Little Sioux River IA 06-LSR-1565

Assessment Comments

Assessment is based on results of DNR ambient monthly water quality monitoring in 2000 & 2001 near Smithland in Woodbury Co. See attached document for details.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed as "fully supported."  Fish consumption uses remain "not assessed."  This assessment is based on results of IDNR monthly ambient monitoring conducted during the 2000-2001 biennial period NE of Smithland (station 10970001 (formerly station 911078)).   EXPLANATION:  Monitoring at the IDNR station near Smithland showed no violations of Class B(WW) water quality criteria for dissolved oxygen (minimum value = 7.0 mg/l) or ammonia nitrogen (maximum value = 1.4 mg/l) in the 24 samples analyzed during the 2000-2001 period.   One of the 24 samples violated the upper Class B(WW) criterion of 9.0 pH units:  the sample collected on November 2, 1999 contained a pH of 9.2 units.   This violation occurred on a day with a very high level of dissolved oxygen (17.4 mg/l) and a water temperature of 8.5C; these readings correspond to percent DO saturation of approximately 140%.   These conditions suggest that the high level of primary productivity resulted in the high level of pH.   Because this violation is more related to natural conditions than to pollution, the occurrence of the high level of pH in this river segment is not seen as a water quality impairment.   Regardless, the U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentage of violations at this station during the 2000-2001 period for pH (7.1%) does not suggest a water quality impairment.   These guidelines allow up to 10% violations of conventional parameters such as pH and dissolved oxygen before impairment of water quality is indicated.   These results suggest "full support" of the Class B(WW) aquatic life uses.   Fish consumption uses remain "not assessed" due to a lack of recent fish contaminant monitoring in this river reach.