Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) results of the IDNR-UHL beach monitoring program in 2006, 2007, and 2008, and (4) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to high levels of indicator bacteria in the past at the beach. In addition, results of the ISU and UHL lake surveys suggest "partial support" of Class A1 uses due to aesthetically objectionable conditions caused by algal blooms. The presence of nuisance aquatic life (cyanobacteria) is also a threat to full support of the Class A1 uses. The Class B(LW) aquatic life uses are assessed (monitored) as “fully supported.” The Class C (drinking water) uses remain “not assessed” due to a lack of information upon which to base an assessment. Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring at this lake. Sources of data for this assessment include: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) results of the IDNR-UHL beach monitoring program in 2006, 2007, and 2008, and (4) information from the IDNR Fisheries Bureau. Note: this lake was drained and its fishery was renovated by IDNR in 2006, and watershed improvements were also made (e.g., installation of silt-retention structures). Positive changes in water quality may result from this renovation which will affect future assessments.
EXPLANATION: Results of IDNR beach monitoring from 2006 through 2008 suggest that the Class A1 uses are "fully supported." Levels of indicator bacteria at Viking Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (22 samples), 2007 (14 samples), and 2008 (27 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
At Viking Lake beach, the geometric means of all 51 thirty-day periods during the summer recreation seasons of 2006, 2007, and 2008 were below the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 14% in 2006, 14% in 2007 and 19% in 2008. Based on DNR's assessment methodology these results suggest "full support" of the Class A1 uses at Viking Lake. However, Viking Lake was assessed as "not supported" for the 2008 assessment/listing cycle due to high levels of bacteria at the beach. Based on IDNR's assessment methodology two consecutive assessment/listing cycles with no violations of the geometric mean standard are necessary to propose de-listing of a bacteria impairment. Therefore, Viking Lake continues to be assessed as impaired for bacteria.
Results from the ISU and UHL lake surveys suggest the Class A1 uses at Viking Lake should be assessed as “partially supported” due to elevated levels of chlorophyll a and nuisance cyanobacteria populations. Using the median values from these surveys from 2004 through 2008 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 64, 66, and 63 respectively for Viking Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Viking Lake in between the eutrophic and hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and moderately high levels of phosphorus in the water column.
The level of inorganic suspended solids is relatively low at Viking Lake and suggests that non-algal turbidity does not contribute to the impairment at this lake. The median inorganic suspended solids concentration at Viking Lake was 3.0 mg/L, which was the 58th lowest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a very large population of cyanobacteria exists at Viking Lake, which contributes to the impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 95% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (37.8 mg/L) was the 24th highest of the 132 lakes sampled. This median is in the worst 25% of the 132 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2004-2008. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
The Class B(LW) (aquatic life) uses at Viking Lake are assessed (monitored) as “fully supported.” Information from the IDNR Fisheries Bureau suggests that since the fishery renovation in 2006 the water quality and fishery at this lake are greatly improved although algae still remains a problem at this lake. Results from the ISU and UHL lake surveys show moderately good chemical water quality at Viking Lake. During 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 22 samples. There were 3 violations of the dissolved oxygen criterion in 23 samples (13%) and 1 violations of the pH criterion in 23 samples (4%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class B(LW) uses at Viking Lake.
The Class C (drinking water) uses remain “not assessed” due to lack of water quality information upon which to base such an assessment. The only parameter collected as part of the ISU and UHL lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU and UHL surveys from 2004-08 show that nitrate levels are very low at this lake (maximum value = 1.4 mg/l; median = 0.1 mg/l) relative to the MCL (10 mg/L), these data are not sufficient for developing a valid assessment of support of the Class C uses.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
