Assessment Comments

Assessment is based on results of IDNR/UHL monthly ambient water quality monitoring conducted during the 2008-2010 assessment period east of Malvern (STORET station 10650001).

Basis for Assessment

[Note:  Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(WW) 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 due to the completion of a Use Attainability Analysis in 2007, 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" due to levels of indicator bacteria that violate state water quality criteria.   The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring from 2008 through 2010.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment.   This assessment is based on results of IDNR/UHL monthly ambient water quality monitoring conducted during the 2008-2010 assessment period east of Malvern (STORET station 10650001 (formerly station 822310)).  

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 21 samples collected during the recreational seasons of 2008 through 2010 at the West Nishnabotna River near Malvern were as follows:  the 2008 geometric mean was 1170 orgs/100 ml, the 2009 geometric mean was 915 orgs/100 ml and the 2010 geometric mean was 1218 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Sixteen of the 21 samples (76%) 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 are assessed (monitored) as “fully supporting.”  Results of routine ambient monitoring at the IDNR/UHL station near Malvern showed no violations of Class B(WW1) water quality criteria during the 2008-2010 assessment period for dissolved oxygen (minimum = 7.4 mg/L) and ammonia-nitrogen (maximum = 2.6 mg/L) in the 31 samples analyzed.   One of the 30 samples violated the Class B(WW1) criterion for pH:  the pH of the sample collected on January 6, 2009 was 10.9 units, thus violating the Class B(WW1) criterion of 9.0 pH units.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentage of violations at this station during the 2008-2010 period for pH (3%) does not suggest a water quality impairment.   These guidelines allow up to 10% violations of conventional parameters such as dissolved oxygen, pH, and water temperature before impairment of water quality is indicated.   These results suggest "full support" of the Class B(WW1) aquatic life uses.  

Fish consumption uses remain "not assessed" due to the lack of fish tissue monitoring in this river segment.