Assessment Comments

Assessment is based on results of (1) IDNR/UHL ambient monthly monitoring near Oskaloosa from 2000-02 and (2) U.S. EPA/IDNR fish contaminant monitoring near Oskaloosa in 1999.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported / threatened" due to a single violation of state water quality criterion for the pesticide chlorpyrifos.   The Class C (drinking water) uses are assessed (monitored) as "partially supported" due to violations of state water quality criteria for nitrate.   Fish consumption uses remain assessed (monitored) as "fully supported" based on results of fish contaminant monitoring in 1999.   The sources of data for this assessment include (1) the results of monthly monitoring from January 2000 through December 2002 at the IDNR/UHL ambient monitoring station located at the Highway 63 bridge north of Oskaloosa (STORET station 10620001; formerly station 821035) and (2) the results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 1999.  

EXPLANATION:  The Class B(WW) aquatic life uses were assessed as "fully supported / threatened" based on results of monitoring from the IDNR ambient station north of Oskaloosa in from 2000 through 2002.   Monitoring at this station showed no violations of Class B(WW) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 36 samples analyzed.     One of the nine samples analyzed for pesticides, however, contained the pesticide chlorpyrifos at a level that exceeded the state water quality criterion.   These results suggest full support of the Class B(WW) aquatic life uses.   The sample collected on December 3, 2001, contained 0.068 ppb of chlorpyrifos; this level exceeds the Class B(WW) chronic criterion of 0.041 ppb.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), this single violation of chronic criteria chlorpyrifos does not suggest an impairment of aquatic life uses.   Based on DNR's assessment methodology for Section 305(b) reporting, however, this violation does suggest that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened."

The Class C (drinking water) uses are assessed as "partially supported" due to violations of state water quality criteria for nitrate.   Results of IDNR ambient monthly monitoring at the Oskaloosa station show seven violations of the nitrate MCL (=state water quality standard) in the 36 samples collected (19%) during the 2000-2002 assessment period (mean=5.9 mg/l; maximum=17.0 mg/l).   According to IDNR's assessment guidelines, this dataset suggests that the Class C uses are "partially supported."  That is, if between 10% and 25% of the samples exceed the MCL for nitrate,  the Class C uses are assessed as “partially supported.”  In addition two of 36 samples contained atrazine at a level above the MCL of 3.0 ug/l:  the sample collected on June 1, 2000 contained 13.0 ug/l of atrazine, and the sample collected on June 4, 2002 contained 7.2 ug/l.   The average level of atrazine in the 36 samples was 0.8 ug/l.   Based on DNR's Section 305(b) assessment methodology, if the average contaminant level in source water is less than the MCL, but the level in one or more samples is above the MCL, the Class C (drinking water) uses of the source water should be assessed as "fully supported / threatened."  Thus, based on the percentage of violations of the Class C criterion for nitrate, the Class C uses of this river segment are assessed as "partially supported."  

Fish consumption uses remain assessed as "fully supported" based on results of U.S.  EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 1999 (see assessment for the 2000 report ).