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, (4) results of a fish kill investigation in August 2007, and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 1999.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to (1) extremely poor water transparency that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions and (2) violations of Iowa's water quality criterion for pH. The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of water quality criteria for dissolved oxygen and pH. Violations of the Class B(LW) chronic criteria for ammonia suggest an additional potential impairment of the aquatic life uses. A fish kill at this lake in 2007 also suggests impairment of the aquatic life uses. Fish consumption uses remain (evaluated) as “fully supported” based on fish tissue monitoring in 1999. 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, (4) results of a fish kill investigation in August 2007, and (5) results of U.S. EPA/IDNR fish contaminant monitoring in 1999.
Note: A TMDL for siltation and organic enrichment at Silver Lake was prepared by IDNR and approved by EPA in 2002; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle. Because not all of the Section 303(d) impairments identified for the 2006 or the 2008 assessment/listing cycles (algal growth, pH, and dissolved oxygen) are addressed by the TMDL, this waterbody is moved from IR Category 4a to IR Category 5a (impaired; TMDL required) for the 2006 and 2008 assessment/listing cycles.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Silver Lake are “not supported.” 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 73, 74, and 88 respectively for Silver Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Silver Lake in the hypereutrophic category. These values suggest very high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and extremely high levels of phosphorus in the water column.
The level of inorganic suspended solids is relatively low at Silver Lake and does not suggest the impairment is due to high non-algal turbidity. The median inorganic suspended solids concentration at Silver Lake was 3.0 mg/L, which was the 49th lowest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a large population of cyanobacteria exists at Silver Lake, which contributes to impairment at this lake. These data show that cyanobacteria comprised 90% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (82.9 mg/L) was also the 11th 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 (monitored) as “partially supported” due to violations of water quality criteria for dissolved oxygen and pH and also due to a fish kill on August 8, 2007. The ISU and UHL lake surveys results show very poor chemical water quality at Silver Lake. Data from the ISU and UHL lake surveys show 5 violations of the Class B(LW) criterion for dissolved oxygen in 22 samples (23%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore indicate impairment (partial support/monitored) of the Class B(LW) uses of Silver Lake. There were 13 violations of the Class A,B(LW) criterion for pH in 23 samples (57%). Based on IDNR’s assessment methodology these violations were significantly greater than 10% of the samples and therefore also constitute an impairment (partial support/monitored) of the Class B(LW) uses of Silver Lake.
During 2002-2006, there were 3 violations of the Class B(LW) criterion for ammonia in 17 samples, thus suggesting a potential impairment of the aquatic life uses of this lake. Based on IDNR’s 2008 Section 303(d) listing methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s chronic criterion for ammonia and thus do not suggest an impairment of the Class B(LW) aquatic life uses. Because, however, more than one violation of the chronic criterion for ammonia occurred, this potential impairment will be added to Iowa's list of waters in need of further investigation. Note: Based on consultation with other Region 7 states and U.S. EPA Region 7 staff in 2007, IDNR's methodology for assessing impairments due to violations of chronic criteria for toxic parameters (e.g., ammonia and toxic metals) was changed. Prior to the 2008 listing cycle, IDNR followed a U.S. EPA recommendation (U.S. EPA 1997b, page 3-18) that more than one violation of a water quality criterion for a toxic pollutant in an abundant data set indicates an impairment of aquatic life uses. Because no state in Region 7 collects ambient data with the frequency necessary to accurately identify compliance with a chronic criterion (i.e., to allow calculatiion of a short-term (4-day or 30-day averages)), Region 7 states concluded that another assessment approach (e.g., the 10% rule) was more appropriate. Thus, for the 2008 listing cycle, IDNR identified aquatic life impairments for toxic parameters when significantly more than 10% of the samples exceeded a chronic criterion.
A fish kill that occurred before August 8, 2007 also suggests impairment of the Class B(LW) uses at Silver Lake. The cause of the fish kill was unknown, however natural causes were suspected. A dissolved oxygen profile was taken on August 8, 2007, two days after the fish kill was first reported: dissolved oxygen was 3.6 mg/L at the surface and 0.2 mg/L at 9 ft. The number of fish killed was estimated to be 826 and consisted mainly of bluegills. According to the IDNR assessment/listing methodology, the occurrence of a single pollutant-caused fish kill, or a fish kill of unknown origin, on a waterbody or waterbody reach during the most recent assessment period (2004-2007) indicates a severe stress to the aquatic community and suggests that the aquatic life uses should be assessed as “impaired.” If a cause of the kill was not identified during the IDNR investigation, or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.” Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5). Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Silver Lake in 1999. 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 support of fish consumption uses in Iowa’s rivers and lakes. Fish contaminant data generated from the 1999 RAFT sampling conducted at Silver Lake show that levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting “full support” of fish consumption uses.
