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 from the IDNR-county voluntary beach monitoring program in 2006-08.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to exceedances of the Iowa's indicatior bacteria standard. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to nutrients in the water column and siltation impacts, especially in the upper portions of the lake. Fish consumption uses are assessed (evaluated) as “fully supported” based on fish contaminant monitoring in 1996. 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 from the IDNR-county voluntary beach monitoring program in 2006-08.
Note: A TMDL for organic enrichment and siltation at Don Williams Lake was prepared by IDNR and approved by EPA in 2005. Because not all Section 303(d) impairments identified for the 2010 assessment/listing cycle (indicator bacteria) are addressed by the TMDL, this waterbody is placed in IR Category 5a (impaired; TMDL needed).
EXPLANATION: Results of IDNR county beach monitoring from 2006 through 2008 suggest that the Class A1 uses are assessed (monitored) as "not supported." Levels of indicator bacteria at Don Williams Lake beach were monitored approximately once per week during the primary contact recreation seasons (May through August) of 2006 (16 samples), 2007 (15 samples), and 2008 (13 samples) as part of the IDNR county beach monitoring program. According to IDNR’s assessment methodology, all thirty-day geometric means for the three-year assessment period must be less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses. If a 5-sample, 30-day geometric mean exceeds the state criterion of 126 orgs/100 ml during the three-year assessment period, the Class A1 uses should be assessed as “not supported”. This assessment approach is based on U.S. EPA guidelines (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
At Don Williams Lake beach, the geometric means of 1 thirty-day periods during the summer recreation seasons of 2008 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 0% in 2006, 7% in 2007 and 23% in 2008. According to IDNR’s assessment methodology and U.S. EPA guidelines, the exceedance of the geometric mean standard suggests nonsupport of the Class A1 (primary contact recreation) uses.
For the 2010 reporting cycle, results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program suggest "full support" of the Class A1 uses. Using the median values from these surveys from 2004 through 2008 (approximately 28 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 59, 59, and 61 respectively for Don Williams Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place Don Williams Lake at the upper end of the eutrophic category, while the total phosphorus value places Don Williams Lake in between the eutrophic and hypereutrophic categories. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and moderately high levels of phosphorus in the water column.
The level of inorganic suspended solids is moderately high at this lake and does not suggest impairment due to high non-algal turbidity. The median inorganic suspended solids concentration at Don Williams Lake was 5.3 mg/L, which was the 50th highest of the 132 monitored lakes.
Populations of cyanobacteria are very low at this lake and do not suggest impairment due to nuisance aquatic life. Data from the ISU and UHL lake surveys show that the median cyanobacteria wet mass at Don Williams Lake (0.6 mg/L) is the 2nd lowest of all 132 monitored lakes. The data also show that cyanobacteria comprised only 8% of the phytoplankton wet mass at this lake.
The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to excessive nutrient loading to the water column, high levels of non-algal turbidity, and siltation in the lake. Information from the IDNR Fisheries Bureau suggests that siltation and non-algal turbidity cause the aquatic life uses to be “partially supported” at Don Williams Lake. A large gizzard shad population also exists in this lake.
The ISU and UHL lake survey results show generally good chemical water quality at Don Williams Lake. From 2004-2008 there were no violations of the Class B(LW) criterion for ammonia (28 samples), or dissolved oxygen (28 samples), and only one violation of the Class A1,B(LW) criterion for pH in 28 samples. According to IDNR’s assessment methodology this pH violation is not significantly greater than 10% of the samples and therefore does not constitute an impairment of the Class B(LW) uses of Don Williams Lake.
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 1996. While these results suggest that levels of contaminants were low at Don Williams Lake, they are now too old (greater than 10 years) to be used for an assessment.
