Assessment Comments
Assessment is based on results of water quality monitoring from April 2011 to May 2012 at Minor Creek Site 3 at 318th Avenue (STORET station 15960043).
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 (2014) 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” (IR 5p) due to levels of indicator bacteria that exceed state water quality criteria. This is a new impairment for this previously unassessed stream segment. The Class B(WW2) aquatic life uses are assessed (evaluated) as “fully supported” (IR 2a) based on results of water quality monitoring from April 2011 to May 2012. The data used for this assessment are from Minor Creek Site 3 at 318th Avenue (STORET station 15960043).
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 10 samples collected during the recreational seasons of 2011 and 2012 at monitoring station 15960043 were as follows: the 2011 geometric mean of six samples was 612 orgs/100 ml and the 2012 geometric mean of four samples was 3,582 orgs/100 ml. Both geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Seven of the 10 samples (70%) 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.”
Results of chemical/physical water quality monitoring in 2011 and 2012 suggest "full support" of the Class B(WW2) aquatic life uses. Monitoring at station 15960043 showed no violations of Class B(WW2) water quality criteria for ammonia or pH in the six samples analyzed from April to June of 2011 (note: this amount of data is not sufficient for a higher confidence (“monitored”) assessment; thus, the assessment of the Class B(WW2) aquatic life uses is considered “evaluated” (of lower confidence)). One of the six samples analyzed for dissolved oxygen violated the Class B(WW2) criterion of 5 mg/l. The sample collected on June 28 contained 3.7 mg/l of dissolved oxygen and thus violated the Class B(WW2) standard. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as pH and dissolved oxygen suggests impairment of aquatic life uses. Based on IDNR’s assessment methodology, however, these results suggest that the frequency of violations is not significantly greater than 10 percent; thus, these results do not suggest impairment of the Class B(WW2) aquatic life uses. Thus, the Class B(WW2) aquatic life uses are assessed (evaluated) as “fully supported”.
