Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation in May 2007.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to poor water transparency and high levels of chlorophyll a (algae) that violate Iowa’s narrative criteria protecting against aesthetically objectionable conditions. Also, the presence of large populations of cyanobacteria suggests an additional impairment of these uses due to nuisance aquatic life. The Class B(LW) aquatic life uses are assessed (evaluated) as “partially supported” due to sediment related turbidity and the results of a fish kill investigation in May 2007. Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring at this lake. Sources of data for this assessment include: (1) results of the statewide survey of Iowa lakes conducted from 2004 through 2007 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, and (4) results of a fish kill investigation in May 2007.
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 2010 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 2010 cycle.
EXPLANATION: Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses at Williamson Pond are “not supported” due to poor water transparency caused by both algal and non-algal turbidity. Using the median values from these surveys from 2004 through 2008 (approximately 22 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 73, 70, and 81 respectively for Williamson Pond. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Williamson Pond in the hypereutrophic category. These values suggest very 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 level of inorganic suspended solids is moderately high at Williamson Pond and suggests that non-algal turbidity may contribute to the impairment at this lake. The median inorganic suspended solids concentration at Williamson Pond was 5.5 mg/L, which was the 47th highest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a large population of cyanobacteria exists at Williamson Pond, which contributes to the impairment due to nuisance aquatic life. These data show that cyanobacteria comprised 84% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (39.4 mg/L) was the 22nd highest of the 132 lakes sampled. This median is in the worst 25% of the 132 lakes sampled. The presence of a large population of cyanobacteria at this lake suggests a potential violation of Iowa’s narrative water quality standard protecting against the occurrence of nuisance aquatic life. This assessment is based strictly on the distribution of the lake-specific median cyanobacteria values from 2004-2008. Median levels greater than the 75th percentile of this distribution were arbitrarily considered to represent potential impairment. No other criteria exist, however, upon which to base a more accurate identification of impairments due to cyanobacteria. The assessment category for assessments based on level of cyanobacteria will be considered "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence) to account for this lower level of confidence.
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 (2006-2009) 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 and UHL lake surveys however, show relatively good chemical water quality at Williamson Pond. Results from these surveys show that during 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 21 samples and one violation of the dissolved oxygen criterion in 22 samples. There were 3 violations of the Class A1,B(LW) criterion for pH in 22 samples (14%). Based on IDNR’s assessment methodology these violations are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class A1 and Class B(LW) uses at Williamson Pond.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
