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 of beach monitoring in 2008 and 2009.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to (1) poor water transparency caused by inorganic suspended solids that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions and (2) levels of indicator bacteria at a swimming beach that exceed state water quality criteria. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. Fish consumption uses are "not assessed" due to a lack of information upon which to base an assessment. 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 of beach monitoring in 2008 and 2009.
Note: A TMDL for turbidity at Storm Lake was prepared by IDNR and approved by EPA in 2005; thus, this impairment is placed in IR Category 4a (impaired; TMDL approved) for the 2010 cycle. Because, however, the bacteria impairment is not addressed by the TMDL, this impairment will be placed in Category 5a of the 2012 Integrated Report (i.e., Section 303(d) list).
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Storm Lake are “not supported.” 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 70, 57, and 65 respectively for Storm Lake. According to Carlson (1977) the Secchi depth value places Storm Lake in the hypereutrophic category, the chlorophyll a value places Storm Lake in the eutrophic category, and the total phosphorus value places Storm Lake in between the eutrophic and hypereutrophic categories. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, very poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids is very high at Storm Lake and suggests that non-algal turbidity contributes to the impairment. The median inorganic suspended solids concentration at Storm Lake was 14.6 mg/L, which was the 8th highest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Storm Lake, which does not contribute to the impairment at this lake. These data show that cyanobacteria comprised 75% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (35.9 mg/L) was the 37th highest of the 134 lakes sampled.
Results of IDNR county beach monitoring in 2008 and 2009 suggest that the Class A1 uses should be assessed (monitored) as "not supported." Levels of indicator bacteria at Awaysis Beach were monitored approximately once per week during the primary contact recreation seasons (May through August) of 2008 (10 samples) and 2009 (13 samples) as part of the IDNR county 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 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 recreation seasons assessed 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 Awaysis Beach, the geometric means of 5 thirty-day periods during summer 2008 and 7 thirty-day period during summer 2009 exceeded 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 less 30% during summer 2008 and was 54% during summer 2009. According to IDNR’s assessment methodology and U.S. EPA guidelines, violation of both the geometric mean and single-sample maximum criteria suggest impairment of the Class A1 (primary contact recreation) uses at this beach.
The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment although information from the IDNR Fisheries Bureau suggests that water quality is generally improving at this lake. The ISU and UHL lake surveys results, however, show relatively good chemical water quality at Storm Lake. During 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 21 samples, no violations for dissolved oxygen in 22 samples, and one violation for pH in 22 samples (5%). Based on IDNR’s assessment methodology the one violation for pH is less than 10% of the samples and therefore does not suggest an impairment of the Class B(LW) uses at Storm Lake.
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 2001. While these results suggest that levels of contaminants were low at Storm Lake, they are now too old (greater than 10 years) to be used for an assessment.
