Assessment Comments
Assessment is based on results of the Iowa Voluntary Atrazine Monitoring Program in 1999-2001.
Basis for Assessment
SUMMARY: The Class B(LW) aquatic life uses and fish consumption uses remain "not assessed." The Class C (drinking water) uses are assessed (monitored) as "fully supported / threatened" with a declining water quality trend (= "impaired" and a candidate for Section 303(d) listing). The source of data for this assessment is the results of the Iowa Voluntary Atrazine Monitoring Program conducted in 2000 and 2001 by Syngenta Crop Protection, Inc. (formerly Novartis Crop Protection, Inc.). EXPLANATION: The Class B(LW) uses remain not assessed due to the lack of water quality information needed to assess these uses. 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 Lake Morris in calendar years 1999, 2000 and 2001 were below the MCL of 3.0 ug/l. The time-weighted mean of atrazine in 1999 was 1.6 ug/l (N=31), in 2000 was 2.69 ug/l (N=31), and in 2001 was 2.99 ug/l (N=31). 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 (0.0016 mg/l) rounds up to 0.002 mg/l, and the time-weighted annual means for 2000 (0.00269 mg/l) and 2001 (0.00299 mg/l) both round up to 0.003 mg/l; none of these three annual means exceed the MCL for atrazine. Nine of the 31 samples collected in 2000 exceeded the MCL of 3.0 ug/l with a maximum value of 4.6 ug/l; 13 of the 31 samples collected in 2001 exceeded the MCL for atrazine (maximum value = 9.6 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." However, because problems with atrazine at this water supply have worsened over the last five year, and because the time-weighted average for 2001 (2.99 ug/l) nearly equals the MCL of 3.0 ug/l, a downward trend in water quality suggested. Based on IDNR guidelines for Section 305(b) assessments, waterbodies assessed as "fully supporting / threatened with a downward trend are candidates for Section 303(d) listing. Fish consumption uses were "not assessed" due to the lack of fish contaminant monitoring at this lake.
