Assessment Comments
Assessment is based on results of (1) results of approximately 16 water quality monitoring events conducted at Rathbun Reservoir during the years 2004 through 2006 by the U.S. Army Corps of Engineers (ACOE) at the downlake station (RA-3) near the dam of Rathbun Reservoir (the results of this monitoring are summarized in the "Rathbun Reservoir Water Quality Reports" for 2004, 2005, and 2006), (2) results from the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the ambient lake monitoring program conducted by the University Hygienic Laboratory (UHL) from 2005 through 2006, (4) results of monitoring from the "Iowa Voluntary Atrazine Monitoring Program" sponsored by Syngenta Crop Protection Inc. in 2004, 2005, and 2006, (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.
Basis for Assessment
[Note: For the 2006 and 2008 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 which developed only one assessment for the entire reservoir.]
SUMMARY: The Class A1 (primary contact recreation), and Class B(WW-1) (aquatic life) uses are assessed (monitored) as "fully supported” based on water quality monitoring from 2002 through 2006. The Class C (drinking water) uses are assessed (monitored) as “fully supported” based on pesticide and nitrate monitoring from 2004 through 2006. Fish consumption uses are 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 approximately 16 water quality monitoring events conducted at Rathbun Reservoir during the years 2004 through 2006 by the U.S. Army Corps of Engineers (ACOE) at the downlake station (RA-3) near the dam of Rathbun Reservoir (the results of this monitoring are summarized in the "Rathbun Reservoir Water Quality Reports" for 2004, 2005, and 2006), (2) results from the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the ambient lake monitoring program conducted by the University Hygienic Laboratory (UHL) from 2005 through 2006, (4) results of monitoring from the "Iowa Voluntary Atrazine Monitoring Program" sponsored by Syngenta Crop Protection Inc. in 2004, 2005, and 2006, (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 "fully supported." Using the median values from the ISU and UHL lake surveys from 2002 through 2006 (approximately 27 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and Secchi depth are 57, 55, and 61, respectively. According to Carlson (1977), the index value for total phosphorus places this reservoir in the upper range of eutrophic lakes; the index value for chlorophyll-a is in the middle range of eutrophic lakes, and the value for Secchi depth is in the lower range between eutrophic and hypereutrophic lakes. These index values suggest relatively low levels of chlorophyll a and suspended algae in the water, good water transparency and relatively low levels of phosphorus in the water column. The 2002-2006 ISU and UHL TSI value for total phosphorus at Rathbun Reservoir (57) is much lower than the corresponding TSIs for Iowa’s other flood control reservoirs (Red Rock Lake (77), Saylorville Lake (73), and especially Coralville Lake (81)), thus suggesting that Rathbun Reservoir likely has fewer water quality problems related to excess algal production than do these other lakes.
Data collected by the U.S. Army Corps of Engineers at Station RA-3 also suggest relatively good water quality and “full support” of the Class A1 uses of main basin of Rathbun Reservoir. Using the median values from the approximately 16 samples collected as part of the ACOE survey from 2004 through 2006, Carlson’s (1977) trophic state indices are 63 for total phosphorus, 60 for chlorophyll-a and 62 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 at the top of the eutrophic range. These index values suggest moderately high levels of phosphorus in the water column, moderately poor water transparency, and moderate levels of chlorophyll-a and suspended algae in the water column. Note: ACOE data for Secchi depth were not available for the summer of 2004.
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 somewhat reduced water clarity. The median level of inorganic suspended solids based on data from the ISU and UHL lake surveys from 2002 through 2006 at Rathbun Reservoir was 6.0 mg/l; this is the 45th highest ISS median of the 131 lakes sampled.
The levels of nuisance (=noxious) algal species (i.e., cyanobacteria) at this lake do not suggest an impairment of Class A1 uses. While data from the ISU and UHL surveys from 2002 through 2006 suggest that cyanobacteria comprise a significant portion (just over 50%) of this lake’s summertime phytoplankton community, these data showed that the median cyanobacteria mass at Rathbun Reservoir was 4.7 mg/L which was the 19th lowest cyanobacteria mass of the 131 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 and UHL lake surveys show that during 2002-2006 there were no violations of the Class B(WW-1) criterion for ammonia in 21 samples. There were 2 violations of the Class B(WW-1) criterion for dissolved oxygen in 27 samples (7%) and one violation of the Class A1,B(WW-1) criterion for pH in 25 samples (4%). 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) uses of Rathbun Reservoir.
Data from the ACOE monitoring at station RA-3 from 2004 through 2006 show 1 violation of the Class B(WW-1) criterion for dissolved oxygen in 14 samples (7%) and 1 violation of the Class A1, B(WW-1) criterion for pH in 14 samples (7%). Based on IDNR’s assessment methodology these results also suggest “full support” of the Class B(WW-1) uses.
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.
The Class C drinking water uses for the main basin of Rathbun Reservoir are assessed (monitored) as "fully supported." Results of pesticide monitoring by ACOE from 2004 through 2006 near the dam (station RA-3) showed no violations of the MCL for atrazine in the 10 samples collected. The mean, minimum, and maximum atrazine levels at station RA-3 during the 2004-2006 period were 1.2 ug/l, 0.6 ug/l, and 1.7 ug/l, respectively. These results are similar to those for previous assessment periods (see assessments for the 2002, 2004, and 2006 reports). The mean, minimum, and maximum atrazine levels at the reservoir outlet area (station RA-25) during the 2004-2006 period were 1.2 ug/l, 0.6 ug/l, and 1.8 ug/l. Levels of atrazine did not exceed the MCL, in the 10 samples collected during the 2004-2006 period at the uplake station (RA-7) in the Chariton Arm of the reservoir (mean/maximum levels of 1.5/2.6 ug/l). Levels of atrazine during the 2004-2006 period did, however, occasionally exceed the atrazine MCL at the uplake station in the South Fork Arm (RA-8) with mean/maximum levels of 3.5/24.1 ug/l in the 10 samples collected. The mean level of atrazine at this site (RA-8) exceeds the MCL and therefore suggests an impairment of the Class C uses for the South Fork Arm of the lake. Results of ACOE monitoring suggest very low levels of nitrate in Rathbun Reservoir. The average level of nitrate in the 16 samples collected from station RA-3 during 2004-06 was 0.3 mg/l; the maximum sample value during this period was 0.8 mg/l. Similarly, results of ISU and UHL lake surveys also show very low levels of nitrate in the main basin of Rathbun Reservoir. The average level of nitrate in the 27 samples collected from the ISU and UHL station from 2002 through 2006 was 0.3 mg/l; the maximum sample value during this period was 0.8 mg/l. The results of nitrate monitoring from ACOE Station RA-3 and from the ISU and UHL lake surveys are far below the nitrate MCL of 10 mg/l and thus do not suggest any impairments or threats to full support of the Class C uses.
Results from Syngenta's "Iowa Voluntary Atrazine Monitoring Program" also show relatively low levels of atrazine in the main lake basin of Rathbun Reservoir. This monitoring showed that the maximum level of atrazine in the 98 samples collected over the 2004-2006 period was 1.6 ug/l (both mean and median = 0.9 ug/l). All moving annual averages for atrazine during this period were well-below the 3 ug/l MCL (maximum average = 1.2 ug/l). Based on DNR's Section 305(b) assessment methodology, if all levels of the monitored toxics are less than the MCL, the drinking water uses should be assessed as “fully supported.” The only parameter collected as part of the ISU and UHL lake surveys relevant to support of Class C (drinking water) uses is nitrate. The results of the ISU and UHL surveys from 2002-06 show that nitrate levels are very low at this lake (maximum value = 0.8 mg/l; median = 0.2 mg/l).
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 this reservoir. 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 were assessed (monitored) 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. 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. Prior to 2006, IDNR used action levels published by the U.S Food and Drug Administration to determine whether consumption advisories should be issued for fish caught as part of recreational fishing in Iowa. In an effort to make Iowa’s consumption more compatible with the various protocols used by adjacent states, the Iowa Department of Public Health, in cooperation with Iowa DNR, developed a risk-based advisory protocol. This protocol went into effect in January 2006 (see http://www.iowadnr.gov/fish/news/consump.html for more information on Iowa’s revised fish consumption advisory protocol). Because the revised (2006) protocol is more restrictive than the previous protocol based on FDA action levels; fish contaminant data that previously suggested “full support” may now suggest either a threat to, or impairment of, fish consumption uses. This scenario, however, does not apply to the fish contaminant data generated from the 2004 RAFT sampling conducted at Rathbun Reservoir: the levels of contaminants do not exceed any of the new (2006) advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
