Assessment Comments

Assessment is based on results from the DNR monthly water quality monitoring station at Hwy 59 near Shenandoah in 2000 & 2001.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed as "fully supported."  Fish consumption uses remain assessed as "fully supported."  This assessment is based on results of IDNR monthly (plus) ambient monitoring conducted during the 2000-2001 biennial period near Shenandoah (station 10360001 (formerly station 821008)).   EXPLANATION:  Monitoring at the IDNR station near Shendoah showed no violations of Class B(WW) water quality criteria during the 2000-2001 biennial period for ammonia-nitrogen (maximum value 1.4 mg/l), and minor violations for dissolved oxygen and pH, in the 31 samples analyzed during the 2000-2001 period.   One of the 31 samples violated the Class B(WW) criterion for dissolved oxygen of 5.0 mg/l.   The sample collected on January 2, 2001 contained 4.3 mg/l of dissolved oxygen.   Two of the 31 samples violated the upper Class B(WW) criterion for pH of 9.0 pH units:  the sample from July 11, 2000 had a pH level of 9.1 units, and the sample from August 6, 2001 had a pH level of 9.2 units.   These violations occurred on days with high levels of dissolved oxygen and water temperatures:  July 11, 2000:  15.6 mg/l and 29.1 C; August 6, 2001:  13.4 mg/l and 30.1 C.  (19.2 mg/l); these readings of water temperature and pH correspond to percent DO saturations of > 140% on both sample dates.   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 percentages of violations at this station during the 2000-2001 period for pH (6.7%) and dissolved oxygen (3.2%) do 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.   In addition, levels of toxic metals in the four samples analyzed, and levels of pesticides in the 10 samples analyzed, were all below the analytical level of detection.   These results suggest "full support" of the Class B(WW) aquatic life uses.   Fish consumption uses remain "not assessed" due to the lack of recent fish tissue monitoring in this river segment.