Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2004-06, and (5) results from U.S. EPA/DNR fish tissue monitoring in 1996.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported.” 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 2002 through 2006 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2005 through 2006 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2004-06, and (5) results from U.S. EPA/DNR fish tissue monitoring in 1996.
Note: A TMDL for organic enrichment and siltation at Don Williams Lake was prepared by IDNR and approved by EPA in 2005. Because all Section 303(d) impairments identified for the 2008 assessment/listing cycle are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).
EXPLANATION: Results of IDNR county beach monitoring from 2004 through 2006 suggest that the Class A1 uses are assessed (monitored) as "fully 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 2004 (13 samples), 2005 (13 samples), and 2006 (16 samples) as part of the IDNR county beach monitoring program. According to IDNR’s assessment methodology, two conditions need to be met for results of beach monitoring to indicate “full support” of the Class A1 (primary contact recreation) uses: (1) all thirty-day geometric means for the three-year assessment period are less than the state’s geometric mean criterion of 126 E. coli orgs/100 ml and (2) not more than 10 % of the samples during any one recreation season exceeds the state’s single-sample maximum value of 235 E. coli orgs/100 ml. 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.” Also, if significantly more than 10% of the samples in any one of the three recreation seasons exceed Iowa’s single-sample maximum value of 235 E. coli orgs/100 ml, the Class A1 uses should be assessed as “partially 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 all 16 thirty-day periods during the summer recreation seasons of 2004, 2005 and 2006 were below 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 also less than 10% in the all of following recreation seasons: 2004: 8%, 2005: 0%, 2006: 0%. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest full support of the Class A1 (primary contact recreation) uses.
For the 2008 reporting cycle, the Class A1 (primary contact recreation) uses for Don Williams Lake are assessed as “fully supported” based on results from the ISU statewide survey of lakes and the UHL ambient lake monitoring program. Using the median values from these surveys from 2002 through 2006 (approximately 26 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 57, 59, and 60 respectively for Don Williams Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Don Williams Lake at the upper end of the eutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, relatively good water transparency, and relatively low 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 4.3 mg/L, which was the 63rd 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 (0.7 mg/L) is the 3rd lowest of all 132 monitored lakes. The data also show that cyanobacteria comprised only 9% 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.
The ISU and UHL lake survey results show generally good chemical water quality at Don Williams Lake. From 2002-2006 there were no violations of the Class B(LW) criterion for ammonia (20 samples), or dissolved oxygen (25 samples), and only one violation of the Class A1,B(LW) criterion for pH in 26 samples. According to IDNR’s assessment methodology this pH violation does not constitute an impairment of the Class B(LW) uses of Don Williams Lake.
Fish consumption uses were assessed (evaluated) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring at Don Williams Lake in 1996. Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. The fish contaminant data generated from the 1996 RAFT sampling conducted at this lake show that levels of all contaminants from this monitoring were below advisory trigger levels, thus suggesting the continued “full support” of fish consumption uses.
