Assessment Comments

Assessment is based on: (1) results of IDNR/SHL biological sampling in 2004 and 2012, (2) results of IDNR/SHL monthly ambient monitoring station on the Chariton River downstream from Rathbun Dam during the 2010-2012 assessment period (STORET station 10040001), (3) results from Rathbun Watershed station RA-28 at the reservoir outlet, and (4) results of U.S. EPA / IDNR fish contaminant (RAFT) monitoring north of Centerville in 2011 and 2013.

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this river segment was designated for Class B(WW) aquatic life uses, including fish consumption uses and for Class C (drinking water) 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 due to the completion of a Use Attainability Analysis in 2006, 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), for fish consumption uses (now termed Class HH (human health/fish consumption uses), and for Class C (drinking water) uses.]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5a) based on monitoring for indicator bacteria (E.  coli).   The Class B(WW1) aquatic life uses are assessed (evaluated) as "fully supporting" (IR 2a) based on results of chemical, physical, and biological monitoring.   The Class C (drinking water) uses are assessed (monitored) as "fully supporting" (IR 2a) based on results of ambient water quality monitoring for pesticides, toxic metals, and nitrate.   Fish consumption uses are assessed (monitored) as "fully supported" (IR 2a) based on results of fish contaminant monitoring in 2011 and 2013.   The sources of data used for this assessments include (1) results of IDNR/SHL biological REMAP sampling in 2004 and large river biological sampling in 2012, (2) results of IDNR/SHL monthly ambient monitoring station on the Chariton River downstream from Rathbun Dam during the 2010-2012 assessment period (STORET station 10040001), (3) results from Rathbun Watershed station RA-28 at the reservoir outlet, and (4) results of IDNR fish contaminant (RAFT) monitoring north of Centerville in 2011 and 2013.  

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 ambient monthly monitoring site on the Chariton River near Centerville were as follows:  the 2010 geometric mean was 31 orgs/100 ml, the 2011 geometric mean was 83 orgs/100 ml and the 2012 geometric mean was 235 orgs/100 ml.  Only the 2012 geometric mean exceeded the Class A1 criterion of 126 orgs/100 ml.   Five of the 24 samples (21%) 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.”  

The Class B(WW1) aquatic life uses remain assessed as "fully supporting" based on results of IDNR/SHL biological sampling conducted in 2004 and 2012 as part of REMAP and large river sampling projects.   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 benthic macroinvertebrate index (BMIBI) and fish index of biotic integrity (FIBI).   The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).   The 2004 REMAP FIBI score was 26 (fair) and the BMIBI score was 47 (fair).   The 2012 BMIBI score was 52 (fair).   The aquatic life use support was assessed (evaluated) as fully supporting (=FS), 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 33 and the BMIBI BIC for this ecoregion is 41.   This segment passed the FIBI BIC 1/1 times (using the uncertainty adjustment value (UAV) of seven points) in 2004 and passed the BMIBI BIC 3/3 times in the last nine years.   This assessment is considered evaluated because the drainage areas (555 and 550 mi2) above the sampling sites were greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria.   Even though this site met both the FIBI (using the UAV of 7 points) and BMIBI BICs, 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 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 between 2008 and 2012 to be considered “monitored”.   This segment had multiple BMIBI samples collected in the previous nine years (2004-2012).  

Results of ambient chemical/physical water quality monitoring conducted by IDNR/SHL do not suggest any water quality impacts in this river reach.   No violations of Class B(WW1) water quality criteria occurred in the 36 samples collected from 2010 through 2012 at the IDNR monthly ambient monitoring station that were analyzed for dissolved oxygen (minimum = 6.1 mg/L), pH (range = 7.2 to 8.6), and ammonia-nitrogen (maximum = 0.11 mg/L).   These results suggest no impairments of the Class B(WW1) aquatic life uses in this river segment.   Results from the Rathbun Watershed station RA-28 also show no violations of the Class B(WW1) water quality criteria for ammonia (maximum = 0.2 mg/L).    

Class C (drinking water) uses are also assessed (monitored) as "fully supporting."  Results from the Rathbun Watershed station RA-28 at the reservoir outlet show low levels of atrazine.   None of the eight samples collected at station RA-28 from 2010 through 2012 violated the U.S.  EPA MCL for atrazine of 3 ug/l; the maximum atrazine concentration was 2.4 ppb on June 8, 2010.   Regarding nitrate, none of the 36 IDNR samples analyzed for nitrate at IDNR station 10040001 from 2010 to 2012 exceeded the EPA MCL of 10 mg/l (maximum value = 1.6 mg/l).    Also, levels of nitrate at Rathbun Watershed station RA-28 were extremely low during this time period with a maximum value of 1.7 mg/l in the 17 samples collected from 2010 through 2012.   Based on IDNR's Section 305(b) assessment methodology, the results of the IDNR/SHL and Rathbun watershed monitoring both suggest that the Class C (drinking water) uses should be assessed as fully supported.  

Fish consumption uses are assessed (monitored) as "fully supported" based on results of the U.S.  EPA/IDNR fish tissue (RAFT) monitoring north of Centerville in 2011 and 2013.   The 2011 composite samples of fillets from channel catfish had very low levels of contaminants.   Levels of primary contaminants in the composite sample of channel catfish fillets were as follows: mercury: 0.197 ppm; total PCBs: <0.09 ppm; and technical chlordane: <0.03 ppm.   Levels of mercury in the 2011 tissue plug samples of white crappie, however, were above the one meal per week consumption advisory threshold.   The average level of mercury in the tissue plugs from five white crappie was 0.316 ppm (SD=0.043 ppm; maximum=0.386; minimum=0.269).   The average and maximum levels of mercury were well below Iowa’s “do not eat” advisory threshold of 1.0 ppm but were above Iowa’s threshold for issuance of a one meal per week fish consumption advisory.   According to the IDNR/IDPH advisory protocol, two consecutive samplings showing that contaminant levels are above the advisory threshold level are needed to justify issuance of a consumption advisory.   Additional monitoring of levels of mercury in white crappie was conducted in 2013.   Results show that the average level of mercury in the five white crappie was 0.195 ppm (standard deviation = 0.079; maximum level = 0.286).   This level is well below Iowa's threshold for a 1 meal/week consumption advisory.  

The existence of, or potential for, a fish consumption advisory is the basis for Section 305(b) assessments of the degree to which Iowa’s lakes and rivers support their fish consumption uses.   The fish contaminant data generated for this segment of the Chariton River in 2011 and 2013 show that the levels of all contaminants are below the IDPH/IDNR advisory trigger levels, thus suggesting no justification for issuance of a consumption advisory and "full support" of fish consumption uses.