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, and (4) results of the IDNR-county voluntary beach monitoring program.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due levels of indicator bacteria that exceed Iowa’s water quality standard. The Class B(LW) aquatic life uses are assessed (monitored) as “fully supported.” 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 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, and (4) results of the IDNR-county voluntary beach monitoring program.
EXPLANATION: Results of IDNR county beach monitoring from 2004 through 2006 suggest that the Class A1 uses are assessed (monitored) as "not supported." Levels of indicator bacteria Willow Lake Beach were monitored approximately once per week during the primary contact recreation seasons (May through August) of 2004 (14 samples), 2005 (15 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 Willow Lake Beach, the geometric means of 3 thirty-day periods during the summer recreation season of 2004 exceeded the Iowa water quality standard of 126 E. coli orgs/100 ml. No geometric means exceeded the criterion during 2005 and 2006. The percentage of samples exceeding Iowa’s single-sample maximum criterion (235 E. coli orgs/100 ml) was less than 10% in all the recreation seasons: 2004: 7%, 2005: 0%, 2006: 0%. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest nonsupport of the Class A1 (primary contact recreation) uses.
Results from the ISU and UHL lake surveys suggest that the Class A1 (primary contact recreation) uses at Willow Lake are “fully supported.” Using the median values from these surveys from 2002 through 2006 (approximately 23 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 52, 54, and 53 respectively for Willow Lake. According to Carlson (1977) the Secchi depth, chlorophyll a, and total phosphorus values all place Willow Lake in the eutrophic category. These values suggest low levels of chlorophyll a and suspended algae in the water, very good water transparency, and low levels of phosphorus in the water column.
The level of inorganic suspended solids is low at Willow Lake and suggests that non-algal turbidity does not cause water quality problems. The median inorganic suspended solids concentration at Willow was 2.0 mg/L, which was the 23rd lowest of the 132 monitored lakes.
Data from the 2002-2006 ISU and UHL surveys suggest a moderately large population of cyanobacteria exists at Willow Lake, which does not contribute to the impairment at this lake. These data show that cyanobacteria comprised 65% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (13.2 mg/L) was the 50th lowest of the 132 lakes sampled.
The Class B(LW) (aquatic life) uses are assessed (monitored) as “fully supported.” Data from the ISU and UHL lake surveys show relatively good chemical water quality at Willow Lake. Results from these surveys show that during 2002-2006 there were no violations of the Class B(LW) criterion for ammonia in 16 samples or dissolved oxygen in 22 samples. There was 1 violation of the Class A1,B(LW) criterion for pH in 23 samples (4%). However, based on IDNR’s assessment methodology this violation are not significantly greater than 10% of the samples and therefore do not suggest an impairment of the Class A and Class B(LW) uses at Willow Lake. Information from the IDNR Fisheries Bureau also suggests that Willow Lake has excellent water quality.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
