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 2008 through 2010.
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 2008 through 2010. 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 2008 through 2010.
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 and current (2012) assessment/listing cycle are addressed by the TMDL, this waterbody is placed in IR Category 4a (impaired; TMDL approved).]
EXPLANATION: The Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 15 samples collected by the South Dakota DENR during the recreational seasons of 2008 through 2010 at the Big Sioux River near Alcester were as follows: the 2008 geometric mean was 43 orgs/100 ml, the 2009 geometric mean was 113 orgs/100 ml and the 2010 geometric mean was 679 orgs/100 ml. The 2010 geometric mean exceeded the Class A1 criterion of 126 orgs/100 ml. Five of the 15 samples (33%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
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 2008-10 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 = 6.6 mg/L), pH (range = 7.2 to 8.9), or ammonia (maximum = 1.08 mg/L) in the 36 samples analyzed from 2008-10.
Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.
