Dibble Creek IA 01-TRK-202

Assessment Comments

Assessment is based on results of monitoring from May 2004 to August 2012 at station DIB10 (Dibble Creek at Union Street (STORET station 15330004).

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 “fully supported” due to low levels of indicator bacteria (IR 1).   The Class B(WW2) aquatic life uses are assessed (monitored) as “fully supported” (IR 1)  The source of data for this assessment is the results of monitoring from May 2004 to August 2012 at station DIB10 (Dibble Creek at Union Street (STORET station 15330004).  

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 (76 orgs/100 ml), and the geometric mean of the five samples collected during 2012 (69 orgs/100 ml) are both well below the Class A1 criterion of 126 orgs/100 ml.   Two of the combined 12 samples (17%) 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 less than 126 orgs/100 ml., the primary contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   Also, if the percentage of samples exceeding the single-sample maximum criterion is significantly greater than 10%, the contact recreation uses should be assessed as impaired.   The percentage of samples exceeding the Class A1 single-sample maximum at station 15330004, however, is not significantly greater than 10% and thus does not suggest impairment.   Thus, both the geometric means of E.  coli and the percentage of samples exceeding the Class A1 single-sample maximum criterion suggest “full support” of the Class A1 (primary contact) uses.   This is one of the few warmwater steams in Iowa to have been monitored for indicator bacteria where the results indicate "full support" of the Class A1 uses.

The Class B(WW2) aquatic life uses are 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 10 samples analyzed from May 2011 to August 2012.