Assessment Comments
Assessment is based on results of (1) results of approximately 17 water quality monitoring events conducted at Rathbun Reservoir during the years 2008 through 2010 by the U.S. Army Corps of Engineers at up-lake Station RA-8 in the South Fork Arm of the reservoir, (2) an IDNR Fisheries analysis of the influence of water quality conditions on sport fish production at Rathbun Reservoir, and (3) results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Rathbun Reservoir in 2004.
Basis for Assessment
[Note 1: For the 2006, 2008, 2010, and 2012 Section 305(b)/303(d) cycles, 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 of the South Fork Arm are assessed (monitored) as “not supporting” due to extremely poor water transparency caused by algal and non-algal turbidity that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class B(WW-1) (aquatic life) uses are assessed (monitored) as “partially supporting” due to impacts of water quality (poor transparency) on the reservoir’s fish populations. Due to the lack of a Class C designation for Rathbun Reservoir, the previously identified impairment of Class C (drinking water) uses due to atrazine that was first added to Iowa’s 2006 Section 303(d) list has been de-listed. Fish consumption uses remain assessed as “fully supporting” 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 18 water quality monitoring events conducted at Rathbun Reservoir during the years 2008 through 2010 by the U.S. Army Corps of Engineers at up-lake Station RA-8 in the South Fork Arm of the reservoir, (2) an IDNR Fisheries Bureau analysis of the influence of water quality conditions on sport fish production at Rathbun Reservoir, and (3) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Rathbun Reservoir in 2004.
EXPLANATION: The Class A1 (primary contact recreation) uses of this arm of Rathbun Reservoir 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 Reservoir suggest that extremely high levels of turbidity impair the Class A1 uses of this arm of Rathbun Reservoir. Using the median values from the approximately 17 samples collected as part of the ACOE survey from 2008 through 2010, Carlson’s (1977) trophic state indices for chlorophyll a, total phosphorus, and Secchi depth are 60, 81, and 74, respectively. According to Carlson (1977), the index values for Secchi depth and total phosphorus place this arm of the reservoir in the hypereutrophic category while the index value for chlorophyll a places this arm of the reservoir in between the eutrophic and hypereutrophic category. These index values suggest very high levels of phosphorus in the water column, extremely poor water transparency, and relatively low levels of chlorophyll a in the water column. Although the TSI value for chlorophyll (60) for the 2008-10 period is below the impairment threshold of 65, IDNR's de-listing methodology requires that TSI values for chlorophyll be less than 63 for two consecutive Integrated Reporting cycles before the impairment is de-listed. Thus, if the chlorophyll TSI for the next (2014) cycle is at or below 63, this impairment will be de-listed.
For information on cyanobacteria at Rathbun Reservoir, see the assessment developed for the main lake basin (IA 05-CHA-0020-L-1)
The Class B(WW-1) aquatic life uses of this arm of Rathbun Reservoir are assessed (monitored) as “partially supported” 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 35-year 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-1) 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 2008-2010 show generally good water quality. Results of this monitoring show that one of the 15 samples collected violated the Class B(WW-1) criteria for dissolved oxygen (7%). None of the 15 samples collected exceeded Iowa’s Class A1,B(LW) criteria for pH. 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-1) uses.
The average level of atrazine in the 9 samples collected from ACOE station RA-8 from 2008-10 (2.0 ug/l) was below the MCL of 3 ug/l. Two of the 9 samples exceeded the atrazine MCL: the sample collected on May 4, 2010 contained 8.8 ug/l of atrazine and the sample collected on June 8, 2010 contained 4.8 ug/l of atrazine.
The average level of atrazine at Station RA-8 in the South Fork Arm (2.0 ug/l) was considerably higher than at station RA-3 near the dam (0.7 ug/l) and likely reflects inputs of atrazine from the watershed. 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 support of the Class C (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, 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 17 samples collected from 2008-10 was 0.8 mg/l; the maximum sample value during this period was 2.6 mg/l. These results are far below the nitrate MCL of 10 mg/l. The results of monitoring for atrazine and nitrate in this segment of Rathbun Reservoir do not suggest any threats to full support of the Class C uses of the Chariton River downriver from Rathbun Dam.
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.
