Silver Lake IA 06-LSR-1669

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Silver Lake (Dickinson) are assessed (monitored) as "fully supported" based on information from the ISU lake survey. Using the median values from these surveys from 2012-2016 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 61, 60, and 72 respectively for Silver Lake (Dickinson). According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Silver Lake (Dickinson) in between the Eutrophic and the Hypereutrophic categories. These values suggest moderately high levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and very high levels of phosphorus in the water column. The data show 2 violations of the Class A1 criterion for pH in 15 samples (13%).

DNR's assessment methodology indicates that at least two assessment/listing cycles with a TSI value less than or equal to 63 are necessary to suggest de-listing of an impairment. The Secchi depth TSI value for the 2002 assessment listing cycle was 78, which based on DNR's assessment/listing methodology indicated an impairment based on the narrative criteria protecting against aesthetically objectionable conditions. Because the TSI value for the 2016 assessment/listing cycle was less than the trigger of 65 (2016 listing for Secchi depth a was 62), and because the TSI value for the current listing cycle is also 61, based on DNR's methodology, these values suggest "full support" of the Class A1 uses protecting against aesthetically objectionable conditions and the impairment (4a) for Secchi depth is suggested for removal for the 2018 assessment/listing cycle.

Data from the 2012-2016 ISU lake survey suggest a moderate population of cyanobacteria exists at Silver Lake (Dickinson). These data show that cyanobacteria comprised 97% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (9.8 mg/L) was ranked 48th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "fully supported." Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), no violations of the criterion for dissolved oxygen in 15 samples(0%), and 2 violations of the criterion for pH in 15 samples(13%). Based on DNR's assessment methodology these violations are not significantly greater than 10% of the samples and therefore suggest (fully supported/monitored) of the Class B(LW) uses of Silver Lake (Dickinson).

The Class C (drinking water) uses are not assessed due to the lack of recent information upon which to base an assessment. The only parameter collected as part of the ISU lake surveys relevant to support of Class C (drinking water) uses is nitrate. While the results of the ISU surveys from 2012-2016 show that nitrate levels are low at this lake (maximum value = 9 mg/l; median = 0.1 mg/l), these data are not sufficient for developing a valid assessment of support of the Class C uses.

Fish consumption uses were assessed (monitored) as “fully supported” based on results of DNR fish contaminant monitoring at Silver Lake (Dickinson) in 2014. The composite samples of fillets from channel catfish and walleye had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.0275 ppm; total PCBs: 0.6ppm; and technical chlordane: <0.1 ppm. Levels of primary contaminants in the composite sample of walleye fillets were as follows: mercury: 0.049ppm.The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2014sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.

Note:A TMDL for turbidity at Silver Lake was prepared by DNR and approved by EPA in 2009.