Iowa DNR

Water Quality Assessments

Impaired Waters List

West Nishnabotna River IA 05-NSH-1441

from confluence with Silver Cr. (S21, T71N, R41W, Mills Co.) to confluence Farm Cr. in S9, T73N, R40W, Mills Co.

Assessment Cycle
Release Status
Result Period
2004 - 2006
Class A1 Class B(WW-1) Class HH
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 05-NSH-0080_1
Overall Use Support
Not supporting
Aquatic Life Use Support
Fish Consumption
Not assessed
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on results of IDNR/UHL monthly ambient water quality monitoring conducted during the 2004-2006 assessment period east of Malvern (STORET station 10650001 (formerly station 822310)).

Basis for Assessment

[Note:  Prior to the current (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, 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 2004 through 2006.   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 2004-2006 assessment period east of Malvern (STORET station 10650001 (formerly station 822310)).  

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.  coli).   Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed.   Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses:  the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff."  Due to a change in the Standards in July 2003, E.  coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E.  coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.”  Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E.  coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.  

The geometric mean level of indicator bacteria (E.  coli) in the 24 samples collected at the IDNR/UHL ambient monitoring station east of Malvern during summer recreational seasons of 2004-2006 (403 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Fifteen of the 24 samples (62%) exceed Iowa’s single-sample maximum criterion of 235 orgs/100 ml.   According to U.S.  EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if the geometric mean level of E.  coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S.  EPA 1997b).  

Regarding support of the Class B(WW1) aquatic life uses, results of routine ambient monitoring at the IDNR/UHL station near Malvern showed no violations of Class B(WW1) water quality criteria during the 2004-2006 assessment period for dissolved oxygen and ammonia-nitrogen in the 36 samples analyzed.   One of the 36 samples violated the Class B(WW1) criterion for pH:  the pH of the sample collected on August 1, 2006 was 9.1 units, thus violating the Class B(WW1) criterion of 9.0 pH units.   Also, this sample had a water temperature of 33C, thus exceeding the water temperature criterion of 32C.   According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the percentages of violations at this station during the 2004-2006 period for pH and water temperature (both 3%) do 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.   The violations of the criteria for pH and water temperature are both likely related to naturally-occurring conditions (bright sunshine, warm air temperatures, and high levels of primary productivity) and do not reflect the influence of anthropogenic pollutants.   In addition, levels of pesticides in the eight samples analyzed, and levels of toxic metals in the ten samples analyzed, were all below the respective Class B(WW1) criteria.   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.

Monitoring and Methods
Assessment Key Dates
12/4/2006 Fixed Monitoring End Date
1/5/2004 Fixed Monitoring Start Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
420 Water column surveys (e.g. fecal coliform)
Monitoring Levels
Biological 0
Habitat 0
Physical Chemistry 3
Toxic 3
Pathogen Indicators 3
Other Health Indicators 0
Other Aquatic Life Indicators 0
# of Bio Sites 0
BioIntegrity N/A
Causes and Sources of Impairment
Causes Use Support Cause Magnitude Sources Source Magnitude
Pathogens Primary Contact Recreation High
  • Source Unknown
  • High