Assessment Comments
Assessment is based on (1) results of the Iowa Voluntary Atrazine Monitoring Program from 1999-2001, (2) ISU lake survey in 2000-01, (3) ISU report on lake phytoplankton communities, & (4) surveys of the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supporting / threatened." The Class C (drinking water) uses are assessed (monitored) as "not supported." Fish consumption uses remain "not assessed." This lake is not designated for Class A (primary contact recreation) uses. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted in 2000 and 2001 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, (3) information on plankton communities at Iowa lakes in 2000 from Downing et al. (2002), and (4) the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 1999, 2000, and 2001 by Syngenta Crop Protection, Inc. (formerly Novartis Crop Protection, Inc.). EXPLANATION: Results from the ISU statewide survey of Iowa lakes suggest that high levels of phosphorus may threaten full support of the Class B(LW) aquatic life uses of West Lake Osceola. Using the median values from this survey in 2000 and 2001 (approximately six samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 79, 55, and 58, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth, however, place this lake in the mid to upper range, respectively, of eutrophic lakes. These index values suggest (1) extremely high levels of phosphorus, (2) relatively low levels of chlorophyll-a, and (3) relatively good water transparency. According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with relatively low values for chlorophyll-a and secchi depth indicate that some factor (e.g., nitrogen limitation or zooplankton grazing) limits production of algae. Based on median values from ISU sampling in 2000 and 2001, the ratio of total nitrogen to total phosphorus for West Lake Osceola is 8.4. This ratio suggest a good possibility that algal production at this lake is nitrogen-limited. The levels of inorganic suspended solids at this lake are relatively low and do not suggest the potential for impairing designated uses. The median level of inorganic suspended solids in the 130 lakes sampled for the ISU lake survey in 2000 and 2001 was 5.27 mg/l; the median level at West Lake Osceola was 3.3 mg/l. The high TSI value for total phosphorus suggests a threat to the full support of the Class B(LW) uses through presence of aesthetically objectionable blooms of algae. The presence of nuisance algal species (i.e., bluegreen algae) does not appear to be a significant problem at this lake. Data from Downing et al. (2002) suggest that bluegreen algae (Cyanophyta) comprise approximately half of the summertime phytoplankton community of this lake but do not dominate to the degree seen in many other Iowa lakes. Regardless, the production of chlorophyll is relatively low at this lake, and the presence of bluegreen algae likely does not present a significant water quality impact. Thus, the eutrophic conditions at this lake, along with information from the IDNR Fisheries Bureau, suggest that the Class B(LW) aquatic life uses should remain assessed as "fully supported / threatened." due to excessive nutrient loading to the water column and siltation in the lake. Although results of ISU lake monitoring in 2000 and 2001 suggest threats to full support of the designated beneficial uses of this lake, the amount of data available for characterizing water quality is not sufficient for developing an accurate assessment of support of these uses. Additional data for this lake are being generated as part of the ongoing ISU lake survey; these data will be used to improve the accuracy of future water quality assessments. The assessment of support of the Class C (drinking water) uses was based on the results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" in 1999, 2000, and 2001. This monitoring showed that the time-weighted mean levels of atrazine in samples collected from West Lake Osceola in calendar year 2000 was above the MCL of 3 ug/l. The time-weighted mean of atrazine in 1999 was 3.34 ug/l (N=29), in 2000 was 4.5 ug/l (N=29), and in 2001 was 3.33 ug/l (N=30). Rounding is required when comparing levels of atrazine to the MCL. Because the MCL, as published by U.S. EPA, is 0.003 mg/l, any values compared to the MCL must be in the proper MCL units and compared at the same number of significant figures. Thus, the annual time-weighted means of atrazine for 1999 (0.00334 mg/l) and 2001 (0.00333 mg/l) both round down to 0.003 mg/l (= 3 ug/l); neither of these values exceed the MCL for atrazine. The time-weighted annual mean for 2000 (4.5 ug/l) rounds down to 4 ug/l; this mean value is a violation of the MCL. Yearly maximum values at West Lake Osceola were 9.8 ug/l in 1999, 4.5 ug/l in 2000, and 9.0 ug/l in 2001. Samples collected during the period June through August tended to exceed the MCL. Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is greater than the MCL, the Class C (drinking water) uses of the source water should be assessed as "not supported." Thus, based on the annual time-weighted mean value in 2000 greater than the atrazine MCL, the Class C uses of West Lake Osceola remain assessed as "not supported." Fish consumption uses remain "not assessed due to the lack of fish contaminant monitoring at this lake.
