Assessment Comments
Assessment is based on results of water quality monitoring by South Dakota DENR near Hudson, SD. from 2002-04.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "fully supported" based on results of ambient monitoring for indicator bacteria (E. coli) from 2002 through 2004. The Class B(WW) aquatic life are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring 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 Hudson, 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 "fully supported" based on results of ambient monitoring for indicator bacteria (E. coli) in this river segment. 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 15 samples collected (37 orgs/100ml) during the recreational seasons of 2002, 2003, and 2004 is well below the Iowa Class A water quality criterion of 126 orgs/100ml. Three of the 15 samples exceeded Iowa’s single-sample maximum value of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting (pgs 3-33 to 3-35of U.S. EPA 1997b), if levels of E. coli exceed the single-sample maximum value 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, however, these results do not suggest that the violation frequency of Iowa’s single-sample maximum criterion is significantly greater than 10%, and thus these results do not suggest impairment of the Class A uses of this river segment. The results of monitoring for E. coli during the 2002-2004 period suggest that levels of indicator bacteria are relatively low in this assessment segment.
Note: despite the apparent difference in the current (2006) assessment of support of Class A uses for this river segment (“full support”) compared to the assessment for the previous (2004) assessment cycle (“partial support”), both the 2006 and 2004 assessments indicate low levels of indicator bacteria and “full support” of the Class A primary contact recreation uses. Similar to the current (2006) assessment, the geometric mean of indicator bacteria (fecal coliforms) calculated for the previous (2004) assessment (121 orgs/100 ml) was well-below the Iowa Class A criterion of 200 orgs/100 ml. The impairment of the Class A uses for the 2004 cycle was based instead based on the percentage of samples that exceeded the U.S. EPA-recommended single-sample maximum value: two of the 11 samples (18%) exceeded the EPA-recommended single-sample maximum value of 400 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of the samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are "partially supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). If, however, Iowa’s current (2006) assessment and listing methodology had been applied to these results, the percentage of samples exceeding the EPA-recommended single-sample maximum value would not have been significantly greater than 10% and thus the Class A uses of this river segment would have been assessed as “fully supported”.
Regarding support of the Class B(WW) aquatic life uses, results of monitoring at the Hudson station showed that three of the 36 samples collected during the 2002-2004 period (9%) violated the state Class A and Class B(WW) criterion for pH (9.0 units). These three samples were collected on April 14, 2003, October 8, 2003, and September 1, 2004. 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 (9%) 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(WW) criteria for dissolved oxygen or ammonia-nitrogen occurred at this station during the 2002-2004 assessment period.
Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.
