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
Release Period
2012 - 2014
10/3/2016 10:07:56 AM
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Last Updated
10/7/2016 2:50:31 PM
Use Support
Class A1
Primary contact
Partially Supporting
Bacteria: Indicator Bacteria, E. coli
Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Geometric mean criterion exceeded
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier III
Class B(WW-1)
Aquatic Life
Warm Water Type 1
Partially Supporting
Biological: low aquatic macroinvertebrate IBI
Impairment Code
3b-u - Use potentially biologically impaired based on uncalibrated IBI metrics.
Cause Magnitude
Unknown: Source Unknown
Source Confidence
Cycle Added
Impairment Rationale
Low Biotic Index
Data Source
Biological monitoring: Iowa DNR WQMA
Class HH
Human Health
Not Assessed
General Use
General Use water
Not Assessed
Impairment Delistings
No delistings for this assessment cycle.
Assessment Summary

The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially 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" based on results of 2012 IDNR/SHl biological sampling data. 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 IDNR/SHL monthly ambient water quality monitoring conducted during the 2012-2014 assessment period east of Malvern (STORET station 10650001 (formerly station 822310)) and on 2012 IDNR/SHL biological sampling conducted near Malvern. Water quality conditions in this segment appear to have improved during the 2011-12 period. Levels of indicator bacteria dropped from the thousands (from 915 to 1,218 orgs/100 ml) during the 2008-10 period to the low hundreds (196 and 169 orgs/100 ml) during 2011 and 2012. Also, the maximum ammonia concentration dropped from 2.6 mg/l during the 2008-10 period to 0.17 during the 2010-12 period.

Assessment Explanation

The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially 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 2012 through 2014 at the West Nishnabotna River near Malvern were as follows: the 2012 geometric mean was 169 orgs/100 ml, the 2013 geometric mean was 317 orgs/100 ml, and the 2014 geometric mean was 489 orgs/100 ml.  All three recreation season geometric means exceeded the Class A1 geometric mean criterion of 126 orgs/100 ml.  Twelve of the combined 24 samples (50%) exceeded Iowa’s 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 the geometric mean is greater than 126 orgs/100 ml., the primary contact recreation uses should be assessed as "impaired" (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 "partially supported."

Based on just the water quality sampling data, the Class B(WW1) aquatic life uses should be assessed (monitored) as “fully supporting.” Results of routine ambient monitoring at the IDNR/SHL station near Malvern showed  no violations of Class B(WW1) water quality criteria for 36 Ammonia samples (maximum = 0.6 mg/L), 36 Dissolved Oxygen samples (minimum = 6.1 mg/L), 36 pH samples (range = 7.5 to 8.8), 36 Temperature samples (maximum = 31.4° c), 36 Chloride samples (maximum = 30 mg/L), or 36 Sulfate samples (maximum = 45 mg/L) occurred during monitoring from January 2012 to December 2014. According to U.S.  EPA guidelines for Section 305(b) water quality assessments (U.S.  EPA 1997b, page 3-17), the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated. Thus, these results thus suggest “full support” of the Class B(WW1) aquatic life uses.

In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting". This evaluated biological assessment was based on data collected in 2012 as part of the IDNR/SHL stream large river sampling 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 collected in the stream sampling reach. The biological metrics were combined a benthic macroinvertebrate index (BMIBI). The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2012 BMIBI score was 39 (fair). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI score with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008. The BMIBI BIC for this ecoregion is 54. This assessment is considered evaluated because the drainage area (1092 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 segment failed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because the site used for the assessment 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 fish tissue monitoring in this river segment.

Monitoring and Methods
Assessment Key Dates
9/4/2012 Biological Monitoring
1/4/2012 Fixed Monitoring Start Date
12/1/2014 Fixed Monitoring End Date
230 Fixed station physical/chemical (conventional plus toxic pollutants)
315 Regional reference site approach
320 Benthic macroinvertebrate surveys
420 Water column surveys (e.g. fecal coliform)