Assessment Comments

Assessment is based on results of IDNR monthly water quality monitoring N of Centerville.

Basis for Assessment

SUMMARY:  Class B(WW) aquatic life uses were assessed as "fully supporting / threatened," and Class C (drinking water) uses were assessed as "fully supporting / threatened."  Fish consumption uses were assessed as "fully supporting."  EXPLANATION:  Class B(WW) aquatic life uses were assessed as "fully supporting" due to the lack of violations of water quality criteria (1) in the 24 samples analyzed for conventional and toxic parameters (dissolved oxygen, pH, and ammonia-nitrogen) and (2) in the two samples analyzed for toxic metals, at the DNR monthly monitoring station on the Chariton River downstream from Rathbun Dam during the 1998-1999 biennial period.   Class C (drinking water) uses were assessed as "fully supporting / threatened" due to the relatively low average level of atrazine (0.9 ug/l) and due to the single violation (1) of the U.S.  EPA MCL for atrazine.   The only one of the 23 samples that contained atrazine above the 3.0 ug/l MCL (4.80 ug/l) was collected on May 26, 1998; the sample with the next highest concentration contained 1.6 ug/l of atrazine.   The average level of atrazine in the 23 samples was 0.9 mg/l (standard deviation = 9.0 mg/l) (median = 0.6 ug/l).   None of the 24 samples analyzed for nitrate exceeded the EPA MCL of 10 mg/l; no other Class C water quality criteria were exceeded.   Based on DNR's Section 305(b) assessment methodology, the results of monitoring suggest that the drinking water uses are fully supported / threatened.   Fish consumption uses remain assessed as "fully supporting" based on results from EPA/DNR fish tissue (RAFT) monitoring in 1996 that showed levels of organochlorine contaminants and toxic metals in composite samples of fillets from channel catfish and white crappie were below ½ of the respective FDA action levels and DNR levels of concern.