Assessment Comments

Assessment is based on results of (1) IDNR ambient monthly monitoring near Oskaloosa in 2000 & 2001 and (2) EPA / DNR (RAFT) in 1999.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses were assessed as "fully supported."  The Class C (drinking water) uses were assessed as "fully supported/threatened."  Fish consumption uses remain assessed as "fully supported."  The sources of data for this assessment are (1) the results of monthly monitoring from October 1999 through September 2001 at the IDNR ambient city monitoring station located at the Highway 63 bridge north of Oskaloosa (station 10620001; formerly station 821035) and (2) the results of EPA/IDNR fish tissue (RAFT) monitoring in 1999.   This assessment is the same as for the adjacent downstream segment of the South Skunk River (IA 03-SSK-0010-2).  

EXPLANATION:  The Class B(WW) aquatic life uses were assessed as "fully supported" based on results of monitoring from the IDNR ambient station north of Oskaloosa in 2000 and 2001.   Monitoring at this station showed no violations of Class B(WW) water quality criteria for for dissolved oxygen, pH, or ammonia-nitrogen in the 24 samples analyzed, for toxic metals in the four samples analyzed, or for pesticides and other toxic organic compounds in the six samples analyzed.   These results suggest full support of the Class B(WW) aquatic life uses.   The Class C (drinking water) uses were assessed as "fully supported/threatened."  Results of IDNR ambient monthly monitoring at the Oskaloosa station show two violations of the nitrate MCL in the 24 samples collected (8.3%) during the 2000-2001 biennial period (mean=4.0 mg/l; maximum=16.0 mg/l).   According to IDNR's assessment guidelines, this dataset suggests that full support of the Class C use is "threatened."  That is, if the MCL is exceeded during the biennial period, but no more than 15% of samples exceed the MCL, the full support of drinking water uses is considered "threatened."  In addition one of 24 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 (the next highest level of atrazine during the two-year period was 1.5 ug/l).   The average level of atrazine in the 24 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, the Class C uses of this river segment were assessed as "fully supported / threatened."  Fish consumption uses remain assessed as "fully supported" based on results of EPA/IDNR fish tissue (RAFT) monitoring near Oskaloosa in 1999 (see assessment for the 2000 report).