Assessment Comments

Assessment is based on: (1) IDNR monthly ambient monitoring conducted during the 2010-2012 assessment period at the County Road C-70 bridge north of Sioux City (STORET station 10750001 (formerly station 950110)) and (2) IDNR/SHL biological sampling in 2000, 2001 and 2008.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this river segment was designated only for Class B(WW1) aquatic life uses, including fish consumption 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) and the results of an Use Attainability Analysis, this segment is also now designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5a) due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW1) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-u) based on results of biological monitoring in 2000, 2001 and 2008 that suggest potential impairments of the Class B(WW1) aquatic life uses.   Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this river segment.   This assessment is based on results of (1) IDNR monthly ambient monitoring conducted during the 2010-2012 assessment period at the County Road C-70 bridge north of Sioux City (STORET station 10750001 (formerly station 950110)) and (2) IDNR/SHL biological sampling in 2000, 2001 and 2008.

EXPLANATION:  The Class A1 (primary contact recreation) are assessed 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 24 samples collected during the recreational seasons of 2010 through 2012 at the Floyd River near Sioux City were as follows:  the 2010 geometric mean was 789 orgs/100 ml, the 2011 geometric mean was 280 orgs/100 ml and the 2012 geometric mean was 235 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Thirteen of the 21 samples (62%) 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 a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S.  EPA 1997b).   Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”  

Regarding support of the Class B(WW1) aquatic life uses, monitoring at the IDNR/SHL monthly ambient monitoring station north of Sioux City showed no violations of Class B(WW1) water quality criteria for ammonia (maximum = 0.6 mg/L), dissolved oxygen (minimum = 7.5 mg/L), or pH (range = 7.8 to 8.6) in the 36 samples analyzed.   Although violations of Class B(WW1) criteria for ammonia have occasionally occurred during past assessment cycles (most recently in July 2002), the absence of violations during the past eight years of monthly monitoring suggests that levels of ammonia do not represent a water quality concern in this segment of the Floyd River.  

Biological sampling results suggest that the aquatic life uses should be assessed (evaluated) as partially supporting based on IDNR/SHL biological sampling in 2000, 2001 and 2008.   This evaluated biological assessment was based on data collected in 2000, 2001 and 2008 as part of the IDNR/SHL large river sampling project.   A series of biological metrics that 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 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 2000 FIBI score was 45 (fair).   The 2000, 2001 and 2008 BMIBI scores were 29 (poor), 45 (fair) and 56 (good).   The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008.   The FIBI BIC for this ecoregion is 31 and the BMIBI BIC for this ecoregion is 54.   This segment passed the FIBI BIC 1/1 times in 2000 and passed the BMIBI BIC 1/3 times in the last 13 years.   This assessment is considered evaluated because the drainage area (886 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   Even though this site passed the FIBI BIC and failed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size.

This aquatic life assessment is now also 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 in a recent five year period to be considered “monitored”.   This segment had multiple samples collected in the previous 13 years (2000-2010); however, the samples were not collected during a recent five year period.   According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report).   IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation).

Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river reach.