Assessment Comments
Assessment is based on results of IDNR ambient monthly monitoring east of Grimes from 2008 through 2010.
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, 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 (2012) 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 2008-2010. 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 2008 through 2010.
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 means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10770001 were as follows: the 2008 geometric mean was 463 orgs/100 ml, the 2009 geometric mean was 349 orgs/100 ml, and the 2010 geometric mean was 1,484 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Seventeen of the 22 samples (77%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
Monitoring at the IDNR/UHL station east of Grimes showed no violations of Class B(WW2) water quality criteria during the 2008-2010 assessment period for dissolved oxygen, pH, or ammonia-nitrogen in the approximately 30 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). For the 2008-10 period, none of the 29 samples collected exceeded the former general criterion of 750 mg/l (maximum value: 480 mg/l; mean: 377 mg/l).
As part of the revisions to the Iowa Water Quality Standards, the previous TDS general use criterion of 750 mg/l was replaced with criteria for chloride and sulfate. 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. Similarly for the 2008-10 monitoring period, none of the 33 samples analyzed violated Iowa’s Class B(WW2) criteria for chloride or sulfate. The mean and maximum chloride levels were 20.3 and 35 mg/l, respectively; the mean and maximum sulfate levels were 18.8 and 34 mg/l, respectively. Iowa’s Class B(WW2) chloride criterion is 389 mg/l (assumes hardness of 200 mg/l).
