Assessment Comments

Assessment is based on results of IDNR ambient monthly monitoring near LaPorte City.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed as "partially supported;" the fish consumption uses remain assessed as "fully supported."  EXPLANATION:  The DNR quarterly water quality monitoring station on Wolf Creek at LaPorte City, upon which previous (1994, 1996 and 1998) assessments of the Class B(WW) uses were based, has not been monitored since the 1992-93 biennial period.   These data are now considered too old (greater than five years) for developing a valid assessment of support of the Class B(WW) uses.   As part of DNR's expanded water quality monitoring program, monthly monitoring at the LaPorte City station began in October 1999.   Results from this monitoring will allow development of an updated assessment of support of beneficial uses for the 2002 report.   The assessment of support of the Class B(WW) uses for the 2000 report is based on results of monitoring conducted on Wolf Creek near Dysart from March 1996 to September 1998 by USGS as part of the National Water Quality Assessment Program (NAWQA) (eastern Iowa river basins study unit, station 05464220).   This monitoring showed (1) no violations of Class B(WW) water quality criteria for pH, dissolved oxygen, or ammonia-nitrogen in the 54 samples collected, (2) a single violation of the Class B(WW) chronic water quality criterion for chlorpyrifos in the 52 samples analyzed, and (3) three violations of the Class B(WW) chronic water quality criterion for dieldrin in 52 samples analyzed.   Although the results for pH, dissolved oxygen, and ammonia-nitrogen suggest full support of the aquatic life uses, the results of pesticide monitoring (dieldrin) indicate only partial support of these uses.   According to U.S.  EPA guidelines for Section 305(b) reporting (U.S.  EPA 1997b, page 3-18), the one violation of the chronic criterion for chlorpyrifos does not suggest an impairment of the aquatic life uses.   The EPA guidelines, however, specify that more than one violation of a water quality criterion for a toxic contaminant within a three-year period indicates that the aquatic life uses are not fully supported.   Thus, the three violations of the chronic criterion for dieldrin (6% violation) indicate "partial support" of the Class B(WW) uses.   Fish consumption uses remain assessed as "fully supported" based on results of the USGS/NAWQA fish tissue monitoring in September 1995 that showed levels of organochlorine contaminants were less than ½ of the respective FDA action levels and DNR levels of concern (see assessment for the 1998 report above).