Assessment Comments

Assessment is based on results of IDNR ambient monthly monitoring east of Grimes from 2000 through 2004.

Basis for Assessment

SUMMARY:  The Class B(LR) aquatic life uses remain assessed (monitored) as "partially supported" based on results of ambient chemical/physical water quality monitoring from 2002-04 that show violations of Iowa’s general use criterion for total dissolved solids.   The source of data for this assessment is the results from the IDNR/UHL ambient monthly monitoring station on Beaver Creek east of Grimes (STORET station 10770001 (formerly station 420307) from 2000 through 2004.  

EXPLANATION:  Monitoring at the IDNR/UHL station east of Grimes showed no violations of Class B(LR) water quality criteria during the 2002-2004 assessment period for pH or ammonia-nitrogen in the 36 samples analyzed, for toxic metals in the ten samples analyzed, or for pesticides in the seven samples analyzed.   Two of the 36 samples analyzed for dissolved oxygen during the 2002-04 period, however, violated the Class B(WW) criterion of 5.0 mg/l.   These violations occurred on February 24, 2004 (4.1 mg/l) and on March 5, 2003 (2.1 mg/l).   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 dissolved oxygen nonetheless suggest "full support" of aquatic life uses.   Thus, the percentages of violations of the dissolved oxygen criterion at this station (6%) does not suggest an impairment of aquatic life uses in this stream segment.   Nonetheless, the occurrence of these violations suggests a potential impact.   If these violations continue to occur in future assessment/listing cycle, the aquatic life uses of this waterbody should be assessed as impaired.

Four of the 36 samples analyzed for total dissolved solids from 2000 through 2002 exceeded Iowa’s general use criterion of 750 mg/l.   These samples were collected on October 11, 2000 (960 mg/l), January 10, 2001 (880 mg/l), October 9, 2001 (780mg/l), and January 8, 2002 (860 mg/l).   The similarity in the timing of these violations suggest some type of scheduled, reoccurring releases to this stream.   Based on these results, this stream segment was assessed as “partially supported” for the 2004 assessment/listing cycle.   A review of TDS data for the current (2002-2004) assessment period, however, shows no violations of Iowa’s general use criterion in the 35 samples collected from February 2002 through December 2004 (maximum value = 650 mg/l).   If subsequent monitoring during the next (2004-2006) assessment period continues to show a lack of these violations, good cause would exist for removing this waterbody from Iowa’s Section 303(d) list due to existence of more recent and accurate data that demonstrates that this waterbody meets and maintains applicable water quality standards.