Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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. 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 2009-2012 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted in 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 2014 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 2008-2012 (approximately 14 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 56, 58, and 73, respectively for Silver Lake. According to Carlson (1977) the chlorophyll a and Secchi depth values place Silver Lake in the eutrophic category, while the total phosphorus value places Silver Lake in the hypereutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and very high levels of phosphorus in the water column. Based on IDNR's assessment methodology, median-based TSI values for both chlorophyll a and Secchi depth must be 63 or less for two consecutive assessment/listing cycles before a lake can be removed from the state's Section 303(d) list (IR Category 5). The median TSI values for the 2012 assessment and listing cycle for chlorophyll a and Secchi depth were 69 and 69, respectively. Therefore, Silver Lake will remain assessed as "partially supported" for the 2014 Integrated Report.
The level of inorganic suspended solids is 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 2.1 mg/L, and ranked 11th of the 134 monitored lakes.
Data from the 2008-2012 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 63% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (4.3 mg/L) and ranked 7th of the 134 lakes sampled.
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 in 2004. The ISU and UHL lake surveys results show poor chemical water quality at Silver Lake. Data from the ISU and UHL lake surveys show 2 violations of the Class B(LW) criterion for dissolved oxygen in 14 samples (14%). 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. Becasuse there were not significant violations for dissolved oxygen in the 2012 and current (2014) assessment/listing cycle. Therefore, the impairment for dissolved oxygen is suggested for de-listing. There were 4 violations of the Class A,B(LW) criterion for pH in 14 samples (29%). 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 2008-2012, there was 1 violation of the Class B(LW) criterion for ammonia in 14 samples (7%), thus suggesting a potential impairment of the aquatic life uses of this lake. Based on IDNR’s 2014 Section 303(d) listing methodology, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s chronic criterion for ammonia.
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 2014 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.
