Shell Rock River IA 02-SHL-783

Assessment Comments

Assessment is based on results of IDNR ambient monthly monitoring at Shell Rock, Butler Co. in 2000 & 2001.

Basis for Assessment

[NOTE:  For the 2002 report, the previous waterbody segment IA 02-SHL-0010-2, which extended 43 miles from the south corporate limit of Shell Rock to the Winnebago River, was split into two subsegments:  (1) south corporate limit of Shell Rock to Flood Creek (IA 02-SHL-0010-2) and (2) Flood Creek to Winnebago River (IA 02-SHL-0010-3).   The assessments developed for previous Section 305(b) reports (see above) refer to the previous 43-mile assessment segment.]

SUMMARY:  The Class B(WW) aquatic life uses were assessed as "fully supported."  Fish consumption uses were not assessed.   The sources of data for this assessment is the results monthly monitoring from October 1999 through September 2001 at the IDNR ambient station located at the County Road C45 bridge at Shell Rock, Butler Co.  (station 10120001).   EXPLANATION:  Monitoring at this station showed no violations of Class B(WW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 24 samples collected, for toxic metals in the four samples collected, or in toxic organic compounds in the six samples analyzed during this biennial period.   One of the 24 samples, however, had a pH value greater than 9.0 units:  the sample collected on April 12, 2000, had a pH of 9.1 units.   This violation occurred on a day when the level of dissolved oxygen (15.3 mg/l) was relatively high and the percent DO saturation was approximately 120%.   These conditions suggest that the high level of primary productivity resulted in the high level of pH.   Because these violations are more related to natural conditions than to pollution, the occurrence of high levels of pH in this river segment is not seen as a water quality impairment.   Fish consumption uses were not assessed due to lack of recent fish tissue monitoring in this river segment.