Bennett Creek IA 02-IOW-6263

[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 assessment, the available water quality monitoring data was compared to the applicable Class A1 and Class B(WW1) water quality criteria.] 

EXPLANATION:  The presumptive Class A1 uses remain assessed (evaluated) as "not supported" based on results of USGS ambient monitoring near Tama in 2006.  The geometric mean level of indicator bacteria (E. coli) in the six samples collected (2,564 orgs/100ml) far exceeded the Iowa Class A1 water quality criterion of 126 orgs/100ml.  All six samples exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml.  According to U.S. EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if the geometric mean level of E. coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S. EPA 1997b).  Note:  these data were collected in 2006 and thus should have been included in Iowa’s 2008 Section 305(b)/303(d) assessment/listing cycle.  These monitoring data were overlooked by IDNR staff when preparing assessments and identifying impairments for the 2008 cycle. 

Because the data upon which this assessment is based are now older than five years, this assessment is considered of lower confidence (“evaluated).  As data age beyond five years, their ability to accurately characterize current water quality conditions is increasingly suspect.  In addition, the number of bacteria samples collected at this station (six) is relatively low and does not meet IDNR guidelines for developing a “monitored” (higher confidence) assessment (i.e., at least 10 samples collected over a three-year period).  Despite the relatively low number of samples, the relatively high geometric mean of indicator bacteria in this stream segment (2,564 orgs/100 ml) constitutes overwhelming evidence of impairment of the presumptive Class A1 uses.  According to Iowa’s listing methodology, overwhelming evidence of bacterial impairment occurs when the E. coli geometric mean of at least five samples collected at regular intervals over a summer recreational season that meets credible data requirements would exceed Iowa’s geometric mean criterion even if the remainder of the 10 samples needed for a high-confidence (“monitored”) assessment all had E. coli levels equal to the IDNR’s detection level for E. coli (i.e., 10 orgs/100 ml).  If four samples all with E. coli levels of 10 orgs/100 ml are added to the Bennett Creek dataset for 2006, the resulting geometric mean (279 orgs/100 ml) still exceeds the Class A1 geometric mean criterion of 126 orgs/100 ml and thus suggests impairment of the presumptive Class A1 use. 

Results of the USGS ambient water quality monitoring from April through December 2006 showed no violations of Class B(WW1) water quality criteria in the approximately six samples analyzed for dissolved oxygen, pH, and ammonia.  In addition, none of the approximately seven samples analyzed for toxic metals and pesticides exceeded the respective Class B(WW2) chronic criteria.  Thus, the presumptive Class B(WW1) aquatic life uses were assessed (evaluated) as “fully supporting”.