Rock Creek IA 02-CED-586

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses.  Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is also now presumptively designated for Class A1 (primary contact recreation) uses.  This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" due to violations of Iowa’s water quality criteria for indicator bacteria.  The geometric means of indicator bacteria (E. coli) in the 14 samples collected during the recreational seasons of 2013 and 2014 at station 14000132 were as follows:  the 2013 geometric was 182 orgs/100 ml and the 2014 geometric mean was 271 orgs/100 ml.  Both geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml.  Seven of the 14 samples (50%) 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 should be assessed as "impaired".  Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.” 

Results of biological sampling in 2002, 2009 and 2013 suggest that the aquatic life uses should be assessed (evaluated) as “fully supported”.  This assessment was based on biological data collected in 2002, 2009 and 2013 as part of the IDNR/SHL stream biocriteria project.  The 2002 FIBI score was 72 (excellent) and the BMIBI score was 72 (good).  The 2009 FIBI score was 76 (excellent) and the BMIBI score was 80 (excellent).  The 2013 FIBI score was 80 (excellent) and the BMIBI score was 68 (good).  The aquatic life use support was assessed as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports.  The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008.  The riffle habitat FIBI BIC for this ecoregion is 65 and the natural substrate BMIBI BIC for this ecoregion is 70.  Overall, this segment passed the FIBI BIC 3/3 times and passed the BMIBI BIC 2/3 times in the last 13 years.  The biological assessment was conducted on August 21, 2002, approximately one month following the July fish kill (see below).  Thus, the potential exists that the kill influenced results of the biological assessment and may be related to the comment on the field sheet that numbers of fish were relatively low. 

This aquatic life assessment is now considered "evaluated" based on a change in the 2010 IDNR assessment methodology.  IDNR now requires a segment have two or more biological samples collected from the segment in multiple years over a recent five-year period to be considered “monitored”.  This segment had multiple samples collected in a recent 13 year period (2002-2014). 

The Class B(WW1) aquatic life uses are also assessed as “fully supported” based on results of chemical/physical monitoring at station 14000132.  Results of ambient water quality monitoring at this station during 2013 showed no violations of Class B(WW1) water quality criteria in the eight samples analyzed for ammonia, dissolved oxygen, pH, and water temperature.  

A fish kill occurred on in this stream segment on July 27, 2002.  The kill occurred after a rainfall event that followed a period of hot, dry weather.  No cause or source was determined, although area feedlots were a suspected source of organic matter that potentially degraded water quality and caused the kill.  Approximately 1 mile of stream was affected, and an estimated 970 fish were killed.  According to IDNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill within the most recent three-year period indicates ongoing impairment of the aquatic life uses.  Also, according to IDNR’s assessment methodology, if a cause of a fishkill was not identified during the IDNR investigation (= "unknown"), or if the kill was attributed to non-pollutant causes (e.g., winterkill), the assessment type will be considered “evaluated.”  Such assessments, although suitable for Section 305(b) reporting, lack the degree of confidence to support addition to the state Section 303(d) list of impaired waters (Category 5 of the Integrated Report).  Waterbodies affected by such fish kills would be placed in IR subcategories 2b or 3b and could be added to the state’s list of “waters in need of further investigation”. 

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.