Assessment Comments

Assessment remains based on results of (1) IDNR/UHL chemical/physical water quality monitoring conducted from May 2004 to November 2006 and (2) IDNR bacterial monitoring from January 2008 to September 2008 at the County Road W4B crossing (STORET station 15030008) as part of the Yellow River Watershed Project.

Basis for Assessment

[Note:  Prior to the 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, 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.]

SUMMARY:  The presumptive Class A1 (primary contact recreation) uses remain assessed (monitored) as “not supported” (IR 4a) due to levels of indicator bacteria that exceed state water quality criteria.   The presumptive Class A2 (secondary contact recreation) uses, however, remain assessed (monitored) as “fully supported” (IR 2a).   The Class B(CW1) aquatic life uses remain assessed (monitored) as "not supported" (IR 5a) due to violations of the water quality criterion for dissolved oxygen.   Fish consumption uses remain “not assessed” (IR 3a) due to the lack of fish contaminant monitoring in this stream segment.   The source of data for this assessment is (1) 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 and (2) results of IDNR/UHL monitoring for indicator bacteria at station 15030008 from April 2006 to September 2008.  

Note: A TMDL for the bacterial impairment for streams in the Yellow River basin was prepared by Iowa DNR in 2012 and was approved by U.S.  EPA in February 2013 (see http://www.iowadnr.gov/Environment/WaterQuality/WatershedImprovement/WatershedResearchData/WaterImprovementPlans/PublicMeetingsPlans.aspx).  Due to completion of the TMDL, the Integrated Report category for the Class A1 recreational uses is changed from 5p (impaired; TMDL needed) to 4a (impaired; TMDL approved).  Because not all impairments identified for this assessment segment are covered by the TMDL (biological impairment of aquatic life uses due to low dissolved oxygen), this segment remains in Category 5a of Iowa's Integrated Report (impaired and TMDL required).

EXPLANATION:  The presumptive Class A1 (primary contact recreation) uses remain 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.”  The geometric mean of E.  coli in the 38 samples collected at the YRWP site at county road W4B (station 15030008) during the recreational seasons of 2006 through 2008 was 278 orgs/100 ml.   This geometric mean exceeds the Class A1 criterion of 126 orgs/100 ml.   Seventeen of the 38 samples (45%) 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).  

The geometric mean of the 47 samples collected at this station during calendar years of 2006 through 2008 was 149 orgs/100 ml; this geometric mean is far below the Class A2 criterion of 630 orgs/100 ml.   Three of the 47 samples (6%) 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, the contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).  

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.   Note:  although IDNR station 15030008 was monitored for E.  coli through September 2008, monitoring for dissolved oxygen was not conducted after November 2006.  

Water temperatures measured in this coldwater stream from April 2004 through November 2006, while not indicating impairment, do suggest a potential impact on coldwater aquatic life.   Six of the 79 temperature measurements (8%) exceeded the Class B(CW1) maximum temperature criterion of 20C.   All violations occurred during summer months, especially July.   According to U.S.  EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if less than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen, the aquatic life uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).   Thus, According to IDNR’s assessment/listing methodology, the results from YRWP site 15030008 (8% violation) do not indicate that significantly greater than 10% of the samples exceed the Class B(CW1) criterion for water temperature.   Regardless, water temperatures in Norfolk Creek tend to be on the warm side for a coldwater stream and may suggest a need to reexamine this streams designation as a coldwater stream in the Iowa water quality standards..  

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