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), and (3) information from the IDNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported” due to poor water transparency caused mainly by non-algal turbidity that violates Iowa’s narrative water quality standard protecting against aesthetically objectionable conditions. The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” The Class C (drinking water) uses are “not assessed” due to the lack of recent information upon which to base an assessment. 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), and (3) information from the IDNR Fisheries Bureau.
Note: A TMDL for turbidity at Silver Lake was prepared by IDNR and approved by EPA in 2009. Because all Section 303(d) impairments identified for the 2010 and current (2012) assessment/listing cycles (turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved) for the 2012 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 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 66, 53, and 68 respectively for Silver Lake. According to Carlson (1977) the Secchi depth and total phosphorus values place Silver Lake in between the eutrophic and the hypereutrophic categories, while the chlorophyll a value places Silver Lake in the eutrophic category. These values suggest low levels of chlorophyll a and suspended algae in the water, poor water transparency, and high levels of phosphorus in the water column.
The level of inorganic suspended solids is high at Silver Lake and suggests the impairment is due to high non-algal turbidity. The median inorganic suspended solids concentration at Silver Lake was 5.0 mg/L, which was the 54th highest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a moderately high population of cyanobacteria exists at Silver Lake, which does not contribute to impairment at this lake. These data show that cyanobacteria comprised 92% of the phytoplankton wet mass at this lake. However, the median cyanobacteria wet mass (29.0 mg/L) was only the 53rd highest of the 134 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed as “fully supported” based on information from the IDNR Fisheries Bureau and results of the ISU and UHL lake surveys. Information from the IDNR Fisheries Bureau suggests that cyanobacteria blooms and little rooted aquatic vegetation are concerns at this lake. The ISU and UHL lake surveys results show relatively good chemical water quality at Silver Lake. During 2006-2010 there was 1 violation of the Class B(LW) criterion for ammonia in 22 samples. Based on IDNR’s assessment methodology a single violation of the ammonia criterion does not suggest impairment of the Class B(LW) uses. Data from the ISU and UHL lake surveys also show no violations of the Class B(LW) criterion for dissolved oxygen in 21 samples, and 2 violations of the Class A1,B(LW) criterion for pH in 22 samples (9%). Based on IDNR’s assessment methodology the pH violations are not significantly greater than 10% of the samples and therefore do not suggest impairment of the Class B(LW) uses of Silver Lake.
The Class C (drinking water) uses are "not assessed" due to lack of monitoring data upon which to base an assessment. The only parameter collected as part of the ISU and UHL lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU and UHL surveys from 2006-10 show that nitrate levels are generally low at this lake (maximum value = 5.7 mg/l; median = 1.3 mg/l) compared to the drinking water MCL (10 mg/L), these data are not sufficient for developing a valid assessment of support of the Class C uses.
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.
