Assessment Comments
Assessment is based on results of the Iowa Voluntary Atrazine Monitoring Program from 2000 through 2004.
Basis for Assessment
[Note: Two lakes (Morris and Ellis) have been used as the raw water source for the Chariton public water supply. Based on information from Chariton CWS received by IDNR in April 2006, Lake Morris is the primary water source.]
SUMMARY: The Class B(LW) aquatic life uses and fish consumption uses remain "not assessed" due to the lack of water quality information upon which to base an assessment. The Class C (drinking water) uses remain assessed (monitored) as "not supported" due to average levels of atrazine that exceeded state water quality standards during the previous (2004) Section 305(b) reporting cycle. Data from the current (2006) reporting cycle, however, indicate declining levels of atrazine at this lake. The source of data for this assessment is the results of the Iowa Voluntary Atrazine Monitoring Program conducted from 2000, through 2004 by Syngenta 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 Class C (drinking water) uses 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, however, show a decline in atrazine levels at this lake. Results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2000 through 2002 showed that the time-weighted mean levels of atrazine in samples collected from Chariton raw water source (primarily Lake Morris) in calendar years 2000, 2001, and 2002 were below the MCL of 3.0 ug/l. According to IDNR's Section 305(b) assessment methodology, if the average contaminant level in source water (either a time-weighted annual average or a moving annual average) is less than the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported." Seven of the 47 moving annual averages calculated for the 2000-2002 period, however, were above the MCL of 3 ug/l. These elevated running annual averages resulted from high levels of atrazine that occurred from May through September of 2001. Based on DNR's Section 305(b) assessment methodology, the Class C (drinking water) uses of Lake Morris were assessed as "not supported" for the 2004 reporting cycle.
Results of the Syngenta "Iowa Voluntary Atrazine Monitoring Program" from 2002 through 2004 show that the time-weighted mean levels of atrazine in samples collected in calendar years 2002 through 2004 were also below the MCL of 3.0 ug/l. The time-weighted mean of atrazine in 2002 (N=16) was 2.1 ug/l, in 2003 (N=25) was 1.9 ug/l, and in 2004 (N=33) was 1.6 ug/l. The overall mean (1.8 ug/l) and median (1.7 ug/l) atrazine levels over this three-year period (N=74) were both less than the MCL of 3 ug/l. The maximum sample value for this three-year period was 3.4 ug/l on May 20, 2002. None of the 66 moving annual averages for atrazine for the years 2002 through 2004 at Lake Morris exceeded the MCL (maximum moving annual average = 2.5 ug/l). Based on DNR's Section 305(b) assessment methodology, if the average contaminant levels 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 Lake Morris 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 (moving annual) levels of atrazine that exceeded the MCL during the 2000-2002 period and (2) levels of atrazine tend to fluctuate significantly in some Iowa lakes, and (3) while annual and moving anuual average levels of atrazine were below the MCL during the 2002-04 period, the moving annual averages sometime approach the MCL, thus suggesting the likelihood of again exceeding the MCL. 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”.
Fish consumption uses were "not assessed" due to the lack of fish contaminant monitoring at this lake.
