Assessment Comments

Assessment is based on results of IDNR ambient monthly monitoring east of Grimes from 2006 through 2008.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life uses.   Due to changes in Iowa’s surface water classification that were approved by U.S.  EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is now presumptively designated for Class A1 (primary contact recreation) uses.   The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses).   Thus, for the current (2010) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW2) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring from 2006-08.   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 2006 through 2008.  

EXPLANATION:  The presumptive Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.   coli).   The geometric mean level of indicator bacteria (E.   coli) in the 22 samples collected (302 orgs/100ml) during the recreational seasons of 2006 through 2008 exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Fourteen of the 22 samples (64%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if the geometric mean level of E.   coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).  

Monitoring at the IDNR/UHL station east of Grimes showed no violations of Class B(WW2) water quality criteria during the 2006-2008 assessment period for dissolved oxygen, pH, or ammonia-nitrogen in the 34 samples analyzed or for toxic metals in the two samples analyzed.   These results suggest that the Class B(WW2) aquatic life uses should be assessed as “fully supported.”  

This stream segment has been previously identified as Section 303(d)-impaired due to levels of total dissolved solids (TDS) that exceeded Iowa’s general water quality of 750 mg/l.   Due to occasional violations of Iowa’s former TDS criterion, this stream segment was assessed as “partially supported” for the 2004 assessment/listing cycle and added to Iowa’s 2004 Section 303(d) list.   A review of TDS data from 2002 through 2004 for the 2006 assessment/listing cycle, however, showed no violations of Iowa’s general use criterion in the 35 samples collected from 2002 through 2004.   Although retained on Iowa’s 2006 Section 303(d) list, IDNR’s 2006 assessment for this stream noted that if subsequent monitoring during the next [2004-2006] assessment period continued 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.   Thus, this impairment was removed (de-listed) from Iowa’s 2008 Section 303(d) list.   None of the 33 samples collected at station 10770001 during the 2006-08 assessment period exceeded the former general criterion of 750 mg/l (maximum value during the 2006-08 period was 490 mg/l).

As part of the revisions to the Iowa Water Quality Standards, the previous TDS general use criterion of 750 mg/l will be replaced with criteria for chloride and sulfate.   Although not yet fully adopted into the Iowa Standards, data for chloride and sulfate from the period 2006-08 from station 10770001 at Beaver Creek were compared to the proposed criteria for chloride and sulfate.   None of the 33 samples collected during this period contained either anion at levels that violated the proposed criteria.   The average level of chloride in the 33 samples was 27 mg/l with a maximum value of 44 mg/l; the average value of sulfate was 23.8 mg/l with a maximum value of 41 mg/l.