station RA-3) showed no violations of the MCL for atrazine in the 127 samples collected. The average level of nitrate in the 40 samples collected from station RA-3 during 2010-2012 was 1.0 mg/l; the maximum sample value during this period was 1.9 mg/l. These results are similar to those for previous assessment periods. The mean, minimum, and maximum atrazine levels at the reservoir outlet area (station RA-25) during the 2010-2012 period were 0.5 ug/l, 0.09 ug/l, and 2.4 ug/l. Results of ACOE, UHL, and ISU monitoring suggest very low levels of nitrate in Rathbun Reservoir.
Although average levels of atrazine as reported in the ACOE and Syngenta monitoring networks are, and have historically been, well below the MCL at the stations near the dam and at the outlet area (the vicinity of the drinking water intake), the atrazine levels at the ACOE’s uplake stations (RA-7 and RA-8) are occasionally higher, and levels in reservoir tributaries are routinely higher, thus suggesting a continued concern regarding atrazine and the drinking water uses of the Chariton River downriver from Rathbun Dam. Based on approximately eight years of monitoring, the typical pattern for atrazine levels in Rathbun Reservoir and its watershed is occurrence of relatively low levels of atrazine near the dam (well-below the atrazine MCL), with progressively higher levels proceeding uplake and upstream into the lakes tributaries.
Fish consumption uses of the entire reservoir are assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Rathbun Reservoir in 2004. The composite samples of fillets from channel catfish and white crappie had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.031 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of white crappie fillets were as follows: mercury: <0.0181 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2004 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
