Big Sioux River IA 06-BSR-1524

Assessment Comments

Assessment is based on 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 2006 through 2008.

Basis for Assessment

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "fully supported" based on results of ambient monitoring for indicator bacteria (E.  coli) from 2006 through 2008.   The Class B(WW1) aquatic life are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring from 2006 through 2008.   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 2006 through 2008.  

EXPLANATION:  The Class A1 uses are assessed (monitored) as "fully supported" based on results of ambient monitoring for indicator bacteria (E.  coli) in this river segment.   The geometric mean level of indicator bacteria (E.  coli) in the 15 samples collected (33 orgs/100ml) during the recreational seasons of 2006, 2007, and 2008 is well below the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Two of the 15 samples (13%) 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 less than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S.   EPA 1997b).   The results of monitoring for E.  coli during both the 2002-2004 and the 2004-2006 assessment periods also suggest that levels of indicator bacteria are relatively low in this assessment segment.  

Regarding support of the Class B(WW1) aquatic life uses, results of monitoring at the Hudson station showed that two of the 35 samples collected during the 2006-2008 period (6%) violated the state Class A1 and Class B(WW1) criterion for pH (9.0 units).   These  two samples were collected on July 17, 2006 and September 22, 2008.   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 (6%) 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(WW1) criteria for dissolved oxygen (minimum = 8.6 mg/L) or ammonia-nitrogen (maximum = 0.92 mg/L) occurred at this station in the 36 samples collected during the 2006-2008 assessment period.  

Fish consumption uses remain "not assessed" due to a lack of fish contaminant monitoring in this river segment.