Assessment Comments

Assessment is based on: (1) results of IDNR/UHL TMDL-related monitoring from June through September 2006 at TMDL station NuCr1 (STORET station 11330001) and (2) results of IDNR/UHL biological monitoring in 2006.

Basis for Assessment

[Note:  Prior to the current (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 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), 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 (2008) 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 were assessed (monitored) as "not supported" due to levels of indicator bacteria that exceeded state water quality criteria.   The Class B(WW2) aquatic life uses are assessed (evaluated) as "fully supported" based on results of (1) IDNR/UHL biological (biocriteria) sampling in 2006 (2) IDNR/UHL TMDL-related water quality monitoring from June to September 2006.   The sources of data for this assessment include (1) results of IDNR/UHL TMDL-related monitoring from June through September 2006 at TMDL station NuCr1 (STORET station 11330001) and (2) results of 2006 IDNR/UHL biological monitoring.

EXPLANATION:  The presumptive Class A1 (primary contact recreation) uses were assessed (monitored) as "not supported" due to violations of Iowa’s water quality criteria for indicator bacteria.   Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed.   Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A1 criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses:  the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff."  Due to a change in the Standards in July 2003, E.  coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E.  coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.”  Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E.  coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.  

The geometric mean of E.  coli in the nine samples collected during the 2006 recreational season (March 15 to November 15) at TMDL station NuCr1 was 5,440 orgs/100 ml.; seven of the nine samples (78%) 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 of E.  coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   This assessment is considered a “monitored” assessment (i.e., higher confidence) even though the IDNR assessment/listing methodology requires at least 10 samples be collected over a three-year period for such assessments.   The extremely high geometric mean value of E.  coli in the nine samples (5,440 orgs/100 ml) suggest that additional sampling during the 2006 recreation season would not have lowered the geometric mean such that the Class A1 criterion (126 orgs/100 ml) would have been met.  

The results of IDNR/UHL water quality monitoring at TMDL station NuCr1 from June to September 2006 suggest “full support” of the Class B(WW2) aquatic life uses.   None of the six samples collected violated the Class B(WW2) criteria for dissolved oxygen, pH, or ammonia, and the level of chlorpyrifos analyzed in one sample was below the Class B(WW2) chronic criterion.   According to the IDNR assessment methodology a minimum of ten samples per site is required for a “monitored” (higher confidence) assessment based on results of water quality monitoring.   Assessments developed with fewer than 10 samples are considered “evaluated” (lower confidence) and are not typically appropriate for Section 303(d) listing.   Thus, because fewer than 10 samples were available for the assessment of aquatic life uses at this monitoring station, IDNR considers the assessment of support of the Class B(WW2) uses is of relatively low confidence (i.e., “evaluated”).  

This assessment is also based on biological data collected in 2006 as part of the DNR/UHL stream biocriteria project.   A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data.   The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were 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 2006 FIBI scores were 68 (good), 50 (fair) and the BMIBI scores were 74 (good), 52 (fair).   The FIBI average was 59 and the BMIBI average was 63.   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 1994-2004.   The FIBI BIC for this ecoregion is 52 and the BMIBI BIC for this ecoregion is 61.