North Skunk River IA 03-NSK-854

Assessment Comments

Assessment is based on results of IDNR/UHL ambient water quality monitoring southwest of Sigourney from 2002-04.

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses are assessed as "fully supporting " based on results of ambient water quality monitoring.   The assessments of support of beneficial uses are based on results of IDNR ambient monthly water quality monitoring conducted on the North Skunk River southwest of Sigourney in Keokuk Co.  (STORET station 10540001) during the 2002-2004 assessment period.   Data from this monitoring station were also used to assess support of the Class B(WW) uses of the the adjacent downstream segment of the North Skunk River (IA 03-NSK-0010-1:  mouth to the confluence with Cedar Creek near Sigourney).

EXPLANATION:  None of the approximately 35 samples collected during the 2002-2004 assessment period violated Class B(LR) water quality criteria for dissolved oxygen or ammonia-nitrogen.   None of the approximately 10 samples analyzed for pesticides or toxic metals violated state water quality criteria.   One of the 35 samples analyzed for pH, however, exceeded the Class B(LR) criterion of 9.0 pH units (this sample contained a pH of 10.3 pH units).   According to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), however, a violation frequency of less than 10 % for conventional parameters such as pH nonetheless suggest "full support" of aquatic life uses.   Thus, the percentages of violations of the dissolved oxygen criterion at this station (3%) does not suggest an impairment of aquatic life uses in this stream segment.   Although one of the four samples analyzed for toxic metals during the previous (2004) assessment period exceeded the Class B(LR) chronic criterion for lead, none of the 10 samples analyzed during the current (2006) assessment period exceeded criteria for lead or any other toxic metal.  Thus, the level of support of the Class B(LR) uses was upgraded from “fully supported / threatened” in the 2004 assessment to “fully supported” for the current (2006) assessment.