Assessment Comments

Assessment is based on: (1) results of IDNR/UHL monthly ambient monitoring conducted during the 2008-2010 assessment period near Shenandoah (STORET station 10360001 at Hwy 59 bridge) and (2) results of IDNR/UHL biological monitoring conducted in 2002 and 2008 as part of the stream biocriteria project.

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.   Although results of ambient water quality monitoring suggest “full support,” the Class B(WW1) aquatic life uses for this assessment segment remain assessed (evaluated) as "partially supported" based on results of IDNR/UHL biological monitoring in 2002 and 2008.   Fish consumption uses remain “not assessed” due to the lack of fish contaminant monitoring in this river segment.   This assessment is based on (1) results of IDNR/UHL monthly ambient monitoring conducted during the 2008-2010 assessment period near Shenandoah (STORET station 10360001 (at Hwy 59 bridge; formerly station 821008)) and (2) results of IDNR/UHL biological monitoring conducted in 2002 and 2008 as part of the stream biocriteria project.  

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 22 samples collected during the recreational seasons of 2008 through 2010 at the East Nishnabotna River near Shenadoah were as follows:  the 2008 geometric mean was 742 orgs/100 ml, the 2009 geometric mean was 756 orgs/100 ml and the 2010 geometric mean was 1461 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Fifteen of the 22 samples (68%) 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.”  

This evaluated biological assessment was based on data collected in 2002 and 2008 as part of the IDNR/UHL biological monitoring project.   A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological 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 2002 and 2008 BMIBI scores were 38 and 41 (both fair).   Fish samples were not collected at this site.   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 biological data collected at stream ecoregion reference sites from 1994-2004.   The BMIBI BIC for this ecoregion is 54.   This segment passed the BMIBI BIC 0/2 times in the last nine years.   This assessment is considered evaluated because the drainage area (1022 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 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 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 2006 and 2010 to be considered “monitored”.   This segment had multiple BMIBI samples collected in the previous nine years (2002-2010); however, the samples were not collected during 2006-2010.   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).

In contrast to results of biological monitoring, results of chemical/physical water quality monitoring at the IDNR/UHL station near Shenandoah continue to suggest “full support” of the Class B(WW1) uses.   Levels of ammonia-nitrogen (maximum = 2.2 mg/L) and dissolved oxygen (minimum = 7.0 mg/L) were below the respective Class B(WW1) criteria in all 32 samples analyzed during the 2008-2010 period.   There was one violation of the Class A1,B(WW1) criteria for pH in 31 samples (3%).   Based on IDNR's assessment methodology, this single violation does not represent an impairment of the aquatic life uses.   In addition, levels of pesticides in the three samples analyzed were below their respective Class B(WW1) criteria.

One of the three samples analyzed for toxic metals violated the Class B(WW1) chronic criterion for chromium:  the sample collected on July 9, 2008, contained 50 ug/l of total chromium, thus violating both the Class B(WW1) chronic (11 ug/l) and acute (16 ug/l) criteria for chromium VI.   This sample was the only one with a detectable level of chromium in the 25 samples from this station analyzed for chromium since 1998.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b) and according to IDNR’s assessment/listing methodology, impairment due to toxics in indicated when more than one violation of an acute criterion occurs within a three-year period.   Because the violation of the acute criterion that occurred on July 9, 2008 was the only chromium violation during the 2008-10 period, this violation does not indicate impairment of the Class B(WW1) aquatic life uses.  

In addition, one of the three samples analyzed for toxic metals violated the Class B(WW1) chronic and acute criteria for zinc (both = 120 ug/l based on an assumed hardness of 100 mg/l).   This sample was also collected on July 9, 2008, and contained 210 ug/l of total zinc.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b) and according to IDNR’s assessment/listing methodology, impairment due to toxics in indicated when more than one violation of an acute criterion occurs within a three-year period.   Because the violation of the acute criterion that occurred on July 9, 2008 was the only zinc violation during the 2008-10 period—and is the only violation, acute or chronic, in the 20 samples collected at this station between 2002 and 2008—this violation does not indicate impairment of the Class B(WW1) aquatic life uses.   A relatively high level of total suspended solids in the July 9, 2008 sample (3,100 mg/l) may have influenced the level of metals in this sample, thus contributing to the violations of the Class B(WW1) metals criteria.   These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”  

In addition, one of the three samples analyzed for toxic metals violated the Class B(WW1) chronic criteria for cadmium (0.4 ug/L).   This sample was also collected on July 9, 2008, and contained 2 ug/l of total cadmium.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b) and according to IDNR’s assessment/listing methodology, impairment due to toxics in indicated when more than one violation of an acute criterion occurs within a three-year period.   Because this sample did not violate the acute criterion, this violation does not indicate impairment of the Class B(WW1) aquatic life uses.  These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”  

In addition, one of the three samples analyzed for toxic metals violated the Class B(WW1) chronic and acute criteria for copper (chronic = 14.68 ug/L, acute = 23.08 ug/L).   This sample was also collected on July 9, 2008, and contained 60 ug/l of total copper.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b) and according to IDNR’s assessment/listing methodology, impairment due to toxics in indicated when more than one violation of an acute criterion occurs within a three-year period.   Because the violation of the acute criterion that occurred on July 9, 2008 was the only cadmium violation during the 2008-10 period—and is the only violation, acute or chronic, in the 20 samples collected at this station between 2002 and 2008—this violation does not indicate impairment of the Class B(WW1) aquatic life uses.  These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”  

In addition, one of the three samples analyzed for toxic metals violated the Class B(WW1) chronic  criteria for lead (6.26 ug/L).   This sample was also collected on July 9, 2008, and contained 40 ug/l of total lead.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b) and according to IDNR’s assessment/listing methodology, impairment due to toxics in indicated when more than one violation of an acute criterion occurs within a three-year period.   Because this sample did not violate the acute criterion, this violation does not indicate impairment of the Class B(WW1) aquatic life uses.  These results suggest that the Class B(WW1) aquatic life uses should be assessed as “fully supported.”

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