Silver Lake IA 01-MAQ-17

For the 2018 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Silver Lake (Delaware) are assessed (monitored) as "not supported" due to frequent violations of the state criterion for pH 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 63, 64, and 77 respectively for Silver Lake (Delaware). According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Silver Lake (Delaware) 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 extremely high levels of phosphorus in the water column. The data show 7 violations of the Class A1 criterion for pH in 15 samples (47%).

Note:A TMDL for siltation and organic enrichment at Silver Lake was prepared by DNR and approved by EPA in 2002 and an additional TMDL for algae, pH, turbidity and ammonia was completed in 2008 and approved by EPA in 2009. The pH impairment is covered under the 2009 TMDL, thus, this waterbody is placed into IR Category 4a (TMDL approved) for the 2018 assessment/listing cycle.

The level of inorganic suspended solids was low at Silver Lake (Delaware), and does not suggest water quality problems due to non-algal turbidity. The median level of inorganic suspended solids in Silver Lake (Delaware) (1.6 mg/L) was ranked 9th among the 138 lakes by the ISU lake survey.

Data from the 2012-2016 ISU lake survey suggest a small population of cyanobacteria exists at Silver Lake (Delaware). These data show that cyanobacteria comprised 32% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (4.9 mg/L) was ranked 29th of the 138 lakes sampled.

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, 2010, 2012, 2014 and 2016 assessment/listing cycles. The continuance of the IR Category 3b listing for this impairment is based on DNR's 2018 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 (2012-2015) 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 DNR 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 3b and will be added to the state list of waters in need of further investigation. Thus, this impairment will remain in Category 3b of Iowa’s 2016 Integrated Report.

The Class B(LW) (aquatic life) uses are assessed (monitored) as "not supported" due to violations of the Class A1,B(LW) criterion for pHand also due to a fish kill in 2007 and 2004. Results of the ISU lake survey from 2012-2016 show there were no violations of the criterion for ammonia in 15 samples(0%), 3 violations of the criterion for dissolved oxygen in 15 samples(20%), and 7 violations of the criterion for pH in 15 samples(47%). Based on DNR's assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest impairment (not supported/monitored) of the Class B(LW) uses of Silver Lake (Delaware).

Fish consumption uses were assessed (monitored) as “fully supported” based on results of Iowa DNR fish contaminant monitoring at this lake.The composite plug samples from largemouth bass had low levels of contaminants in 2015. Levels of primary contaminants in the composite sample of largemouth bass plugs were as follows: mercury: 0.050 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 2015 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.