Assessment Comments
Assessment is based on results of (1) the 1999-2001 "Iowa Voluntary Atrazine Monitoring Program, (2) ISU lake survey in 2000-01, (3) ISU report on lake phytoplankton communities, and (4) surveys of the DNR Fisheries Bureau.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (evaluated) as "fully supporting / threatened." The Class B(LW) aquatic life uses remain assessed (evaluated) as "fully supported / threatened." The Class C (drinking water) uses remain assessed (monitored) as "fully supported / threatened." Fish consumption uses remain "not assessed." 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) 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 A uses of Diamond Lake. 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 71, 55, and 56, respectively. According to Carlson (1977), the index value for total phosphorus places this lake in the lower range of hyper-eutrophic lakes; the index values for chlorophyll-a and secchi depth are in the middle range of eutrophic lakes. These index values suggest (1) very high levels of phosphorus, (2) relatively low levels of chlorophyll-a, and 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 Diamond Lake is 13.6. This ratio suggests that algal production at this lake may be nitrogen-limited. The levels of inorganic suspended solids at this lake are very 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 Diamond Lake was 1.4 mg/l. This median value is the fifth lowest of the 130 lakes sampled. Despite the indications of good water quality, the high TSI value for total phosphorus suggests a threat to the full support of the Class A (primary contact) 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 a relatively small portion of the summertime phytoplankton community of this lake. The wet mass of bluegreen algae did not exceed 25% in summer 2000. 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." 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 2000 and 2001. This monitoring showed that the time-weighted mean levels of atrazine in samples collected from the Montezuma raw water source in calendar years 1999, 2000, and 2001 were below the MCL of 3.0 ug/l. The time-weighted mean of atrazine in 1999 (N=31) was 1.9 ug/l, in 2000 (N=30) was 2.0 ug/l, and in 2001 (N=31) was 1.3 ug/l. Two the 31 samples collected in 1999, and two of the 30 samples collected in 2000, exceeded the MCL of 3.0 ug/l with a maximum value of 5.4 ug/l; none of the 31 samples collected in 2001 exceeded the MCL for atrazine (maximum value = 2.4 ug/l). Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, but a level in one or more samples is above the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported / threatened." Thus, based on the MCL violations in 1999 and 2000, the Class C uses of Diamond Lake were assessed as "fully supported / threatened." Fish consumption uses were "not assessed due to the lack of fish contaminant monitoring at this lake.
