Brush Creek IA 01-VOL-317

Assessment Comments

Assessment based on results of (1) IDNR project monitoring at county road C24 (site BRU 10, STORET station 15330002) from May 2011 to November 2012 and (2) 2005 & 2012 IDNR/SHL biological sampling data: FIBI = 75, 80 (excellent), BMIBI = 62 (good), 77 (excellent): FIBI BIC = 52, BMIBI BIC = 61.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life 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.   The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses).   Thus, for the current assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses are assessed as “fully supporting" (IR 1) based on results of bacteria monitoring during 2011 and 2012.   The Class B(WW2) aquatic life uses are assessed (evaluated) as "fully supporting"  (IR 1) based on results chemical/physical water quality monitoring during 2011 and 2012 and on results of IDNR/SHL biological sampling in 2005 and 2012.   The sources of data for this assessment include results of (1) IDNR project monitoring at county road C24 (site BRU 10, STORET station 15330002) from May 2011 to November 2012 and (2) IDNR/SHL biological sampling in 2005 and 2012.   This is one of the few Iowa waterbodies where all designated uses are assessed as "fully supporting" and thus this waterbody is placed in Category 1 of Iowa's 2014 Integrated Report.

EXPLANATION:  The presumptive Class A1 (primary contact recreation) uses are assessed as "fully supported" based on levels of indicator bacteria are below, and thus meet, state water quality criteria.   The geometric means of indicator bacteria (E.  coli) in the 15 samples collected during the recreational seasons of 2011 and 2012 at monitoring station BRU 10 were as follows:  the 2011 geometric mean of seven samples was 52 orgs/100 ml and the 2012 geometric mean of eight samples was 113 orgs/100 ml.   Both geometric means are below the Class A1 criterion of 126 orgs/100 ml.   Three of the 15 samples (20%) 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 the recreation season geometric means are below the respective water quality criterion, the contact recreation uses should be assessed as "fully supporting" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).  

Further, if the percentage of samples with levels of indicator bacteria that exceed the single-sample maximum criterion is significantly greater than 10%, the contact recreation uses should be assessed as impaired.   According to the IDNR assessment/listing methodology, however, the percentage of samples exceeding the Class A1 single-sample maximum at station BRU 10 is not significantly greater than 10% and thus does not suggest impairment.   Thus, both the geometric means of E.  coli and the percentage of samples exceeding the Class A1 single-sample maximum criterion suggest “full support” of the Class A1 (primary contact recreation) uses.   This is one of the few monitored warmwater small streams in the state where bacteria monitoring has shown “full support” of Class A1 (primary contact recreation) uses.

Results of both chemical/physical and biological monitoring suggest that the Class B(WW2) aquatic life uses of this segment of Brush Creek should be assessed as “fully supporting”.   Results of chemical/physical water quality monitoring in 2011 and 2012 do not suggest impairment of the Class B(WW2) aquatic life uses.   Monitoring at station BRU 10showed no violations of Class B(WW2) water quality criteria for ammonia or dissolved oxygen in the 15 samples analyzed.   One of the 15 samples (7%) analyzed violated the Class B(WW2) the pH criterion of 9.0 pH units.   The sample collected on June 1, 2011 had a pH of 9.7 units.   According to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), a violation frequency of greater than 10% for conventional parameters such as pH suggests impairment of aquatic life uses.   Based on IDNR’s assessment methodology, however, these results suggest that the frequency of violations for this parameters in this segment of Brush Creek is not significantly greater than10 percent; thus, these results do not suggest impairment of the Class B(WW2) aquatic life uses.   Thus, the Class B(WW2) aquatic life uses are assessed (monitored) as “fully supported”.

In addition to results of chemical/physical monitoring, results of IDNR/SHL biological sampling conducted in 2005 and 2012 as part of the stream biocriteria project also suggest “full support” of the aquatic life uses.   The 2005 FIBI score was 80 (excellent) and the BMIBI score was 62 (good).   The 2012 FIBI score was 75 (excellent) and the BMIBI score was 77 (excellent).   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 FIBI BIC for this ecoregion is 52 and the BMIBI BIC for this ecoregion is 61.   This segment passed the FIBI and BMIBI BICs 2/2 times in the last eight years.   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 five-year period to be considered “monitored”.   This segment had multiple samples collected in the previous eight years (2005-2012); however, the multiple samples were not collected during a five-year period.   Despite this change in assessment methodology and type, this waterbody remains in IR Category 2a.