Prairie Creek IA 03-SSK-6598

Assessment Comments

Assessment is based on the results of monitoring at Iowa DNR monitoring station SC-13 (STORET station 908022) conducted as part of Iowa DNR water quality snapshot monitoring from 2008 through 2012.

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 as “not supporting” (IR 5p) due to levels of indicator bacteria that exceed Iowa water quality criteria.   The presumptive Class B(WW1) aquatic life uses remain “not assessed” (IR 3a) due to lack of information upon which to base an assessment.   The source of data for this assessment is the results of monitoring at Iowa DNR monitoring station SC-13 (STORET station 908022) conducted as part of Iowa DNR water quality snapshot monitoring from 2008 through 2012.  

EXPLANATION: The Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria.   The geometric mean of indicator bacteria (E.  coli) in the nine samples collected during the recreational seasons of 2008 through 2012 at station SC-13 was 1,070 orgs/100 ml, thus exceeding the Class A1 criterion of 126 orgs/100 ml.   All nine of the samples (100%) 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 the geometric mean of at least 10 samples collected over five years of snapshot monitoring exceeds the respective water quality criterion, the contact recreation uses should be assessed as "not supported".   Although only nine samples were collected over the five-year period, the reltively high geometric mean and the occurence of greater than the single-sample maximum criterion (235 orgs/100 ml) in all nine samples suggests that the presumptive Class A1 uses should be assessed as "not supporting" (impaired).   Thus, the Class A1 uses are assessed as “impaired.”  

None of the other parameters collected at this station over the 2008-2012 period have Class B(WW1) water quality criteria to which monitoring results can be compared.   Thus, the presumptive Class B(WW1) aquatic life uses of this segment remain "not assessed".