Unnamed Tributary to Big Creek Lake IA 04-UDM-6546

Assessment Comments

Assessment is based on results of monitoring at STORET station 13770003 (BCW-8) conducted as part of a Clean Water Act Section 319 water quality project from March to September 2011.

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 presumptively designated as Class A1 and Class B(WW1) waters.  Thus, for the current assessment, perennial flow is presumed, and the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW1) water quality criteria.]

SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported” (IR 2a) based on results of bacteria monitoring in 2011.  The presumptive Class B(WW1) aquatic life uses remain "not assessed" (IR 3a) due to the lack of sufficient water quality information upon which to base an assessment.  The source of data for this assessment is the results of monitoring at STORET station 13770003 (BCW-8) conducted as part of a Clean Water Act Section 319 water quality project from March to September 2011.  

EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as “fully supported" (IR 2a) due to levels of indicator bacteria (E.  coli) that meet the Class A1 geometric mean criterion.  The geometric mean of indicator bacteria (E.  coli) in the seven samples collected at Site BCW-8 during the recreation season of 2011 was 102 orgs/100 ml.  Two of the seven samples (29%) 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 is below the the respective criterion (126 orgs/100 ml for Class A1), the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).  According to the Iowa DNR assessment/listing methodology, too few samples (< 10) were collected during the 2010-12 assessment period in order to determine whether the percentage of samples exceeding Iowa's Class A1 single-sample maximum criterion of 235 orgs/100 ml is significantly greater than 10%.      

The presumptive Class B(WW1) aquatic life uses remain "not assessed".  Other than indicator bacteria, the only other parameter monitored that is supported by Class B(WW1) criteria is ammonia.   However, the supporting water quality data needed to identify violations of the ammonia (water temperature and pH) were not collected as part of this monitoring.  Regardless, six of the seven sample values for ammonia were reported as less than the detection level of 0.05 mg/l.   Thus, due to insufficient parameter coverage, the presumptive Class B(WW1) aquatic life uses remain "not assessed".