West Nishnabotna River IA 05-NSH-1441

Assessment Comments

Assessment is based on results of IDNR/UHL ambient monthly water quality monitoring near Malvern from 2000-02.

Basis for Assessment

SUMMARY:  The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.   This assessment is based on results of IDNR/UHL monthly ambient water quality monitoring conducted during the 2000-2002 assessment period east of Malvern (STORET station 10650001 (formerly station 822310)).  

EXPLANATION:  Monitoring at the IDNR station near Malvern showed no violations of Class B(WW) water quality criteria during the 2000-2002 assessment period for dissolved oxygen and ammonia-nitrogen (maximum value 1.4 mg/l), and only minor violations for pH, in the 36 samples analyzed during the 2000-2002 period.   One of the 36 samples violated the lower Class B(WW) criterion for pH of 6.5 pH units:  the sample from January 2, 2001, had a pH level of 5.0 units.   This level of pH is atypical for Iowa streams and rivers.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentage of violation for pH at this station (3%) does not suggest a water quality impairment.   These guidelines allow up to 10% violations of conventional parameters such as pH before impairment of water quality is indicated.   In addition, levels of pesticides in the nine 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 fish tissue monitoring in this river segment.