Unnamed Tributary to Dry Run IA 02-CED-6294

Assessment Comments

Assessment is based on results of monitoring conducted from June 2005 through November 2010 at Site DRC-6D1 at Cedar Falls (STORET station 11070008). This assessment may be in error: station DRC-6D1 may not be in this segment.

Basis for Assessment

Note 1:  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.    

Note 2:  Monitoring site DRC-6D1 is located Dry Run Creek and is used to assess segment IA 02-CED-0390_0; thus, this assessment may be in error:  station DRC-6D1 may not be in this segment.  

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 2005 through 2010.   The source of data for this assessment is the results of chemical/physical/bacteria monitoring conducted from June 2005 through November 2010 at Site DRC-6D1 at Cedar Falls (STORET station 11070008).  

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 70 samples (from 7 to 17 per season) collected during the recreational seasons of 2005 through 2010 at Site DRC-6D1 were as follows:  

2005:  17 recreation season samples, geometric mean = 604;
2006:  10 recreation season samples, geometric mean = 343;
2007: 14 recreation season samples, geometric mean = 577;
2008: 7 recreation season samples, geometric mean = 64;
2009: 10 recreation season samples, geometric mean = 613;
2010:  11 recreation season samples, geometric mean = 498.

Only the 2008 geometric mean met the Class A1 criterion of 126 orgs/100 ml.   Over the six recreation seasons monitored, from 29 to 82% 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 2005 through 2010.   No violations of Class B(WW1) criteria for temperature, pH, or ammonia occurred in the approximately 50 samples analyzed at station DRC-6D1 over this period.   pH values ranged from 7.2 to 8.4 and the maximum temperature being only 23.4 C.   Four of the 59 samples analyzed for dissolved oxygen violated the Class B(WW1) criterion of 5.0 mg/l.   All four samples were collected in 2005 with three of the four samples collected during summer months.   According to U.S.  EPA assessment guidelines, if more than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen, the aquatic life (Class B) uses should be assessed as "impaired" (see pgs 3-17 of U.S.  EPA 1997b).   The results from Site DRC-6D1, however, do not suggest that more than 10 percent of the samples violated Iowa’s dissolved oxygen criteria and thus do not suggest an impairment of the Class B(WW1) uses of this stream segment.