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 the IDNR-county voluntary beach monitoring program in 2006, 2007, and 2008.
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 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 the IDNR-county voluntary beach monitoring program in 2006, 2007, and 2008.
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 Willow 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 (14 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 Willow Lake Beach, the geometric means of all 28 thirty-day periods during the summer recreation seasons of 2006, 2007, and 2008 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 0% in 2006, 13% in 2007, and 14% in 2008. According to IDNR’s assessment methodology and U.S. EPA guidelines, these results suggest "full support" of the Class A1 (primary contact recreation) uses. However, Willow Lake was assessed as “not supporting” in the 2008 assessment/listing cycle due violations of the geometric mean criterion and therefore remains "partially supported." Based on IDNR’s assessment methodology 2 consecutive assessment/listing cycles with no geometric mean violations and also without significantly greater than 10% of the samples violating the criterion are necessary to propose delisting based on bacteria violations.
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 2004 through 2008 (approximately 25 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 52, 53, 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 1.7 mg/L, which was the 13th lowest of the 132 monitored lakes.
Data from the 2004-2008 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at Willow Lake, which does not contribute to the impairment at this lake. These data show that cyanobacteria comprised 76% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (6.5 mg/L) was the 34th 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 2004-2008 there were no violations of the Class B(LW) criterion for ammonia in 24 samples, dissolved oxygen in 25 samples, or pH in 25 samples. Information from the IDNR Fisheries Bureau also suggests that Willow Lake has good water quality.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
