Cox Creek IA 01-VOL-303

Assessment Comments

Assessment is based on results of monitoring from May 2011 to November 2012 at station Cox 20 at Hwy 13 (STORET station 15220010).

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 5a) due to violations of Iowa water quality criteria for indicator bacteria.   This is a new impairment for this previously unassessed segment.   The Class B(WW2) aquatic life uses are assessed (monitored) as “fully supported” (IR 2a).   The source of data for this assessment is the results of monitoring from May 2011 to November 2012 at station Cox 20 at Hwy 13 (STORET station 15220010).  

EXPLANATION:  The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) 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 seven monthly samples collected during the recreational season of 2011 (718 orgs/100 ml), and the geometric mean of the eight samples collected during 2012 (1,225 orgs/100 ml) both exceeded the Class A1 criterion of 126 orgs/100 ml.   Eleven of the combined 15 samples (73%) exceeded Iowa’s 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 the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).  

The Class B(WW2) aquatic life uses remain assessed (monitored) as “fully supported.”  No violations of Class B(WW2) water quality criteria for ammonia, dissolved oxygen, pH, or temperature occurred in the approximately 15 samples analyzed from May 2011 to November 2012.