Assessment Comments
Assessment is based on (1) 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 2010 through 2012 and (2) results of IDNR/U.S. EPA fish contaminant monitoring near Hawarden in 2011.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 4a) 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” (IR 2a) based on results of ambient water quality monitoring conducted from 2010 through 2012. Fish consumption are assessed as "fully supporting" (IR 2a) based on results of fish contaminant monitoring in 2011. The sources of data for this assessment are (1) 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 2010 through 2012 and (2) results of IDNR/U.S. EPA fish contaminant (RAFT) monitoring in 2011 near Hawarden.
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 2010 through 2012 at the Big Sioux River near Alcester were as follows: the 2010 geometric mean was 679 orgs/100 ml, the 2011 geometric mean was 114 orgs/100 ml, and the 2012 geometric mean was 87 orgs/100 ml. Only the 2010 geometric mean slightly 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.”
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 2010-12 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 8.8), or ammonia (maximum = 0.7 mg/L) in the 35 samples analyzed from 2010-12. These results are similar to those from the previous IR assessment periods.
Fish consumption uses are assessed (monitored) as “fully supported” based on results of U.S.EPA/IDNR fish contaminant (RAFT) monitoring on the Big Sioux River north of Hawarden in 2011. The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of support of fish consumption uses in Iowa’s rivers and lakes. Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.056 ppm; total PCBs: 0.09 ppm; and technical chlordane: <0.03 ppm. [Note: Typically, samples of both bottom-feeding fish (e.g., common carp) and predator species (e.g., largemouth bass) are collected at RAFT status sites such as the Big Sioux River near Hawarden. Predator species, however, are naturally rare in rivers of southern and western Iowa, and RAFT status samples from these rivers typically contain only the bottom-feeder sample.] The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses. The fish contaminant data generated for this segment of the Big Sioux River from the 2011 RAFT show that the levels of contaminants do not exceed any of the IDPH/IDNR advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory.
