Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2001.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water transparency caused by inorganic suspended solids that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. Nuisance cyanobacteria blooms are also a concern at this lake. 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 assessed (evaluated) as “fully supported” based on results of fish contaminant monitoring in 2001. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring in 2001.
Note: A TMDL for turbidity at Storm Lake was prepared by IDNR and approved by EPA in 2005. Because the primary Section 303(d) impairment identified for the 2008 assessment/listing cycle (turbidity) is addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved) for the 2008 cycle.
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 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 75, 60, and 67 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 moderately high levels of chlorophyll a and suspended algae in the water, extremely 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 19.5 mg/L, which was the 7th highest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a large population of cyanobacteria exists at Storm Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 96% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (40.4 mg/L) was also the 29th 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.
The Class B(LW) (aquatic life) uses are assessed as “partially supported” due to excessive nutrient loading to the water column, nuisance blooms of algae, and re-suspension of sediment. The ISU and UHL lake surveys results, however, show relatively good chemical water quality at Storm Lake. During 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 15 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 assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Storm Lake in 2001. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2001 RAFT sampling conducted at Storm Lake showed that the levels of contaminants do not exceed any of the advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory for this waterbody.
