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 2010 through 2012.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 4a) due to violations of Iowa’s water quality criteria for indicator bacteria. The Class B(WW1) aquatic life are assessed (monitored) as "fully supported" (IR 2a) based on results of ambient chemical/physical water quality monitoring from 2010 through 2012. Fish consumption uses remain "not assessed" (IR 3a) 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 2010 through 2012.
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 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 during the recreational seasons of 2010 through 2012 at the Big Sioux River near Hudson were as follows: the 2010 geometric mean was 470 orgs/100 ml, the 2011 geometric mean was 121 orgs/100 ml, and the 2012 geometric mean was 75 orgs/100 ml. Only the 2010 geometric mean exceeded the Class A1 criterion of 126 orgs/100 ml. Eight of the 15 samples (53%) 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.” Note: The 2010 geometric mean of 470 org/100 ml is the only indication of impairment of the Class A1 uses of this segment over the last several IR listing cycles. Levels of idnciator in this segment of the Big Sioux River are typically very low relative to other streams and rivers in or bordering Iowa.
Regarding support of the Class B(WW1) aquatic life uses, results of monitoring at the Hudson station showed that one of the 36 samples collected during the 2010-2012 period (3%) violated the state Class A1 and Class B(WW1) criterion for pH (9.0 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). No violations of Class B(WW1) criteria for dissolved oxygen (minimum = 6.5 mg/L), temperature (maximum = 29C), or ammonia-nitrogen (maximum = 0.5 mg/L) occurred at this station in the 36 samples collected during the 2010-2012 assessment period.
Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.
