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 2004 through 2006.
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. Despite the occurrence of an unusually high level of ammonia-nitrogen in August, 2004, the Class B(WW1) aquatic life are assessed (monitored) as "fully supported” based on results of ambient water quality monitoring conducted from 2004 through 2006. 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 2004 through 2006.
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 2008 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. Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A1 criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses: the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff." Due to a change in the Standards in July 2003, E. coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E. coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.” Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E. coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.
The geometric mean level of indicator bacteria (E. coli) in the nine samples collected during the recreational seasons of 2004, 2005, and 2006 (156 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml. In addition, three of the nine samples (33%) 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 greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). Although IDNR’s assessment/listing methodology requires that at least 10 samples over a three-year period are needed to develop a “monitored” (i.e., higher confidence) assessment that can support a Section 303(d) listing, the availability of the nine samples for indicator bacteria, along with the listing history of this river segment (i.e., impaired due to indicator bacteria), suggests that the Class A1 uses should remain assessed (monitored) as “not supported.”
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 2004-06 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 that one of the 26 samples collected during the 2004-2006 period (3%) violated the Class B(WW1) criterion for ammonia-nitrogen. The sample collected on August 9, 2004 contained 21.3 mg/l of ammonia-nitrogen; this level is extremely high for an ambient water quality sample and exceeded Iowa’s the Class B(WW1) chronic and acute criteria. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-18), however, one violation of a water quality criterion for a toxic pollutant in an abundant data set (at least 10 samples over at three-year period) set does not indicate an impairment of aquatic life uses. Regardless of U.S. EPA guidance, the level of ammonia in this sample (21.3 mg/l) is one of the highest seen in results of ambient water quality monitoring in the state of Iowa in recent years and suggests the potential for significant adverse water quality impacts in this river segment. Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen or pH in the 26 samples analyzed from 2004-06.
Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.
