Assessment Comments

Assessment is based on results of (1) IDNR/UHL biocriteria sampling in 2002, (2) IDNR/UHL monthly ambient water quality monitoring from 2000-02, and (3) Rathbun Watershed water quality project monitoring at outlet of Rathbun Reservoir, and (4) U.S. EPA/IDNR fish contaminant monitoring in 1996.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed (evaluated) as "fully supporting / threatened" based on results of biological monitoring.   The Class C (drinking water) uses were assessed (monitored) as "fully supporting / threatened" based on results of ambient water quality monitoring that show a single violation of state water quality criteria for atrazine.   Fish consumption uses are assessed (evaluated) as "fully supported" based on results of fish contaminant monitoring in 1996.   The sources of data used for this assessments include (1) results of IDNR/UHL biological (biocriteria) monitoring in 2002, (2) results of IDNR/UHL monthly ambient monitoring station on the Chariton River downstream from Rathbun Dam during the 2000-2002 assessment period (STORET station 10040001), (3) results of monitoring at the outlet of Rathbun Reservoir (station RA-28) as part of the Rathbun Reservoir Water Quality Project, and (4) results of U.S.  EPA / IDNR fish contaminant (RAFT) monitoring north of Centerville in 1996.  

EXPLANATION:  The Class B(WW) aquatic life uses were assessed (evaluated) as "fully supporting / threatened" based on results of IDNR/UHL biological monitoring conducted in 2002 as part of the stream biocriteria project.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa that were collected in the stream sampling reach.   The biological metrics were combined to make a benthic macroinvertebrate index (BM-IBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The BM-IBI score was 46 (Fair).  The aquatic life use support was assessed as Fully Supported/Threatened (=FS/T), based on a comparison of the BM-IBI scores with biological assessment criteria established for previous Section 305(b) reports.  The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001.   The assessment is considered "evaluated" due to lack of IBI values for both fish and macroinvertebrates.

Results of ambient chemical/physical water quality monitoring conducted by IDNR/UHL and by ISU/ACOE do not suggest any water quality impacts in this river reach.   No violations of Class B(WW) water quality criteria occurred (1) in the 36 samples from the IDNR monthly ambient monitoring station analyzed for conventional and toxic parameters (dissolved oxygen, pH, and ammonia-nitrogen) and (2) in the nine samples analyzed for pesticides during the 2000-2002 assessment period.   In addition, no violations of Class B(WW) criteria for dissolved oxygen, pH, or ammonia-nitrogen occurred in the 13 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 (0.8 ug/l) and the occurrence of a single violation of the U.S.  EPA MCL for atrazine during the 2000-2002 assessment period.   Only one of the 36 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 36 samples was 0.8 mg/l.  

Monitoring at Rathbun Project station RA-28 produced similar results:  one of 18 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 18 samples was 1.15 ug/l.   None of the 36 IDNR samples analyzed for nitrate exceeded the EPA MCL of 10 mg/l (maximum value = 2.0 mg/l).   Similarly, none of the 18 samples from Rathbun Project site RA-28 exceeded the MCL (maximum value = 1.5 mg/l)  No other Class C water quality criteria were exceeded during the 2000-2002 assessment period at either station.   Based on DNR's Section 305(b) assessment methodology, the results of monitoring suggest that, due to the single violations of the MCL for atrazine at the IDNR/UHL and ISU/ACOE stations, the drinking water uses are assessed as "fully supported / threatened"; that is,  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 remain assessed (evaluated) as "fully supported" based on results of the U.S.  EPA/IDNR fish tissue (RAFT) monitoring north of Centerville in 1996 (see assessment for the 2000 report).   However, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is changed from "monitored" (indicating an assessment with relatively high confidence) to "evaluated" (indicating an assessment with relatively low confidence).   This site was again monitored as part of the 2003 RAFT program; the results from this monitoring, however, are not yet available.