Assessment Comments
Assessment remains based on 2005 IDNR/SHL biological sampling data: FIBI = 49 (fair), BMIBI = 68 (good). Non-riffle substrate FIBI BIC = 44; artificial substrate BMIBI BIC = 52. Assessment is also based on results of ambient monitoring in 2010 and 2011 at 61st Street (STORET station 15060001).
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 (monitored) as “not supported” (IR 5p) due to violations of Iowa water quality criteria for indicator bacteria. This is a new impairment for this assessment segment. The Class B(WW2) aquatic life uses are assessed (evaluated) as "fully supported" (IR 2a) based on results of IDNR/SHL biological (biocriteria) sampling in 2005 and based on results of water quality monitoring at STORET station 15060001 from May 2010 to July 2011.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 11 samples collected at station 15060001 during the recreational season of 2010 (655 orgs/100 ml), and the geometric mean of the 12 samples collected during the recreation season of 2011 (638 orgs/100 ml) both exceeded the Class A1 criterion of 126 orgs/100 ml. Twenty-one of the combined 23 samples (91%) exceeded Iowa’s 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 geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
The Class B(WW2) aquatic life uses remain assessed (evaluated) as "fully supporting" based on data collected in 2005 as part of the IDNR/SHL stream REMAP project. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2005 FIBI score was 49 (fair) 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 non-riffle substrate FIBI BIC for this ecoregion is 44 and the artificial substrate BMIBI BIC for this ecoregion is 52. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. This segment passed both the FIBI and BMIBI BICs in 2005.
This aquatic life assessment is considered "evaluated" because there were not two or more samples collected from this segment in multiple years over a five-year period. Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). Despite this change in assessment methodology and type, this waterbody remains in IR Category 2a.
The results of ambient water quality monitoring at station 15060001 in 2010 and 2011 indicate good chemical water quality in this stream segment, are consistent with the results of biological monitoring, and thus suggest “full support” of the Class B(WW2) aquatic life uses. No violations of Class B(WW2) water quality criteria for dissolved oxygen, pH or temperature occurred in the 23 samples analyzed from May 2010 to July 2011.
