Assessment Comments

Assessment based on: (1) IDNR/SHL biological sampling conducted in 2003, 2004 and 2012 and (2) IDNR/SHL monthly ambient monitoring conducted during the 2010-2012 assessment period at the Highway 141-175 bridge at Mapleton (STORET station 10670002 (formerly station 911040)).

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 2003, 2004, and 2012.   Results of ambient water quality monitoring from 2010 through 2012 suggest relatively good water quality in this river segment.   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/SHL biological sampling conducted in 2003, 2004, and 2012 and (2) IDNR/SHL monthly ambient monitoring conducted during the 2010-2012 assessment period at the Highway 141-175 bridge at Mapleton (STORET station 10670002 (formerly station 911040)).  

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 Maple River near Mapleton were as follows:  the 2010 geometric mean was 437 orgs/100 ml, the 2011 geometric mean was 206 orgs/100 ml and the 2012 geometric mean was 212 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Eleven of the 24 samples (46%) 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, results of ambient chemical/physical monitoring at the IDNR/SHL station at Mapleton showed no violations of Class B(WW1) water quality criteria during the 2010-2012 assessment period for dissolved oxygen (minimum value = 7.6 mg/l), ammonia nitrogen (maximum value = 0.12 mg/l), pH (range = 7.9 to 8.7 units), temperature (maximum = 28.4C), chloride, or sulfate in the 36 samples collected.   These results suggest “full support” of the Class B(WW1) aquatic life uses.

Despite results of ambient physical/chemical monitoring that suggest “full support” of aquatic life uses, results of biological monitoring suggest only “partial support” of these uses.   This assessment was based on biological data collected in 2003, 2004, and 2012 as part of the IDNR/SHL REMAP and large river sampling projects.   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 2003 FIBI score was 9 (poor).   The 2004 FIBI score was 14 (fair) and the 2004 BMIBI score was 50 (fair).   The 2012 BMIBI score was 34 (fair).   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 0/2 times in the last 10 years and passed the BMIBI BIC 0/2 times in the last nine years.   This assessment is considered “evaluated” because the drainage areas (669, 725 and 732 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 failed the FIBI 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.   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.