Assessment Comments

Assessment is based on results of IDNR ambient monthly water quality monitoring near Malvern in 2000 & 2001.

Basis for Assessment

[Note:  For the 2002 report, the previous waterbody segment for the West Nishnabotna River (IA 05-NSH-0080-2), which extended 46 miles from Silver Creek in Mills County to East Branch West Nishnabotna R.  in Pottawattamie County, was split into two subsegments:  (1) Silver Creek to Farm Creek near Henderson (IA 05-NSH-0080-1) and (2) Farm Creek to East Branch West Nishnabotna River near Avoca (IA 05-NSH-0080-2).   See above for previous Section 305(b) assessments for the original 46-mile stream segment.]  

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 east of Malvern (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 2000-2001 biennial period for dissolved oxygen and ammonia-nitrogen (maximum value 1.4 mg/l), and only minor violations for pH, in the 24 samples analyzed during the 2000-2001 period.   One of the 24 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 (4.2%) 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 toxic metals in the four samples analyzed, and levels of pesticides in the six 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.