West Lake (Osceola) IA 04-LDM-1082

Assessment Comments

Basis for Assessment

SUMMARY:  The Class B(LW) aquatic life uses remain assessed as "fully supported / threatened."  The Class C (drinking water) uses were assessed as "not supported."  Fish consumption uses remain "not assessed."  EXPLANATION:  The previous (1998) assessment of support of the Class B(LW) uses was reviewed and approved by the DNR Fisheries Bureau in 2000.   The assessment of support of the Class C (drinking water) uses as "not supported" was based on the the 1998 results of the Novartis "Iowa Voluntary Atrazine Monitoring Program."  This monitoring showed that the time-weighted mean level of atrazine in samples collected from the Osceola raw water source from January to December 1998 (3.7 ug/l, N=30, maximum=7.3 ug/l) was above the MCL of 3.0 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, the Class C uses of West Lake Osceola were assessed as "not supported."  The high levels of atrazine in West Lake Osceola suggest that this lake should remain on Iowa's Section 303(d) list of impaired waters.   Fish consumption uses remained "not assessed" due to lack of fish contaminant monitoring at this lake.