Iowa DNR

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
Release Status
Result Period
2008 - 2010
Class A1 Class B(WW-2)
Assessment Type
Integrated Report
Category 5a
Legacy ADBCode
IA 03-NSK-0010_2
Overall Use Support
Not supporting
Aquatic Life Use Support
Not supporting
Primary Contact Recreation
Not supporting
Assessment Comments

Assessment is based on results of IDNR ambient monthly water quality monitoring conducted on the North Skunk River southwest of Sigourney in Keokuk Co. (STORET station 10540001) during the 2008-2010 assessment period.

Basis for Assessment

[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 (see 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).]

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 remain assessed as "not supporting" due to violations of state water quality criteria for chromium.   Fish consumption uses remain "not assessed" 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 Co.   (STORET station 10540001) during the 2008-2010 assessment period.   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).  

EXPLANATION:  The Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E.  coli).   The geometric means of indicator bacteria (E.  coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10540001 were as follows:  the 2008 geometric mean was 580 orgs/100 ml, the 2009 geometric mean was 453 orgs/100 ml and the 2010 geometric mean was 556 orgs/100 ml.   All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml.   Seventeen of the 22 samples (77%) 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(WW2) aquatic life uses remain assessed (monitored) as “not supported due to violations of the acute criterion for chromium.   None of the approximately 30 samples collected at station 10540001 during the 2008-2010 assessment period violated Class B(WW2) water quality criteria for dissolved oxygen, pH or ammonia-nitrogen.   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(WW2) 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(WW2) acute criterion for chromium was 0.30 mg/l, and the chronic criterion was 0.20 mg/l.]

Monitoring and Methods
Assessment Key Dates
12/7/2010 Fixed Monitoring End Date
1/7/2008 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
Chromium Aquatic Life Support Moderate
  • Source Unknown
  • Moderate
Pathogens Primary Contact Recreation Moderate
  • Source Unknown
  • High