Silver Lake IA 06-LSR-1669

For the 2016 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 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 62, 61, 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.While the TSI value for Secchi depth in Silver Lake (Dickinson) is below the impairment trigger of 65 for the 2016 reporting cycle, Silver Lake (Dickinson) was listed as "partially supporting" the Class A1 uses for the 2014 reporting cycle due to aesthetically objectionable conditions caused by poor water clarity.   Based on IDNR's assessment methodology, median-based TSI values for 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).    Therefore, Silver Lake (Dickinson) remains assessed as "partially supported" due to poor water clarity for the 2016 assessment cycle.

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.  

The level of inorganic suspended solids was moderately high at Silver Lake (Dickinson), and does suggest that non-algal turbidity may contribute to the impairment at this lake. The median level of inorganic suspended solids in Silver Lake (Dickinson) (4.4 mg/L) was ranked 84th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderately large population of cyanobacteria exists at Silver Lake (Dickinson). These data show that cyanobacteria comprised 99% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (31.3 mg/L) was ranked 96th 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 2010-2014 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 IDNR'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 2010-2014 show that nitrate levels are relatively low at this lake (maximum value = 3.9 mg/l; median = 0.5 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 IDNR 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: 0.6 ppm; and technical chlordane: <0.1 ppm.   Levels of primary contaminants in the composite sample of walleye fillets were as follows: mercury: 0.049 ppm.  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 2014 sampling 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.