Assessment Comments
Assessment is based on results of monthly ambient water quality monitoring conducted on the Big Sioux River near Brandon, 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 uses are assessed (monitored) as "fully supported" (IR 2a) based on results of ambient chemical/physical water quality monitoring from 2008 through 2010. Fish consumption uses remain not assessed (IR 3a) due to a lack of information upon which to base an assessment. The source of data for this assessment is results of monthly ambient water quality monitoring conducted on the Big Sioux River near Brandon, 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. Due to the 2008 approval of the TMDL for bacteria impairments in the Big Sioux River, however, this impairment is placed in Category 4a of Iowa's Integrated Report (impaired but TMDL not required). 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 Brandon were as follows: the 2010 geometric mean was 144 orgs/100 ml, the 2011 geometric mean was 257 orgs/100 ml, and the 2012 geometric mean was 116 orgs/100 ml. The 2010 and 2011 geometric means slightly exceed 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.”
Regarding support of the Class B(WW1) aquatic life uses, results of monitoring at the Brandon station during the 2008-2010 period show no violations of Class B(WW1) criteria for dissolved oxygen (minimum = 8.1 mg/L), pH (minimum: 7.3, maximum: 9) or ammonia (maximum = 0.91 mg/L) in the 36 samples collected. These results suggest that the Class B(WW1) aquatic life uses should be assessed as "fully supported."
