Assessment Comments
Assessment is based on (1) results of the Iowa Voluntary Atrazine Monitoring Program from 2000-02, (2) ISU lake survey from 2000-04, (3) ISU reports on lake plankton communities from 2000-05, and (4) surveys of the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses are assessed (monitored) as "fully supported". [Note: this lake is not designated for Class A (primary contact recreation) uses.] The Class C (drinking water) uses remain assessed (monitored) as "not supported" due to high levels of atrazine in this lake during the 2000-2002 period. More recent monitoring (2003 and 2004) has shown improvements (lowering) of atrazine levels in this lake. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake. The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes sponsored by IDNR and conducted by Iowa State University (ISU) from 2000 through 2004, (2) surveys by IDNR Fisheries Bureau, and (3) information on plankton communities collected at Iowa lakes from 2000 through 2005 as part of the ISU lake survey, and (4) results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2003 and 2004 by Syngenta Crop Protection, Inc.
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 assessment/listing cycle. Because all Section 303(d) impairments identified for the 2006 assessment/listing cycle (atrazine) are addressed by the TMDL, this waterbody remains in IR Category 4a.
EXPLANATION: Using the median values from the ISU survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 71, 55, and 59, 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 (and much less than expected) 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, zooplankton grazing, or some other factor) other than phosphorus limits production of algae. The results of the ISU lake survey suggest that algal production at this lake may be limited by nitrogen availability; levels of inorganic suspended solids and zooplankton grazers are low at this lake and likely do not influence algal populations. Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for West Lake Osceola is 13. This ratio is low and suggests the possibility that algal production at this lake is, at times, nitrogen-limited.
Relative to other Iowa lakes, the levels of inorganic suspended solids at this lake are low and thus suggest little potential for both limiting algal production or contributing to reductions in water transparency. The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l; the median level at West Lake Osceola was 3.6 mg/l.
In terms of all Iowa lakes sampled, data from the ISU survey show extremely small populations of zooplankton species at this lake that graze on algae. Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised only about 25% of the dry mass of the zooplankton community of this lake; the average per summer sample mass of Cladoceran taxa (21 mg/l) was the 9th lowest level of the 131 lakes sampled. These results suggest little if any non-phosphorus limitation due to zooplankton grazing at West Lake Osceola.
The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of designated uses at this lake. While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (approximately 70%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at West Lake Osceola (8.4 mg/l) was the 45th lowest of the 131 lakes sampled. This level is in the lower half of the 131 Iowa lakes sampled. The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life. This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period. Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the designated uses of Iowa lakes. No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae. Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported". Nutrient loading to the water column and siltation, however, remain water quality concerns at this lake. Results of ISU monitoring from 2000 through 2004 suggest generally good chemical water quality at this lake. Results of this monitoring show that two of the 15 samples collected exceeded the Class B(LW) criteria for dissolved oxygen and that only one of the 13 samples collected exceeded Iowa’s Class A,B(LW) criteria for pH (maximum = 9.0; minimum = 7.7 pH units). Based on IDNR’s assessment methodology, these results do not suggest violation frequencies are significantly greater than 10% for either parameter and thus do not suggest impairment of either the Class A and Class B(LW) uses of West Lake Osceola. The violations of the pH criterion likely reflect primary productivity at West Lake Osceola and does not reflect the input of pollutants into this lake.
The Class C (drinking water) uses at West Lake Osceola remain assessed (monitored) as "not supported " based on results of sampling conducted as part of the Iowa Voluntary Atrazine Monitoring Program conducted in 2000, 2001, and 2002 by Syngenta Crop Protection, Inc. Results of more recent (2002-2004) monitoring show declines in atrazine levels at this lake. According to IDNR's Section 305(b) assessment methodology, if the average contaminant levels in source water (either a time-weighted annual average or a moving annual average) are less than the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported."
The assessment of support of the Class C (drinking water) uses for the current (2006) assessment cycle was based on the results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" in 2000, 2001, and 2002. This monitoring showed that the time-weighted mean level of atrazine in samples collected from West Lake Osceola in calendar year 2000 was below the MCL of 3 ug/l. The time-weighted mean of atrazine in 2000 was 3.34 ug/l (N=29), in 2001 was 3.33 ug/l (N=30), and in 2002 was 2.09 mg/l. [Note: 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. For example, the annual time-weighted means of atrazine for 2001 (0.00333 mg/l) rounds down to 0.003 mg/l (= 3 ug/l); this value does not exceed the MCL for atrazine. The time-weighted annual mean for 2000 (3.34 ug/l) similarly rounds down to 0.003 mg/l. 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.
Twenty-one of the 46 moving annual averages for the years 2000, 2001, and 2002, however, exceeded the state water quality criterion and the U.S. EPA MCL of 3 ug/l (range: 2.8 to 4.1 ug/l). 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 moving annual average values that exceed the atrazine MCL during the period 2000-2002, the Class C uses of West Lake Osceola were assessed as "not supported" for the 2004 assessment cycle.
Results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2003 through 2004 suggest improvements (lowering) in the levels of atrazine in this lake. This monitoring shows that the time-weighted mean levels of atrazine in samples collected from the Osceola raw water source in calendar years 2003 and 2004 were well-below the MCL of 3.0 ug/l. The mean and median atrazine levels over this two-year period (N=69) were 2.1 and 2.1 ug/l, respectively. The maximum value for this two-year period was 4.0 ug/l. None of the 64 moving annual averages for atrazine for the years 2003 and 2004 at West Lake Osceola exceeded the MCL (maximum average = 2.7 ug/l). Based on DNR's Section 305(b) 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." For the following reasons, however, the Class C (drinking water) uses of West Lake Osceola will remain assessed as “not supported” for the current (2006) Section 305(b) reporting cycle: (1) the Class C uses of this lake were assessed as “not supported” for the 2004 Section 305(b) reporting cycle due to average levels of atrazine that exceeded its MCL and (2) levels of atrazine tend to fluctuate significantly in some Iowa lakes. Thus, to ensure that the improving trend in atrazine levels at this lake is real and sustained, the Class C uses will remain assessed as “not supported”.
The only parameter collected as part of the ISU lake survey relevant to support of Class C (drinking water) uses is nitrate. The results of the ISU survey from 2000-04 show that nitrate levels are very low at this lake (N = 15; maximum value = 1.7 mg/l; median = 0.2 mg/l) and are well-below the MCL of 10 mg/l.
Fish consumption uses remain "not assessed due to the lack of fish contaminant monitoring at this lake.
