Assessment Comments
Assessment is based on results of (1) water quality monitoring conducted by the U.S. Army Corps of Engineers from 2000-04 as part of the Rathbun Water Quality Project, and (2) 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) uses of the South Fork Arm are assessed (monitored) as “not supporting” due to extremely poor water transparency that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class B(WW/aquatic life) uses are assessed (monitored) as "partially supported” due to impacts of water quality (poor transparency) on the reservoir’s fish populations. The Class C (drinking water) uses are assessed as “fully supported” based on ambient monitoring for atrazine and nitrate. 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 up-lake Station RA-8 in the South Fork Arm of the reservoir and (2) 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 of this arm of Rathbun Lake are assessed (monitored) as "not supported" due to poor water transparency that violates Iowa’s narrative water quality criteria protecting against “aesthetically objectionable conditions”. Data collected by the U.S. Army Corps of Engineers at the up-lake station (RA-8) in the South Fork Arm of Rathbun Lake suggest that extremely high levels of turbidity impair the Class A uses of this arm 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 79 for both total phosphorus and secchi depth. According to Carlson (1977), these index values place this arm of the reservoir solidly in the range of hyper-eutrophic lakes. These index values suggest the following: (1) extremely high levels of phosphorus in the water column and (2) extremely poor 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. In addition, this methodology states that trophic state index values for chlorophyll or secchi depth greater than five points above the impairment trigger level of 65 constitute overwhelming evidence of impairment. The trophic state index value for Secchi depth in the South Fork Arm (79), although based only on less than 10 samples over the 2000-2004 period, far exceeds the impairment trigger level of 65, thus indicating an impairment appropriate for addition to the state’s list of impaired waters. 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. Data for Secchi depth are not available for the summers of 2003 or 2004.
Data do not exist from Station RA-8 for either chlorophyll-a or inorganic suspended solids. Thus, a determination cannot be made regarding whether the extremely poor turbidity in this arm of Rathbun Lake is due to suspended algae, inorganic turbidity, or a combination of both. Regardless--relative to results of the ISU statewide survey of 131 lakes--the TSI for Secchi depth (84) places this arm of the lake in the worst 5% of Iowa lakes and suggests some of the most turbid water of any lake in the state.
For information on zooplankton and bluegreen algae (Cyanobacteria) at Rathbun Lake, see the assessment developed for the main lake basin (IA 05-CHA-0020-L-1)
The Class B(WW) aquatic life uses of this arm of Rathbun Lake are assessed as "impaired" due to the impact or poor water quality (water transparency) on the lake’s fish populations. A comparison of water quality data to production of the primary sport fish at Rathbun Reservoir (crappie) suggests only “partial support” of the aquatic life uses of the South Fork Arm. 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.
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. Of the main arms of the reservoir, the Honey Creek Arm typically has the best crappie production and best water transparency (average Secchi depth of 0.66 m), followed by the Buck Creek and Chariton arms (average of 0.5 m), and South Fork Chariton arm (average of 0.3 m). The poorest crappie production occurs in the Chariton and South Fork arms. The reduced water transparencies and relatively poor crappie production in the arms of the reservoir is attributed primarily to greater influence of nonpoint source runoff from the reservoir’s watershed on the water quality in the reservoir’s arms. Additional sources of turbidity in the reservoir’s arms include re-suspension of bottom sediments and wave action. In summary, results of the analysis of fisheries and water quality data at Rathbun Reservoir suggest that (1) poor water quality (transparency) in adversely affects the reservoir’s sport fish populations, (2) the aquatic life uses of the Chariton and South Fork arms are impaired due to high levels of turbidity, and (3) improving water transparency through control of nonpoint source runoff (i.e., through implementation of best management practices (BMPs)) could provide significant benefits to the sport fishery of this reservoir. Based on this information, the Class B(WW) aquatic life uses of the South Fork Arm are assessed (monitored) as “partially supported”.
Results of ACOE chemical/physical water quality monitoring in this arm of Rathbun Reservoir from 2002-2004 show generally good water quality. Results of this monitoring show that only two of the 13 samples collected violated the Class B(WW) criteria for dissolved oxygen. Levels of dissolved oxygen violated the Class B(WW) criterion of 5.0 mg/l in samples collected on June 19, 2003 (4.4 mg/l) and June 22, 2004 (4.8 mg/l). None of the 13 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 8.3; minimum = 6.7 pH units). According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), "partial support" of aquatic life uses is indicated if criteria are exceeded in from 11% to 25% of the samples for conventional parameters (e.g., pH, temperature, or dissolved oxygen). Based on IDNR’s assessment methodology, these results do not suggest that the violation frequency for dissolved oxygen is significantly greater than 10% and thus do not suggest impairment of the Class B(WW) uses.
Results of monitoring for atrazine and nitrate do not suggest impairment of the Class C drinking water uses in the South Fork Arm of Rathbun Reservoir. The average level of atrazine in the 12 samples collected from ACOE station RA-8 from 2002-04 (2.9 ug/l) was slightly below the MCL of 3 ug/l. According to Iowa’s assessment/listing methodology, if average levels of pesticides are less than their respective MCLs, the Class C drinking water uses should be assessed as “fully supported”. Two of the 12 samples exceeded the atrazine MCL: the sample collected on May 13, 2002 contained 19.5 ug/l and the sample collected on June 6, 2002 contained 4.6 ug/l of atrazine. The average level at Station RA-8 in the South Fork Arm was considerably higher than at station RA-3 near the dam (1.2 ug/l) and likely reflects inputs of atrazine from the watershed. 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, with progressively higher levels proceeding uplake and upstream into the lakes tributaries. Results of ACOE monitoring suggest very low levels of nitrate in the South Fork Arm. The average level of nitrate in the 15 samples collected from 2002-04 was 0.7 mg/l; the maximum sample value during this period was 2.9 mg/l. These results 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.
Fish consumption uses for 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.
