Assessment Comments
Assessment is based on results of IDNR/UHL monthly ambient monitoring downstream from Sac City from 2000-02.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality standards. The Class B(WW) aquatic life uses are assessed (monitored) as "partially supported" based on the results from the IDNR/UHL ambient monthly monitoring in 2001 and 2002. The fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment. The source of data for this assessment include the results from the IDNR/UHL ambient monthly monitoring station downstream from Sac City in Sac County (STORET station 10810001 (formerly station 423014)) located approximately 5 miles south of Sac City.
EXPLANATION: The Class A uses are assessed (monitored) as "not supported" due to levels of indicator bacteria (fecal coliforms) that exceed state water quality criteria. For purposes of Section 305(b) assessments, DNR uses the long-term average monthly flow plus one standard deviation of this average to identify river flows that are materially affected by surface runoff. According to the Iowa Water Quality Standards (IAC 1990:8), the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) does not apply "when the waters are materially affected by surface runoff." Twenty-four of the 34 samples collected from the Sac City station during the 2000, 2001, and 2002 recreational seasons were collected at flows not materially affected by surface runoff. The geometric mean level of indicator bacteria (fecal coliforms) in these 24 non-runoff-affected samples (215 orgs/100ml) is above the Iowa Class A water quality criterion of 200 orgs/100ml. Seven four of the 24 samples (30%) exceeded the U.S. EPA-recommended single-sample maximum value of 400 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, geometric means for fecal coliforms that exceed 200 organisms/100 ml indicate “nonsupport” of primary contact recreation uses (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
Results of IDNR/UHL ambient chemical/physical water quality monitoring at the Sac City station during the 2000-2002 assessment period, however, suggest relatively good water quality in this river segment. Results of this monitoring show no violations of Class B(WW) (aquatic life) water quality criteria in the 52 samples analyzed for dissolved oxygen and pH, in the four samples analyzed for toxic metals, or in the 10 samples analyzed for pesticides and other toxic organic compounds. The sample collected on March 19, 2001, however, contained 4.1 mg/l of ammonia-nitrogen and thus violated the Class B(WW) chronic water quality criterion. According to U.S. EPA guidelines for Section 305(b) reporting (U.S. EPA 1997b, page 3-18), this one violation of a water quality criterion for a toxic parameter in an "abundant" data set does not suggest an impairment of the aquatic life uses. Based on IDNR's assessment methodology for Section 305(b) reporting, however, this violation suggests that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened." Also, one of the 19 samples analyzed for pesticides contained the pesticide chlorpyrifos at a level that exceeded the state water quality criterion. The sample collected on November 27, 2001, contained 0.78 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."
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment. The most recent fish contaminant monitoring in this river segment was conducted in 1993 as part of the U.S. EPA/IDNR RAFT program; these data are too old to accurately characterize current contaminant levels.
