Williamson Pond IA 04-LDM-1058

For the 2016 assessment/listing cycle, the Class A1 (primary contact recreation) uses of Williamson Pond are assessed (monitored) as "not supported" due to poor water transparency and aesthetically objectionable conditions caused by algae blooms 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 70, 67, and 83 respectively for Williamson Pond. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Williamson Pond in between the Eutrophic and the Hypereutrophic categories. These values suggest high levels of chlorophyll a and suspended algae in the water, very poor water transparency, and extremely high levels of phosphorus in the water column. The data show 3 violations of the Class A1 criterion for pH in 15 samples.

Note:  A TMDL for turbidity and nutrients at Williamson Pond was prepared by IDNR and approved by EPA in 2006.   Because all Section 303(d) impairments identified for the 2016 assessment/listing cycle (turbidity, algae and noxious aquatic plants) are potentially addressed by the TMDL, this waterbody remains in IR Category 4a (impaired; TMDL approved) for the 2016 cycle.

The level of inorganic suspended solids was high at Williamson Pond, and does suggest that non-algal turbidity contributes to the impairment at this lake. The median level of inorganic suspended solids in Williamson Pond (10 mg/L) was ranked 117th among the 138 lakes by the ISU lake survey.

Data from the 2010-2014 ISU lake survey suggest a moderate population of cyanobacteria exists at Williamson Pond. These data show that cyanobacteria comprised 82% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (20.5 mg/L) was ranked 68th of the 138 lakes sampled.

The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to the results of a fish kill investigation in May 2007.   A fish kill that occurred on or before May 14, 2007 suggests impairment of the Class B(LW) uses at Williamson Pond.   The cause of the fish kill was spawning stress related to warm water conditions along with high densities of crappie and bluegill.   Water temperatures had risen approximately 10 degrees in the week preceding the fish kill.   Dissolved oxygen concentrations were found to be normal at the time of the investigation.   The number of fish killed was estimated to be 673.   According to IDNR’s assessment/listing methodology, 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 (2008-2011) 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.”  Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (IR Category 5).   Waterbodies affected by such fish kills will be placed in IR subcategories 2b or 3b and will be added to the state list of waters in need of further investigation.  

Data from the ISU lake survey however, show relatively good chemical water quality at Williamson Pond. Results of the ISU lake survey from 2010-2014 show there were no violations of the criterion for ammonia in 15 samples(0%), one violation of the criterion for dissolved oxygen in 15 samples(7%), and 3 violations of the criterion for pH in 15 samples(20%). 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 Williamson Pond.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.