Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) 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 “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 assessed (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 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of U.S. EPA/IDNR fish contaminant monitoring in 1999.
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 assessment/listing cycles (turbidity) are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved) for the 2010 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 2004 through 2007 (approximately 28 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 66, 53, and 70 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 very high levels of phosphorus in the water column.
The level of inorganic suspended solids is very 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 10.8 mg/L, which was the 19th highest of the 132 monitored lakes.
Data from the 2004-2008 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 99% of the phytoplankton wet mass at this lake. However, the median cyanobacteria wet mass (21.2 mg/L) was only the 46th highest of the 132 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 2004-2008 there was 1 violation of the Class B(LW) criterion for ammonia in 28 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 28 samples, and 2 violations of the Class A1,B(LW) criterion for pH (7%). 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.
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. The 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 the continued “full support” of fish consumption uses.
