Assessment Comments
Assessment is based on results of water quality monitoring conducted by South Dakota DENR near Alcester, SD, from 2002-04.
Basis for Assessment
SUMMARY: The Class A (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(WW) aquatic life are assessed (monitored) as "fully supported” based on results of ambient water quality monitoring conducted from 2002 through 2004. 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 2002 through 2004.
EXPLANATION: The Class A 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 2006 assessment methodology for indicator bacteria has changed. Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A 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 the 2006 Section 305(b) assessments and Section 303(d) listings.
The geometric mean level of indicator bacteria (E. coli) in the 14 samples collected during the recreational seasons of 2002, 2003, and 2004 (206 orgs/100ml) exceeded the Iowa Class A water quality criterion of 126 orgs/100ml. In addition, seven of the 14 samples (50%) 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). Also, the U.S. EPA guidelines state that if levels of E. coli exceed the single-sample maximum criterion in more than 10% of the samples, the primary contact recreation uses should be assessed as “partially supported.” According to IDNR’s assessment/listing methodology, these results suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum value, thus suggesting that the Class A uses should be assessed as “partially supported/impaired”. Thus, both the geometric mean and the percentage of samples exceeding the single-sample maximum criterion suggest continued impairment of the Class A uses in this assessment segment.
Despite occurrence of several isolated violations of Class B(WW) criteria for ammonia, dissolved oxygen, and pH, the results of ambient water quality monitoring from 2002-04 suggest “full support” of the aquatic life uses of this segment of the Big Sioux River. Results of monitoring at the Alcester station showed that one of the 33 samples collected during the 2002-2004 period (3%) violated the Class B(WW) criterion for ammonia-nitrogen. The sample collected on August 9, 2004 contained 21.3 mg/l of ammonia-nitrogen; this level exceeded Iowa’s the Class B(WW) 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(WW) water quality criteria for dissolved oxygen in the 34 samples analyzed from 2002-04 but did show that one of the 34 samples analyzed for pH exceeded Iowa’s Class A,B(WW) criterion of 9.0 pH units (9.2 units). According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), however, the percentages of violations for dissolved oxygen and pH in this river reach (3%) do not suggest water quality impairments: EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated. These results thus suggest “full support” of the Class B(WW) aquatic life uses.
Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.
