Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in summers of 2004, 2005, and 2006, (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 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 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 2004, 2005, and 2006, (2) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
EXPLANATION: Results of IDNR beach monitoring at Green Valley Lake from 2004 through 2006 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 2004 (16 samples), 2005 (23 samples), and 2006 (28 samples) as part of the IDNR beach monitoring program. According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) all thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. 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.” Also, if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A1 uses should be assessed as “partially 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 55 thirty-day periods during the summer recreation seasons of 2004, 2005 and 2006 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 below 10% for all years (2004: 6%, 2005: 9%, 2006: 7%). 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 2002-2006 suggest high populations of cyanobacteria exist at Green Valley Lake, which contribute to impairment at this lake. These data show that cyanobacteria comprised 86% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (40.3 mg/L) was also the 30th 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 2002-2006. 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 2002 through 2006 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 60, 64, and 66 respectively for Green Valley Lake. According to Carlson (1977) the Secchi depth value places Green Valley Lake at the upper end of the eutrophic category, while the chlorophyll a and total phosphorus values 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 (4.4 mg/L) was the 61st 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 2002-2006 show no violations of the Class B(LW) criteria for ammonia in 16 samples, and no violations of the dissolved oxygen criterion in 21 samples. The pH data show 3 violations of the Class A1,B(LW) criterion in 22 samples (14%). 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. Results of physical/chemical monitoring associated with IDNR’s beach monitoring program also indicate “full support” of the Class B(LW) uses. The dissolved oxygen data show no violations of the Class B(LW) criterion in 70 samples. The pH data show 7 violations of the Class B(LW) criterion for pH in 64 samples (11%). Again, these violations are not significantly greater than 10% of the samples and therefore do not constitute an impairment of the Class B(LW) uses of Green Valley Lake.
Note: A fishery renovation is planned for the fall of 2008 at this lake. Watershed improvements are being installed and other water quality improvements are being evaluated and will be installed during the next 18 months. 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 2002-06 show that nitrate levels are low at this lake (maximum value = 2.6 mg/l; median = 0.7 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.
