Assessment Comments

Assessment is based on results of (1) IDNR monthly water quality monitoring N of Centerville in 2000-01 and (2) Rathbun WQ Project at outlet of Rathbun Res. (station RA-28).

Basis for Assessment

SUMMARY:  Class B(WW) aquatic life uses remain assessed as "fully supporting" and the Class C (drinking water) uses were assessed as "fully supporting / threatened."  Fish consumption uses are "not assessed."  The sources of data used for this assessments include (1) the DNR monthly ambient monitoring station on the Chariton River downstream from Rathbun Dam during the 2000-2001 biennial period and (2) results of monitoring at the outlet of Rathbun Reservoir (station RA-28) as part of the Rathbun Reservoir Water Quality Project.   EXPLANATION:  Class B(WW) aquatic life uses were assessed as "fully supporting" due to the lack of violations of Class B(WW) water quality criteria (1) in the 24 samples from the IDNR monthly ambient monitoring station analyzed for conventional and toxic parameters (dissolved oxygen, pH, and ammonia-nitrogen), (2) in the four samples analyzed for toxic metals, and (3) in the 6 samples analyzed for pesticides during the 2000-2001 biennial period.   In addition, no violations of Class B(WW) criteria for dissolved oxygen, pH, or ammonia-nitrogen occurred in the 10 samples collected from Rathbun Project station RA-28.   Class C (drinking water) uses were assessed as "fully supporting / threatened."  Results from the IDNR ambient station show a relatively low average level of atrazine (0.5 ug/l) and the occurrence of a single violation of the U.S.  EPA MCL for atrazine during the 2000-2001 biennnial period.   Only one of the 24 samples from this station contained atrazine above the 3.0 ug/l MCL: the sample collected on June 1, 2001, contained 5.7 ug/l of atrazine.   The average level of atrazine in the 24 samples was 0.5 mg/l.   Monitoring at Rathbun Project station RA-28 produced similar results:  one of 12 samples exceeded the atrazine MCL:  the sample collected on June 13, 2001, contained 3.87 ug/l of atrazine.   The average level of atrazine in the 12 samples was 1.05 ug/l.   None of the 24 IDNR samples analyzed for nitrate exceeded the EPA MCL of 10 mg/l (maximum value = 1.3mg/l).   Similarly, none of the 12 samples from Rathbun Project site RA-28 exceeded the MCL (maximum value = 1.2 mg/l)  No other Class C water quality criteria were exceeded during the 2000-2001 biennial period at either station.   Based on DNR's Section 305(b) assessment methodology, the results of monitoring suggest that, due to the single violation of the MCL for atrazine, the drinking water uses are assessed as "fully supported / threatened":  if the average level of atrazine is less than the MCL, but one or more samples exceeds the MCL, the drinking water uses should be assessed as "fully supported / threatened."  Fish consumption uses are "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.