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, and (4) results from the "Iowa Voluntary Atrazine Monitoring Program" conducted by Syngenta Co. in 2008 and 2009.
Basis for Assessment
SUMMARY: The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” due to violations of the state’s dissolved oxygen criterion. [Note: this lake is not designated for Class A1 (primary contact recreation) uses in the Iowa Water Quality Standards.] The Class C (drinking water) uses are assessed (monitored) as "fully supported." Fish consumption uses remain “not assessed” due to a 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 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 "Iowa Voluntary Atrazine Monitoring Program" conducted by Syngenta Co. in 2008 and 2009.
Note: A TMDL for atrazine at West Lake Osceola was prepared by IDNR and approved by EPA in 2002; thus, this waterbody was placed into IR Category 4a (TMDL approved) for the 2004 through the 2010 Integrated Reporting cycles. Because atrazine levels have been low over the 2004-2010 monitoring period and have shown "full support" of the Class C uses for the 2008, 2010, and 2012 Integrated Reporting cycles, this impairment is being removed and the Class C uses are now moved from Category 4a (impaired; TMDL not required) to Category 2a (fully supported) of Iowa's 2012 Integrated Report. Also, because the new impairment due to low levels of dissolved oxygen was not addressed by this TMDL, this waterbody was placed in Category 5a of Iowa's 2008 Integrated Report and remains in Category 5a for the 2012 Integrated Report.
EXPLANATION: The Class B(LW) (aquatic life) uses are assessed (monitored) as “partially supported” based on data from the ISU and UHL lake surveys. During 2006-2010 there were no violations of the Class B(LW) criterion for ammonia in 17 samples and one violation of the pH criterion in 17 samples. Based on IDNR's assessment methodology the single violation of the pH criterion is not significantly greater than 10% of the samples and therefore does not suggest impairment of the Class B(LW) uses. There were 4 violations of the dissolved oxygen criterion in 17 samples (24%). Based on IDNR’s assessment methodology these violations are significantly greater than 10% of the samples and therefore suggest an impairment of the Class B(LW) uses at West Lake.
Additional results from the ISU and UHL lake surveys show relatively good water quality at West Lake. Using the median values from these surveys from 2006 through 2010 (approximately 17 samples), Carlson’s (1977) trophic state indices for Secchi depth, chlorophyll a, and total phosphorus were 65, 60, and 74 respectively for West Lake. According to Carlson (1977) the Secchi depth and chlorophyll a values place West Lake in between the eutrophic and the hypereutrophic categories while the total phosphorus value places West Lake in the hypereutrophic category. These values suggest relatively low levels of chlorophyll a and suspended algae in the water, moderately poor water transparency, and very high levels of phosphorus in the water column.
The level of inorganic suspended solids is also relatively low at West Lake and suggests that non-algal turbidity does not cause an impairment at this lake. The median inorganic suspended solids concentration at West Lake was 3.0 mg/L, which was the 59th lowest of the 134 monitored lakes.
Data from the 2006-2010 ISU and UHL surveys suggest a relatively small population of cyanobacteria exists at West Lake. These data show that cyanobacteria comprised 87% of the phytoplankton wet mass at this lake. The median cyanobacteria wet mass (29 mg/L) was the 52nd highest of the 134 lakes sampled.
Information from the IDNR Fisheries Bureau suggests that West Lake Osceola does experience severe algae blooms during the summer. However, the Municipal Utilities treat the lake with copper sulfate to suppress the algae, which may affect sampling by the ISU and UHL surveys. Information from the IDNR Fisheries Bureau also suggests that common carp affect water quality at this lake.
The Class C (drinking water) uses are assessed (monitored) as "fully supported" based primarily on results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2008 and 2009. This monitoring showed that the time-weighted mean levels of atrazine were below the MCL of 3.0 ug/L. The mean and median atrazine levels over this two year period (N=43) were 1.4 ug/L and 0.5 ug/L, respectively. The maximum value was 5.8 ug/L. However, none of the 15 moving annual averages for atrazine from 2008 and 2009 at West Lake Osceola exceeded the MCL (maximum average = 1.8 ug/L). Based on IDNR's assessment methodology, if the average contaminant level in source water is less than the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported." In addition results of the ISU and UHL lake surveys from 2006-10 show that nitrate levels are very low at this lake (maximum value = 1.8 mg/l; median = 0.3 mg/l) relative to the MCL (10 mg/L).
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this lake.
