Assessment Comments
Assessment is based on: (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 through 2008 by University Hygienic Laboratory (UHL), (3) information from the IDNR Fisheries Bureau, (4) results from the IDNR-county voluntary beach monitoring program in 2008-10, and (5) results of IDNR/U.S. EPA fish contaminant (RAFT) monitoring in 2010.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported." This assessment represents a de-listing of the previous impairment due to indicator bacteria. The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to siltation impacts, especially in the upper portions of the lake. Fish consumption uses are assessed (monitored) as "fully supported" based on the results of fish tissue monitoring in 2010. Sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2006 through 2010 by Iowa State University (ISU), (2) results of the statewide ambient lake monitoring program conducted from 2006 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 2008-10, and (5) results of IDNR/U.S. EPA fish contaminant (RAFT) monitoring in 2010.
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 2012 assessment/listing cycle are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: Results of IDNR city/county beach monitoring from 2008 through 2010 suggest that the Class A1 uses are “fully supported." Levels of indicator bacteria at Don Williams Lake beach were monitored once per week during the primary contact recreation seasons (May through September) of 2008 (13 samples), 2009 (11 samples), and 2010 (11 samples) as part of the IDNR 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) the geometric mean of the samples from each recreation season of 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 sampling season 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).
NOTE: Based on consultation with EPA Region 7 staff in 2011, IDNR’s methodology for assessing impairments based on the geometric mean water quality criterion was changed. Prior to the 2012 listing cycle, IDNR calculated geometric means for lakes based on a 30-day periods within the recreational season. Any violation of one of these 30-day periods within 3 years resulted in an impairment of the Class A1 uses of that lake. Because water quality standards do not identify a 30 day period but instead a recreational season, Region 7 concurred that the approach used for rivers and streams with less frequent bacteria data (seasonal geometric means) would be appropriate for identifying §303(d) impairments at lake beaches. Thus, for the 2012 listing cycle, IDNR identified primary contact recreation impairments for lakes when the geometric mean of all samples from the recreation season of a given year exceeded the geometric mean criterion. This does not impact the way IDNR assesses beaches for closure to protect the recreating public in the short term.
At Don Williams Lake beach, the geometric means from 2008, 2009, and 2010 were all below the Iowa water quality standard of 126 E. coli orgs/100 ml. The geometric mean was 50 E. coli orgs/100 ml in 2008, 9 E. coli orgs/100 ml in 2009, and 61 E. coli orgs/100 ml in 2010. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was 23% in 2008, 0% in 2009 and 18% in 2010. None of these are significantly greater than 10% of the samples and therefore do not suggest impairment of the Class A1 uses. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest “full support” of the Class A1 uses.
For the 2012 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 2006 through 2010 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 59, 60, 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 5.2 mg/L, which was the 52nd highest of the 134 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 (5.0 mg/L) is the 10th lowest of all 134 monitored lakes. The data also show that cyanobacteria comprised only 29% of the phytoplankton wet mass at this lake.
The Class B(LW) (aquatic life) uses are assessed (evaluated) as “partially supported” due to siltation in the lake. Information from the IDNR Fisheries Bureau suggests that siltation causes 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, however, show generally good chemical water quality at Don Williams Lake. From 2006-2010 there were no violations of the Class B(LW) criterion for ammonia (23 samples), or dissolved oxygen (23 samples), and only one violation of the Class A1,B(LW) criterion for pH in 23 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 assessed (monitored) as “fully supported” based on results of U.S. EPA/IDNR fish contaminant (RAFT) monitoring at Don Williams Lake in 2010. The composite samples of fillets from channel catfish and largemouth bass had low levels of contaminants. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.0443 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. Levels of primary contaminants in the composite sample of largemouth bass fillets were as follows: mercury: 0.0978 ppm. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated from the 2010 RAFT sampling conducted at this lake show that the levels of contaminants do not exceed any of the advisory trigger levels, thus indicating no justification for issuance of a consumption advisory for this waterbody.
