Assessment Comments

Assessment is based on results of IDNR/UHL chemical/physical water quality monitoring conducted from May 2004 to November 2006 at the County Road W4B crossing (STORET station 15030008) as part of the Yellow River Watershed Project.

Basis for Assessment

[Note:  Prior to the current (2008) Section 305(b) cycle, this stream segment was designated only for Class B(CW) 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 also now presumptively designated for Class A1 (primary contact recreation) and Class A2 (secondary contact recreation) uses.   This segment remains designated for coldwater aquatic life use (now termed Class B(CW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption) uses.   This stream segment also remains identified as a “HQR” (high quality resource) water.]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as “not supported” due to levels of indicator bacteria that exceed state water quality criteria.   The Class A2 (secondary contact recreation) uses, however, are assessed (monitored) as “fully supported.”  The Class B(CW1) aquatic life uses are assessed (monitored) as "not supported" due to violations of the water quality criterion for dissolved oxygen.   Fish consumption uses are “not assessed” due to the lack of fish contaminant monitoring in this stream segment.   The source of data for this assessment is the results of IDNR/UHL chemical/physical water quality monitoring conducted from May 2004 to November 2006 at the County Road W4B crossing (STORET station 15030008) as part of the Yellow River Watershed Project.  

EXPLANATION:  The Class A1 (primary contact recreation) uses are assessed (monitored) as impaired due to violations of Iowa’s water quality criteria for indicator bacteria.   The Class A2 (secondary contact recreation) uses, however, are assessed (monitored) as “fully supported.”  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 71 samples collected at the YRWP site at county road W4B (station 15030008) during the recreational seasons of 2004 through 2006 was 312 orgs/100 ml.   This geometric mean exceeds the Class A1 criterion of 126 orgs/100 ml.   Thirty-six of the 71 samples (54%) exceeded Iowa’s 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 geometric mean of E.  coli is greater than the applicable state criterion, the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   Thus, because the geometric mean for IDNR/UHL station 15030008 exceeds the Class A1 geometric mean criterion, these uses are assessed as “not supported.”  

The geometric mean of the 80 samples collected at this station during calendar years of 2004 through 2006 was 232 orgs/100 ml; this geometric mean is below the Class A2 criterion of 630 orgs/100 ml.   Eight of the 80 samples (10%) exceeded Iowa’s Class A2 single-sample maximum criterion (2,880 orgs/100 ml.).   According to U.S.  EPA guidelines for Section 305(b) reporting, if the geometric mean is less than the applicable state criterion, and if less than 10% of samples exceed the single-sample maximum criterion, the contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   According to IDNR’s assessment/listing methodology, the results from this monitoring station suggest that the number of samples exceeding the Class A2 single-sample maximum criterion is not significantly greater than 10%.   Thus, because both the Class A2 geometric mean and single-sample criteria were met, these results suggest that the Class A2 uses should be assessed as “fully supported”.

The results of the IDNR/UHL water quality monitoring from May 2004 to November 2006 show 26 violations of the Class B(CW1) criteria for dissolved oxygen (7.0 mg/l) in the 79 samples collected for a violation frequency of 33%.   The minimum level of dissolved oxygen was 3.4 mg/l.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), impairment of aquatic life uses is indicated if criteria are exceeded in more than 10% of the samples for conventional parameters (e.g., pH, temperature, or dissolved oxygen).   Because the percentage of violations of the dissolved oxygen criterion in this stream segment suggests that significantly greater than 10% of the samples violated the Class B(CW1) criterion, the coldwater aquatic life uses are assessed as impaired.  

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