Assessment Comments
Assessment is based on results of (1) water quality monitoring conducted by the U.S. Army Corps of Engineers from 2002-04 as part of the Rathbun Water Quality Project, (2) the statewide survey of Iowa lakes conducted from 2000-04 by Iowa State University, (3) ISU information on plankton communities at Iowa lakes from 2000-05, (4) results from 2002-04 from the "Iowa Voluntary Atrazine Monitoring Program" sponsored by Syngenta Crop Protection Inc., and (5) an analysis of the lake's water quality and fish populations by the IDNR Fisheries Bureau in 2006.
Basis for Assessment
[Note: For the 2006 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 A (primary contact recreation), Class B(WW/aquatic life) and Class C (drinking water) uses are all assessed (monitored) as "fully supported” based on water quality monitoring from 2002 through 2004. 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 13 water quality monitoring events conducted at Rathbun Lake during the years 2002 through 2004 by the U.S. Army Corps of Engineers at the downlake station (RA-3) near the dam of Rathbun Lake (the results of this monitoring are summarized in the "Rathbun Lake Water Quality Reports" for 2002, 2003, and 2004), (2) results from the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (3) results of 2002, 2003, and 2004 monitoring from the "Iowa Voluntary Atrazine Monitoring Program" sponsored by Syngenta Crop Protection Inc., and (4) an IDNR Fisheries analysis of the influence of water quality conditions on sport fish production at Rathbun Reservoir.
EXPLANATION: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supported". Using the median values from the ISU statewide lake survey from 2000 through 2004 (approximately 15 samples), Carlson’s (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 59, 46, 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 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 to relatively good water transparency. The 2000-2004 ISU TSI value for total phosphorus at Rathbun Lake (59) is much lower than the corresponding TSIs for Iowa’s other flood control reservoirs (Red Rock Lake (79), Saylorville Lake (75), and especially Coralville Lake (82)), thus suggesting that Rathbun Lake 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 marginally to relatively good water transparency and “full support” of the Class A uses of main basin of Rathbun Lake. Using the median values from the nine samples collected as part of the ACOE survey from during June-August periods of 2000 through 2004, Carlson’s (1977) trophic state indices are 71 for total phosphorus and 65 for secchi depth. According to Carlson (1977), the index value for total phosphorus is in the lower range of hyper-eutrophic lakes, and the index value for secchi depth is in the middle range between eutrophic and hyper-eutrophic lakes. These index values suggest the following: (1) high levels of phosphorus in the water column and (2) marginally good water transparency. Although IDNR’s 2006 assessment/listing methodology requires at least 10 lake samples over a the 2000-04 period to develop a high-confidence (“monitored”) assessment appropriate for identifying 303(d)-type impairments, the availability of nine samples for Rathbun Reservoir is considered sufficient for developing a “monitored” assessment of the Class A uses. Note: In order to be comparable with TSI value based on data from the ISU statewide lake survey, ACOE data for only the June-August periods of 2000-2004 were used to calculate TSI values for total phosphorus and Secchi depth. ACOE data for Secchi depth were not available for the summers of 2003 or 2004.
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 in the main basin of 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 the typical to moderately high levels of inorganic suspended solids that occur at this lake. Neither nitrogen limitation nor zooplankton grazing appear to influence algal production.
Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Rathbun Lake is 24. This TN:TP ratio suggests that algal production at this lake is likely limited by phosphorus versus nitrogen availability.
In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at Rathbun Lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 25% of the dry mass of the zooplankton community of this lake. The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (15 mg/l) was the 6th lowest of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Rathbun Lake.
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 transparency. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU statewide lake survey from 2000 through 2004 was 5.2 mg/l; the median level at Rathbun Lake was 6.0 mg/l; this is the 77th highest ISS median of the 131 lakes sampled.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (just over 60%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Rathbun Lake (2.2 mg/l) was the 11th lowest of the 131 lakes sampled. This level is in the lowest 10 percent of the 131 Iowa lakes sampled. The presence of a very small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Rathbun Lake 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 A water quality criterion of 200 orgs/100 ml. These results suggested “full support” of the Class A uses. No additional beach monitoring has been conducted at this lake since 1999.
The Class B(WW) aquatic life uses were assessed as "fully supported " based on results of ACOE monitoring and ISU monitoring from 2000 through 2004 that showed no violations of Iowa Class B(WW) water quality criteria for dissolved oxygen or pH. 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 are assessed (monitored) as "fully supported ". Results of pesticide monitoring by ACOE from 2002 through 2004 near the dam (station RA-3) showed no violations of the MCL for atrazine in the 14 samples collected. The mean, minimum, and maximum atrazine levels at station RA-3 during the 2002-2004 period were 1.2 ug/l, 0.5 ug/l, and 1.9 ug/l, respectively. These results are similar to those for previous assessment periods (see assessments for the 2000, 2002, and 2004 reports). The mean, minimum, and maximum atrazine levels at the reservoir outlet area (station RA-28) during the 2002-2004 period were 1.4 ug/l, 0.9 ug/l, and 2.2 ug/l. Levels of atrazine during the 2002-2004 period did, however, occasionally exceed the atrazine MCL at the uplake station in the South Fork Arm (RA-8) with mean/maximum levels of 2.9/19.5 ug/l in the 12 samples collected. Levels of atrazine did not exceed the MCL, however, in the 11 samples collected during the 2002-2004 period at the uplake station (RA-7) in the Chariton Arm of the reservoir (mean/maximum levels of 1.2 and 2.1 ug/l). Results of ACOE monitoring suggest very low levels of nitrate in Rathbun Reservoir. The average level of nitrate in the 15 samples collected from station RA-3 during 2002-04 was 0.3 mg/l; the maximum sample value during this period was 0.8 mg/l. Similarly, results of ISU lake survey monitoring also show very low levels of nitrate in the main basin of Rathbun Reservoir. The average level of nitrate in the 15 samples collected from the ISU station during summers of 2002 through 2004 was 0.2 mg/l; the maximum sample value during this period was 0.4 mg/l. The results of nitrate monitoring from ACOE Station RA-3 and from the ISU lake survey station 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 78 samples collected over the 2002-2004 period was 2.5 ug/l (both mean and median = 1.2 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 lake survey relevant to support of Class C (drinking water) uses is nitrate. The results of the ISU survey from 2002-04 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 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 Lake 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 Lake 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 Lake: 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.
