Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results from the IDNR-county voluntary beach monitoring program from 2008 through 2010.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to levels of indicator bacteria that exceed Iowa’s water quality standard. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Fish consumption uses remain “not assessed” due to a 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 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results from the IDNR-county voluntary beach monitoring program from 2008 through 2010.
EXPLANATION: Results of IDNR city/county beach monitoring from 2008 through 2010 suggest that the Class A1 uses are “partially supported." Levels of indicator bacteria at Hickory Grove Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2008 (14 samples), 2009 (16 samples), and 2010 (11 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) 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 Hickory Grove Lake beach, the geometric means from 2008, 2009, and 2010 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 64 E. coli orgs/100 ml in 2008, 87 E. coli orgs/100 ml in 2009, and 52 E. coli orgs/100 ml in 2010. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 14% in 2008, 25% in 2009 and 18% in 2010. The percentage of samples exceeding the single-sample maximum was significantly greater than 10% in 2009 and therefore indicates impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “partial support” of the Class A1 uses.
Results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program, however, show good water quality at Hickory Grove Lake. Using the median values from these surveys from 2006 through 2010 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 58, 57, and 60 respectively for Hickory Grove Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Hickory Grove Lake in the eutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately high levels of phosphorus in the water column.
The levels of inorganic suspended solids at this lake were moderately high but do not suggest an impairment due to non-algal turbidity. The median level of inorganic suspended solids in Hickory Grove Lake (4.0 mg/L) was the 63rd highest median of the 134 lakes sampled by the ISU and UHL programs.
Data from the 2006-2010 ISU and UHL surveys suggest a moderately relatively small population of cyanobacteria exists at Hickory Grove Lake, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised 80% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (17.0 mg/L) was the 53rd lowest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on results from the ISU and UHL lake surveys. The data from these surveys from 2006-2010 show no violations of the Class B(LW) criteria for ammonia in 22 samples, or dissolved oxygen in 22 samples. There were two violations of the Class A1,B(LW) criterion for pH in 22 samples (9%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses at Hickory Grove Lake.
Information from the IDNR Fisheries Bureau suggests that high levels of nutrients enter the lake during rain events and that a restoration project is beginning.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
