Assessment Comments
Assessment is based on monitoring at three sites conducted as part of a Section 319 water quality project from January 2010 through 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. 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.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” (IR 5p) due to violations of Iowa water quality criteria for indicator bacteria. The Class B(WW1) aquatic life uses are assessed (monitored) as “fully supporting” (IR 2a) based on results of water quality monitoring in 2010 and 2011. The source of data for this assessment is the results of monitoring conducted from January 2010 through November 2011 at two sites: (1) Site 5m at 120th Street (STORET ID 13700006) and (2) Site 6 at 80th Street (STORET ID 13790007). Note: limited amounts of data are available for an additional station monitored in this assessment segment during the 2010-2012 assessment period: Site WLNT3 (STORET station 11790010). Too few data were collected at these stations (two samples over the three-year period) to be useful for developing Section 305(b) water quality assessments. None of the samples collected at this station, however, violated Class B(WW2) water quality criteria.
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as “not supported” due to levels of indicator bacteria (E. coli) that exceed the Class A1 geometric mean criterion. A total of 25 samples were collected at each of the two monitoring stations: 15 samples were collected during the recreation season of 2010, and 10 samples were collected during the recreation season of 2011. The geometric means of indicator bacteria (E. coli) in the 25 samples collected during the recreational seasons of 2010 and 2011 at Section 319 monitoring stations 5m and 6 were as follows:
1. Site 5m geometric means in 2010 and 2011 were 395 and 912 orgs/100 ml, respectively; 20 of 25 samples (80%) exceeded the Class A1 single-sample maximum criterion;
2. Site 6 geometric means in 2010 and 2011 were 334 and 484 orgs/100 ml, respectively; 15 of 25 samples (60%) exceeded the Class A1 single-sample maximum criterion.
All of these recreation season geometric means exceed Iowa’s Class A1 geometric mean criterion of 126 E. coli orgs/100 ml, thus indicating nonsupport of the Class A1 primary contact recreation uses. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean is greater than 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).
The Class B(WW2) aquatic life uses are assessed as “fully supporting” based on results of chemical/physical monitoring in 2010 and 2011. None of the combined 10 samples collected in 2010 and 2011 at the monitoring stations 5m and 6 violated the Class B(WW2) criteria for dissolved oxygen, ammonia-nitrogen (8 samples), pH, or temperature.
