Assessment Comments
Assessment is based on: 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 2006 through 2008.
Basis for Assessment
SUMMARY: The Class A1 (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(WW1) aquatic life are assessed (monitored) as "fully supported” based on results of ambient water quality monitoring conducted from 2006 through 2008. 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 2006 through 2008.
Note: A TMDL for pathogen indicators in this segment of the Big Sioux River was prepared by IDNR and the South Dakota Dept. of Environment & Natural Resources; this TMDL was approved by EPA in January 2008. Because all Section 303(d) impairments identified for the 2010 assessment/listing cycle are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).]
EXPLANATION: The Class A1 uses were assessed (monitored) as "not supported" due to levels of indicator bacteria (E. coli) that violate state water quality standards. Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed. The geometric mean level of indicator bacteria (E. coli) in the fifteen samples collected during the recreational seasons of 2006, 2007, and 2008 (58 orgs/100ml) was below the Iowa Class A1 water quality criterion of 126 orgs/100ml. Three of the fifteen 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, and according to IDNR’s assessment/listing methodology, if the geometric mean of E. coli is less than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). However, based on IDNR's assessment methodology two assessment/listing cycles with indicator bacteria levels below the geometric mean criteria are necessary to suggest de-listing of a bacteria impairment. Therefore, the Class A1 uses will remain assessed as "not supported" for the 2010 assessment/listing cycle. If bacteria levels remain below the geometric mean criterion for the 2012 assessment/listing cycle, the bacteria impairment at this river segment will be suggested for de-listing. ”
Despite the historical occurrence of several isolated violations of Class B(WW1) criteria for ammonia, dissolved oxygen, and pH, the results of ambient water quality monitoring from 2006-08 suggest “full support” of the aquatic life uses of this segment of the Big Sioux River. Results of monitoring at the Alcester station by the South Dakota DENR showed no violations of Class B(WW1) water quality criteria for dissolved oxygen (minimum = 8.0 mg/L), pH (range = 7.5 to 8.9), or ammonia (maximum = 1.65 mg/L) in the 36 samples analyzed from 2006-08.
Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.
