Assessment Comments
Assessment is based on results of water quality monitoring by South Dakota DENR near Hudson, SD. from 2000-02.
Basis for Assessment
[NOTE: For the 2002 report, the previous waterbody segment for the Big Sioux River (IA 06-BSR-0020-0), which extended 54 miles from its confluence with the Rock River in Sioux County to the Iowa/Minnesota state line, was split into three subsegments: (1) from Rock River to Beaver Creek near Canton, SD and Beloit, IA (IA 06-BSR-0020-1) (this one), (2) Beaver Creek to Ninemile Creek ENE of Harrisburg, SD and west of Larchwood, IA (IA 06-BSR-0020-2), and (3) Ninemile Creek to the Iowa Minnesota state line (IA 06-BSR-0020-3). See assessment information from this subsegment for previous Section 305(b) assessments for the entire 54-mile reach.]
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" due to high levels of indicator bacteria. The Class B(WW) aquatic life are assessed (monitored) as "fully supported / threatened" (minor impacts) based on results of ambient water quality monitoring. 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 March 2000 through December 2002.
EXPLANATION: The Class A uses were assessed (monitored) as "partially supported" due to high levels of indicator bacteria (fecal coliforms) in this river segment. The geometric mean of indicator bacteria (fecal coliforms) for the 11 samples not materially affected by surface runoff during the recreational seasons of 2000, 2001, and 2002 at the Hudson monitoring station was below the Iowa water quality criterion (200 fecal coliform orgs/100ml) to protect primary contact recreation uses; however, the percentage of samples that exceeded the U.S. EPA-recommended single-sample maximum value suggests only "partial support" of the Class A uses. 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." The geometric mean of fecal coliform bacteria in the 11 non-runoff-affected samples was 121 orgs/100 ml, with two samples (18%) exceeding the EPA-recommended single-sample maximum value of 400 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of the samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are "partially supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
Regarding support of the Class B(WW) aquatic life uses, results of monitoring at the Hudson station showed that one of the 36 samples collected during the 2000-2002 period (3%) violated the state Class A and Class B(WW) criterion for pH (9.0 units); the pH of the sample collected on September 12, 2000, was 9.3 units. 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 (5%) 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). This violation occurred on a day with high levels of dissolved oxygen (15.8 mg/l) and water temperature (24.2C) indicating a percent DO saturation of > 140%. These conditions suggest that the high level of primary productivity resulted in the high level of pH. Because this violation is more related to natural conditions than to pollution, the occurrence of the high level of pH in this river segment is not seen as a water quality impairment. No other violations of Class B(WW) criteria occurred at this station during the 2000-2002 assessment period.
Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.
