Assessment Comments
Assessment is based on results of monthly ambient water quality monitoring conducted on the Big Sioux River near Hudson, SD, by the South Dakota Department of Environment and Natural Resources (DENR) from 2004 through 2006.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "fully supported" based on results of ambient monitoring for indicator bacteria (E. coli) from 2004 through 2006. The Class B(WW1) aquatic life are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring from 2004 through 2006. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment. The source of data for this assessment is the results of monthly ambient water quality monitoring conducted on the Big Sioux River near Hudson, SD, by the South Dakota Department of Environment and Natural Resources (DENR) from 2004 through 2006.
EXPLANATION: The Class A1 uses are assessed (monitored) as "fully supported" based on results of ambient monitoring for indicator bacteria (E. coli) in this river segment. Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed. Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A1 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 Section 305(b) assessments and Section 303(d) listings.
The geometric mean level of indicator bacteria (E. coli) in the 10 samples collected (51 orgs/100ml) during the recreational seasons of 2004, 2005, and 2006 is well below the Iowa Class A1 water quality criterion of 126 orgs/100ml. Two of the 10 samples (20%) exceeded Iowa’s single-sample maximum value of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting (pgs 3-33 to 3-35of U.S. EPA 1997b), if levels of E. coli exceed the single-sample maximum value in more than 10% of the samples, the primary contact recreation uses should be assessed as “partially supported.” According to IDNR’s assessment/listing methodology, however, these results do not suggest that the violation frequency of Iowa’s single-sample maximum criterion is significantly greater than 10%, and thus these results do not suggest impairment of the Class A1 uses of this river segment. The results of monitoring for E. coli during both the 2002-2004 and the 2004-2006 assessment periods suggest that levels of indicator bacteria are relatively low in this assessment segment.
Regarding support of the Class B(WW1) aquatic life uses, results of monitoring at the Hudson station showed that two of the 27 samples collected during the 2004-2006 period (7%) violated the state Class A1 and Class B(WW1) criterion for pH (9.0 units). These two samples were collected on September 1, 2004 and July 17, 2006. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), the percentage of violations for pH in this stream reach (7%) does not suggest a water quality impairment (the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated). No violations of Class B(WW1) criteria for dissolved oxygen or ammonia-nitrogen occurred at this station in the 27 samples collected during the 2004-2006 assessment period.
Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.
