Iowa DNR
ADBNet

Water Quality Assessments

Impaired Waters List

North Skunk River IA 03-NSK-854

from confluence with Cedar Cr. (S15, T75N, R12W, Keokuk Co.) to confluence with Middle Cr. in S35, T76N, R14W, Mahaska Co.

Assessment Cycle
2016
Release Status
Final
Result Period
2012 - 2014
Created
9/26/2016 1:14:55 PM
Overall IR Category
5 - Water is impaired or threatened and a TMDL is needed.
Trend
Unknown
Last Updated
9/26/2016 3:05:20 PM
Use Support
Class A1
Recreation
Primary contact

Not Supporting - Bacteria: Indicator Bacteria, E. coli

Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Slight
Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2008
Impairment Rationale
Geometric mean criterion exceeded
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier IV
Class B(WW-2)
Aquatic Life
Warm Water Type 2

Not Supporting - Metals: Chromium

Impairment Code
5a - Pollutant-caused impairment. TMDL needed.
Cause Magnitude
Moderate
Status
Continuing
Source
Unknown: Source Unknown
Source Confidence
N/A
Cycle Added
2010
Impairment Rationale
Violations of acute criterion
Data Source
Ambient monitoring: Iowa DNR-rivers
TMDL Priority
Tier III
General Use
General Use water

Not Assessed

Impairment Delistings
No delistings for this assessment cycle.
Documentation
Assessment Summary

The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" (IR 5a) due to levels of indicator bacteria that slightly exceed state water quality criteria. The Class B(WW2) aquatic life uses remain assessed as "not supporting" (IR 5a) due to violations of state water quality criteria for chromium during the 2004-06 assessment period. Fish consumption uses remain "not assessed" (IR 3a) due to the lack of fish contaminant monitoring in this river segment. The assessments of support of beneficial uses are based on results of IDNR ambient monthly water quality monitoring conducted on the North Skunk River southwest of Sigourney in Keokuk County (STORET station 10540001) during the 2012-2014 assessment period and on 2013 and 2014 IDNR/SHL biological sampling data. Data from this monitoring station were also used to assess support of the Class A1 and B(WW1) uses of the adjacent downstream segment of the North Skunk River (IA 03-NSK-0010-1: mouth to the confluence with Cedar Creek near Sigourney).

Assessment Explanation

[Note: Prior to the 2008 Section 305(b) cycle, this river segment was designated only for Class B(LR) aquatic life uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008 and due to the completion of a 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(WW2) uses).]

EXPLANATION: The Class A1 uses are assessed (monitored) as "partially supported" based on results of ambient monitoring for indicator bacteria (E. coli). The geometric means of indicator bacteria (E. coli) in the 23 samples collected during the recreational seasons of 2012 through 2014 at IDNR station 10540001 were as follows: the 2012 geometric mean was 169 orgs/100 ml, the 2013 geometric mean was 102 orgs/100 ml, and the 2014 geometric mean was 169 orgs/100 ml. The 2012 and 2014 geometric means slightly exceeded the Class A1 criterion of 126 orgs/100 ml; the 2013 geometric mean was below (met) this criterion. Eleven of the 23 samples (48%) 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 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.”

The Class B(WW1) aquatic life uses remain assessed (monitored) as “not supported due to violations of the acute criterion for chromium. None of the 36 samples collected at station 10540001 during the 2010-2012 assessment period violated Class B(WW1) water quality criteria for dissolved oxygen, pH, ammonia-nitrogen, chloride, or sulfate. Monitoring for toxic metals has not been conducted at this monitoring station since April 2006. During the 2004-2006 period, however, ten samples were analyzed for total chromium. Levels of chromium in two of these samples (collected on April 13, 2005 and April 5, 2006) contained 0.02 mg/l of total chromium and thus exceeded the Class B(WW1) aquatic life acute criterion of 0.016 mg/l. The remaining eight samples were reported as less than the detection level of 0.02 mg/l. Based on Iowa DNR’s assessment/listing methodology, more than one violation of an acute water quality criterion for a toxic parameter over a three-year period suggests impairment of aquatic life uses. Thus, the violations of the chromium criterion suggests that the Class B(WW1) aquatic life uses of this assessment segment should be assessed as impaired. [Note: Prior to EPA approval in 2009, Iowa’s Class B(WW1) acute criterion for chromium was 0.060 mg/l, and the chronic criterion was 0.040 mg/l.]

In contrast to the aquatic life use assessment based on the 2004-2006 water quality data, biological sampling data collected in 2013 and 2013 by IDNR/SHL suggests that the aquatic life uses in this segment are "fully supporting".  This evaluated biological assessment was based on data collected in 2013 and 2014 as part of the IDNR/SHL 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 to make a benthic macroinvertebrate index (BMIBI).  The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum).  The 2013 and 2014 BMIBI scores were 56, 57 (both good). The aquatic life use support was assessed (evaluated) as fully supporting (=FS), based on a comparison of the BMIBI scores 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 51.  This assessment is considered evaluated because the drainage area (631 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 passed the BMIBI BIC (2/2), 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.  

Monitoring and Methods
Assessment Key Dates
10/7/2013 Biological Monitoring
8/21/2014 Biological Monitoring
1/7/2004 Fixed Monitoring Start Date
12/4/2006 Fixed Monitoring End Date
1/5/2012 Fixed Monitoring Start Date
12/2/2014 Fixed Monitoring End Date
Methods
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)