Assessment Comments
Assessment is based on: (1) the results of the IDNR-UHL beach monitoring program in the summer of 2008, (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2008 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 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 2008, 2009, and 2010, (2) results of the statewide survey of Iowa lakes conducted from 2006 through 2008 by Iowa State University (ISU), (3) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), and (4) information from the IDNR Fisheries Bureau.
EXPLANATION: Results of IDNR beach monitoring from 2008 suggest that the Class A1 uses are assessed (evaluated) as “fully supported." Levels of indicator bacteria at Green Valley Lake beach were monitored once per week during the primary contact recreation season (May through September) of 2008 (7 samples) as part of the IDNR beach monitoring program. No samples were collected in 2009 or 2010 due a draw down of the lake for restoration purposes. This assessment is considered an "evaluated" assessment because there were too few samples collected to support a "monitored" assessment. 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) the geometric mean of the samples from each recreation season of 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 sampling season 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).
NOTE: Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed. Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season. Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake. Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches. Thus, for the 2012 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion. This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.
At Green Valley Lake beach, the geometric mean from 2008 was below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 16 E. coli orgs/100 ml in 2008. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 14% in 2008. These results are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “full support” of the Class A1 uses.
Results of the ISU and UHL lake surveys also suggest "full support" of the Class A uses of Green Valley Lake. Using the median values from these surveys from 2006 through 2008 (approximately 15 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 62, 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.
While the TSI values for Green Valley Lake are below the impairment trigger of 65 for the 2012 reporting cycle, Green Valley Lake was listed as "partially supporting" the Class A1 uses for the 2004, 2006, 2008, and 2010 reporting cycle due to algal turbidity. Based on IDNR's assessment methodology, median-based TSI values for both chlorophyll-a and Secchi depth must be 63 or less for two consecutive summer seasons before a lake can be removed from the state’s Section 303(d) list (IR Category 5). Therefore, Green Valley Lake remains assessed as "partially supported" for the 2012 assessment cycle.
Data from 2006-2008 suggest moderately high populations of cyanobacteria exist at Green Valley Lake, which may contribute to aesthetically objectionable conditions at this lake. These data show that cyanobacteria comprised 88% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (25.1 mg/L) was the 58th highest of the 134 lakes sampled.
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 56th highest median of the 134 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 2006-2008 show no violations of the Class B(LW) criteria for ammonia in 15 samples, one violation of the Class B(LW) dissolved oxygen criterion in 15 samples (7%), and no violations of the Class A1,B(LW) pH criterion in 15 samples. 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 and the lake is filled.
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 2006-08 show that nitrate levels are low at this lake (maximum value = 4.5 mg/l; median = 1.0 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.
