Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to algal turbidity. 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 are “not assessed” due to a lack of recent fish contaminant monitoring at this lake. Sources of data for this assessment include (1) the results of the IDNR-UHL beach monitoring program in summers of 2006, 2007, and 2008, (2) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
EXPLANATION: Results of IDNR beach monitoring at Green Valley Lake from 2006 through 2008 suggest that the Class A1 uses should be assessed (monitored) as “fully supported.” Levels of indicator bacteria were monitored once per week during the primary contact recreation seasons (May through September) of 2006 (28 samples), 2007 (15 samples), and 2008 (7 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 Green Valley Lake beach, the geometric means of all 38 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. These results suggest generally low levels of indicator bacteria at this lake. Also, the number of samples exceeding the one-time maximum criteria of 235 orgs/100 ml was 7% in 2006, 0% in 2007, and 14% in 2008. Thus, these results do not suggest an impairment of the Class A1 uses of Green Valley Lake.
Results of the ISU and UHL lake surveys, however, show that nuisance algae blooms remain a problem at Green Valley Lake. Data from 2004-2008 suggest high populations of cyanobacteria exist at Green Valley Lake, which contribute to impairment at this lake. These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (33.6 mg/L) was also the 31st 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.
Using the median values from these surveys from 2004 through 2008 (approximately 24 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 64, and 65 respectively for Green Valley Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Green Valley Lake in between the eutrophic and hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and high levels of phosphorus in the water column.
The levels of inorganic suspended solids at this lake were moderate and do not suggest ongoing problems related to non-algal turbidity. The median level of inorganic suspended solids in Green Valley Lake (5.0 mg/L) was the 54th highest median of the 132 lakes sampled by the ISU and UHL programs.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” The ISU and UHL lake surveys data from 2004-2008 show no violations of the Class B(LW) criteria for ammonia in 24 samples, one violation of the Class B(LW) dissolved oxygen criterion in 24 samples (4%), and one violation of the Class A1,B(LW) criterion in 24 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 of Green Valley Lake.
Note: A fishery renovation was completed in 2008. Watershed improvements are being installed and other a sediment removal project is scheduled for the winter of 2009/2010. Water quality improvements are expected as this project progresses.
The Class C (drinking water) uses remain "not assessed" due to a lack of water quality information upon which to base 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 survey from 2004-08 show that nitrate levels are low at this lake (maximum value = 4.5 mg/l; median = 0.9 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 were “not assessed” due to a lack of recent fish contaminant monitoring at this lake. The most recent fish tissue monitoring was conducted in 1995. While these results suggest that levels of contaminants were low at Green Valley Lake, they are now too old (greater than 10 years) to be used for an assessment.
