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
2004 - 2006
HQR 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
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 2004-2006 assessment period.

Basis for Assessment

[Note:  Prior to the current (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(WW2) aquatic life uses are assessed (monitored) as "fully supporting" based on results of ambient water quality monitoring.   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 2004-2006 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).   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 (545 orgs/100ml) far exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Fourteen of the 24 samples (58%) exceed Iowa’s single-sample maximum value 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).  

The Class B(WW2) aquatic life uses are assessed (monitored) as “fully supported.”  None of the approximately 36 samples collected during the 2004-2006 assessment period violated Class B(WW2) water quality criteria for dissolved oxygen or ammonia-nitrogen.   None of the approximately 10 samples analyzed for toxic metals, nor none of the seven samples analyzed for pesticides, violated state water quality criteria.   One of the 36 samples analyzed for pH, however, exceeded the Class B(WW2) criterion of 9.0 pH units (this sample contained a pH of 10.3 pH units).   According to U.S.  EPA guidelines (U.S.  EPA 1997b, page 3-17), however, a violation frequency of less than 10 % for conventional parameters such as pH nonetheless suggests "full support" of aquatic life uses.   Thus, the percentage of violations of the pH criterion at this station (3%) does not suggest an impairment of aquatic life uses in this stream segment.

Monitoring and Methods
Assessment Key Dates
12/4/2006 Fixed Monitoring End Date
1/7/2004 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