Wolf Creek IA 05-CHA-1339

Assessment Comments

Stream use assessment in 1991.

Basis for Assessment

Continue to use assessment of support of the Class B(LR) aquatic life uses developed for the 1996 report (=FST).   This assessment, however, was based on a 1991 stream use assessment.   Because this information is greater than 5 years old, the assessment category should be considered "evaluated" vs.  "monitored" as defined in U.S.  EPA guidelines for preparation of Section 305(b) reports.   Also used results of 1997 sampling by the U.S.  Army Corps of Engineers as part of a Rathbun Lake watershed study that showed no violations of the Class B(LR) WQ criterion for ammonia in the 6 samples collected.  Although Wolf Creek is not designated for Class C drinking water use, the stream is tributary to Rathbun Lake, a Class C lake.  ACOE sampling showed that 2 of the 6 samples exceeded the MCL for atrazine; these samples were both collected in June 1997.  The average level of atrzine in the six samples was 5.35 ug/l; median = 1.98 ug/l, and levels ranged from 0.05 to 20.6 ug/l.  None of the six samples contained levels of alachlor that exceeded the 2.0 ug/l MCL.   A summary of ACOE monitoring in the Rathbun Lake watershed in 1997 is available.   A review of the field sheet for the 1991 DNR stream use assessment shows that sampling was conducted in early March; thus, results of fish sampling are suspect (too early in the year).   The 5 species from 2 families captured contained only half of the expected fish taxa for streams in the Central Irregular Plains (40) ecoregion.   The decision to continue to assess the Class B(LR) uses as FST is based on the relatively high habitat score from the 1991 DNR assessment and the typically poor fish sampling conditions in late winter.   Additional biological monitoring is needed to update this assessment and to determine the status of the aquatic communities and habitats.