Walnut Creek IA 02-IOW-1916

Assessment Comments

Assessment is based on monitoring at two sites conducted as part of a Section 319 water quality project from April 2009 through November 2010.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was classified only for general 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 and for Class B(WW1) aquatic life uses.   Thus, for the current (2012) assessment, the available water quality monitoring data was compared to the applicable Class A1 and Class B(WW1) water quality criteria.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to violations of Iowa water quality criteria for indicator bacteria.   This is a new impairment for this previously unassessed segment.   The Class B(WW1) aquatic life uses remain “not assessed” due to the lack of water quality information upon which to base an assessment.   The source of data for this assessment is the results of monitoring at two sites (WC5M and WC6) conducted as part of a Clean Water Act Section 319 water quality project from April 2009 through November 2010.  

EXPLANATION:  The presumptive Class A1 uses are assessed (monitored) as “not supported due to levels of indicator bacteria (E.  coli) that exceed the Class A1 geometric mean criterion.   The geometric means of indicator bacteria (E.  coli) in the 27 samples collected during the recreational seasons of 2009 and 2010 at Section 319 monitoring stations WC5M and WC6 were as follows:

1.   WC5M geometric means were 620 and 394 orgs/100 ml;
2.   WC6 geometric means were 320 and 334 orgs/100 ml.

All of these recreation season geometric means exceed Iowa’s Class A1 geometric mean criterion of 126 E.  coli orgs/100 ml, thus indicating nonsupport of the Class A1 primary contact recreation uses.   From 56 to 74 percent of the 27 samples collected at each site exceeded Iowa’s 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 is greater than the respective criterion (126 orgs/100 ml for Class A1, the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).