Silver Lake IA 01-MAQ-17

For the 2016 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 2010-2014 (approximately 15 samples), Carlson 's (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 52, 57, and 67 respectively for Silver Lake (Delaware). According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Silver Lake (Delaware) in the Eutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, very good water transparency, and high levels of phosphorus in the water column. The data show 6 violations of the Class A1 criterion for pH in 15 samples (40%).

IDNR'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 and chlorophyll a TSI value for the 2012 assessment listing cycle were 69 and 69 respectively, which based on IDNR's assessment/listing methodology indicated an impairment based on the narrative criteria protecting against aesthetically objectionable conditions.   Because the TSI values for the 2014 assessment/listing cycle were less than the trigger of 65 (2014 listing for Secchi depth and chlorophyll a were 56 and 58 respectively), and because the TSI values for the current listing cycle are also 52 and 57, based on IDNR's methodology, these values suggest "full support" of the Class A1 uses protecting against aesthetically objectionable conditions.   

Note:  A TMDL for siltation and organic enrichment at Silver Lake was prepared by IDNR and approved by EPA in 2002; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 assessment/listing cycle.   Because not all of the Section 303(d) impairments identified for the 2006 or 2008 assessment/listing cycles (algal growth, pH, and dissolved oxygen) are addressed by the TMDL, this waterbody is moved from IR Category 4a to IR Category 5a (impaired; TMDL required) for the 2006 and 2008 assessment/listing cycles.   An additional TMDL for algae, pH, turbidity and ammonia was completed in 2008 and approved by EPA in 2009; thus, this waterbody is placed into IR Category 4a (TMDL approved for the 2016 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) (2 mg/L) was ranked 10th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a small population of cyanobacteria exists at Silver Lake (Delaware). These data show that cyanobacteria comprised 55% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (5.1 mg/L) was ranked 12th 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 and 2014 assessment/listing cycles.   The continuance of the IR Category 2b listing for this impairment is based on IDNR's 2016 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 IDNR 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 2b or 3b and will be added to the state list of waters in need of further investigation.   Thus, this impairment will remain in Category 2b 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 pH and also due to a fish kill in 2007 and 2004. Results of the ISU lake survey from 2010-2014 show there were one violation of the criterion for ammonia in 15 samples(7%), one violation of the criterion for dissolved oxygen in 15 samples(7%), and 6 violations of the criterion for pH in 15 samples(40%). Based on IDNR'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 “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.