Assessment Comments
Assessment is based on results of (1) IDNR/UHL monthly ambient monitoring downstream from Sac City from 2002-04 and (2) IDNR/UHL biological monitoring in 2001.
Basis for Assessment
Note: This assessment is also used for the adjacent upstream segment (IA 04-RAC-0040-6).
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria (E. coli) that violate state water quality standards. The Class B(WW) aquatic life uses are assessed (evaluated) as "partially supporting" based on results of IDNR/UHL biological (biocriteria) monitoring in 2001. The fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment. The sources of data for this assessment include (1) 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 from 2002-2004 and (2) results of IDNR/UHL biological (biocriteria) monitoring in 2001.
EXPLANATION: The Class A uses are assessed as "not supported" due to levels of indicator bacteria (E. coli) that exceed state water quality criteria. Due to recent changes in Iowa’s Water Quality Standards, Iowa’s 2006 assessment methodology for indicator bacteria has changed. Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses: the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff." Due to a change in the Standards in July 2003, E. coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E. coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.” Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E. coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of the 2006 Section 305(b) assessments and Section 303(d) listings.
The geometric mean level of indicator bacteria (E. coli) in the 37 samples collected during the recreational seasons of 2002, 2003, and 2004 (294 orgs/100ml) exceeded the Iowa Class A water quality criterion of 126 orgs/100ml. In addition, 15 of the 37 samples (41%), however, exceeded Iowa’s single-sample maximum value of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, and according to IDNR’s assessment/listing methodology, if the geometric mean of E. coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
The Class B(WW) aquatic life uses were also assessed using biological data collected in 2001 as part of the ambient water monitoring project and the DNR/UHL stream biocriteria project. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (F-IBI) and a benthic macroinvertebrate index (BM-IBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2001 evaluated Fish IBI score was 54 (good) and 2001 evaluated BM-IBI score was 51 (fair), respectively. The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the F-IBI and BM-IBI scores with biological assessment criteria established specifically for the 2002 Section 305(b) report. The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2001. The reason this site was evaluated instead of monitored for this cycle was because of the size of the waterbody. The FIBI and BMIBI criteria were based and calibrated on wadeable streams and rivers. This waterbody is on the upper end of being classified as a wadeable site; therefore, the criteria used to assess this may not accurately reflect the condition of the biological community. The results of this sampling thus do not meet IDNR guidelines for developing a “monitored” assessment of support of the aquatic life uses that is appropriate to support addition of this waterbody to Iowa's Section 303(d) list.
Results of IDNR/UHL ambient chemical/physical water quality monitoring at the Sac City station during the 2002-2004 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 approximately 45 samples analyzed for dissolved oxygen, ammonia, and pH, in the 14 samples analyzed for toxic metals, or in the 15 samples analyzed for pesticides and other toxic organic compounds.
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 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.
