Dry Run IA 02-CED-6293

Assessment Comments

Assessment is based on results of monitoring from March 2006 through November 2010 conducted at station DRC-9 of the Dry Run Creek project.

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.    According to the Iowa Water Quality Standards, all perennial rivers and streams and all intermittent streams with perennial pools that are not specifically listed in the Iowa surface water classification are designated as Class A1 and Class B(WW1) waters.    

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported” due to levels of indicator bacteria that exceed state water quality criteria.   The presumptive Class B(WW1) aquatic life uses are assessed (monitored) as “fully supported” based on data collected from 2006 through 2010.   The source of data for this assessment is the results of chemical/physical/bacteria monitoring conducted on Dry Run Creek from January 2006 through November 2010 at Site DRC-9 at Union Road (STORET station 15070006).   Note:  due to data management issues, this stream segment was not assessed for the previous (2010) Section 305(b) cycle.

EXPLANATION:  The presumptive Class A1 (primary contact recreation) 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 62 samples (from 7to 17 per season) collected during the recreational seasons of 2006 through 2010 at Site DRC-9 were as follows:  

2006:  17 recreation season samples, geometric mean = 296;
2007: 17 recreation season samples, geometric mean = 254;
2008: 7 recreation season samples, geometric mean = 94;
2009: 10 recreation season samples, geometric mean = 906;
2010:  11 recreation season samples, geometric mean = 394.

Only the 2008 geometric mean met the Class A1 criterion of 126 orgs/100 ml.   Over the five recreation seasons monitored, from 29 to 80% of the samples 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.”  

The presumptive Class B(WW1) aquatic life uses are assessed (monitored) as “fully supported” based on results of chemical/physical monitoring conducted for the Dry Run Creek Watershed project from 2006 through 2010.   No violations of Class B(WW1) criterion for dissolved oxygen, temperature, pH, or ammonia occurred in the approximately 54 samples analyzed at station DRC-9 over this period.   These data suggest a relatively stable headwater aquatic environment in terms of chemical/physical water quality, with pH values ranging only from 7.1 to 8.4 and the maximum temperature being only 20.1 C.