Assessment Comments
Assessment is based on results of water quality monitoring conducted by South Dakota DENR near Alcester, SD, from 2000-02.
Basis for Assessment
[NOTE: For the 2002 report, the previous waterbody segment for the Big Sioux River (IA 06-BSR-0010-0), which extended 82 miles from its mouth at Sioux City to confluence with the Rock River in Sioux County, was split into four subsegments: (1) mouth to Broken Kettle Creek in southwestern Plymouth County (IA 06-BSR-0010-1), (2) Broken Kettle Creek to Brule Creek near Richland, SD (and near Westfield, IA) (IA 06-BSR-0010-2), (3) Brule Creek to Indian Creek in northwestern Plymouth Co. (IA 06-BSR-0010-3), and this one (4) Indian Creek to the Rock River in Sioux Co. (IA 06-BSR-0010-4). See assessment information found in this subsegment (IA 06-BSR-0010_4 for previous Section 305(b) assessments for the entire 82-mile reach.]
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 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 Alcester, 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 "not supported" due to levels of indicator bacteria (fecal coliforms) that violate state water quality standards. The geometric mean of indicator bacteria (fecal coliforms) for the 12 samples not materially affected by surface runoff during the recreational seasons of 2000, 2001, and 2002 at the Alcester monitoring station exceeded the Iowa water quality criterion (200 fecal coliform orgs/100ml) to protect primary contact recreation 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 12 non-runoff-affected samples was 584 orgs/100 ml, with seven samples (58%) 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 the geometric mean level of fecal coliforms exceeds 200 orgs/100 ml, the primary contact recreation uses are "not 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 Alcester station showed that one of the 36 samples collected during the 2000-2002 period (3%) violated the Class B(WW) criterion for ammonia-nitrogen. The sample collected on March 27, 2001 contained 1.24 mg/l of ammonia-nitrogen; this level exceeded the Class B(WW) temperature/pH-dependent chronic criterion of 1.10 mg/l. Violations also occurred on this date at two other stations (Richland and Hudson) on the Iowa reach of the Big Sioux River that are monitored by the South Dakota DENR. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-18), however, one violation of a water quality criterion for a toxic pollutant in an abundant data set (at least 10 samples over at three-year period) set does not indicate an impairment of aquatic life uses. Based on Iowa DNR's assessment methodology for Section 305(b) reporting, this violation suggests that the Class B(WW) aquatic life uses should be assessed as "fully supported/threatened." Results of monitoring at the Alcester station also showed that one of the 36 samples collected from November 1999 through September 2001 (4%) violated the state Class A and Class B(WW) criterion for pH of 9.0 units: the pH of the sample collected on September 11, 2000, was 9.2 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 (3%) 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 (11.09 mg/l) and water temperature (23.6C) indicating a percent DO saturation of 130%. 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.
