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 a fish kill investigation in August 2007.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to (1) poor water transparency and high algae levels that violate 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 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 2004 also suggests impairment of the aquatic life uses. Fish consumption uses are "not assessed" due to a lack of recent data 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 a fish kill investigation in August 2007.
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 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. An additional TMDL for algae, pH, turbidity and ammonia was completed in 2008 and approved by EPA in 2009; thus, this waterbody is placed into IR Category 4a (TMDL approved for the 2010 assessment/listing cycle.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 uses at Silver Lake are “partially supported.” Using the median values from these surveys from 2006 through 2010 (approximately 24 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 69, 69, and 87 respectively for Silver Lake. According to Carlson (1977) the chlorophyll a and Secchi depth values place Silver Lake in between the eutrophic and hypereutrophic categories, while the total phosphorus value places Silver Lake in the hypereutrophic category. These values suggest high levels of chlorophyll a and suspended algae in the water, 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 2.4 mg/L, which was the 46th lowest of the 134 monitored lakes.
Data from the 2006-2010 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 88% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (71.9 mg/L) was also the 10th highest of the 134 lakes sampled. This median is in the worst 25% of the 134 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 2006-2010. 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 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 3 violations of the Class B(LW) criterion for dissolved oxygen in 24 samples (12%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not indicate impairment of the Class B(LW) uses of Silver Lake. However, there were significantly greater than 10% of the dissolved oxygen samples violating the standard for the 2010 assessment/listing cycle. Based on IDNR's assessment methodology two consecutive assessment/listing cycles without significantly greater than 10% of the samples violating the standard are necessary to suggest de-listing of an oxygen impairment. Therefore, the impairment for dissolved oxygen will remain for the 2012 assessment/listing cycle. There were 12 violations of the Class A,B(LW) criterion for pH in 24 samples (50%). 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 2006-2010, there were 3 violations of the Class B(LW) criterion for ammonia in 24 samples, thus suggesting a potential impairment of the aquatic life uses of this lake. Based on IDNR’s 2012 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, 2010, and 2012 listing cycles, 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. This is the same assessment as that developed for the 2008 and 2010 assessment/listing cycles. The continuance of the IR Category 2b listing for this impairment is based on IDNR's 2012 assessment methodology that states the following: 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 (2008-2011) 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” and will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation. Thus, this impairment will remain in Category 2b of Iowa’s 2012 Integrated Report.
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 1999. While these results suggest that levels of contaminants were low at Silver Lake, they are now too old (greater than 10 years) to be used for an assessment.
