North Skunk River IA 03-NSK-863

[Note: Prior to the 2008 Section 305(b) cycle, this stream 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 http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is now presumptively designated for Class A1 (primary contact recreation) uses. The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses). Thus, for the current assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]

The Class B(WW-2) aquatic life uses are assessed (evaluated) as "partially supported" based on results of a fish kill investigation in 2006. The kill occurred on or before August 11, 2006 and was attributed to runoff from an open feedlot where 1,300 hogs were housed. The runoff occurred following heavy rains when manure from the open hog lot had overflowed from a retention structure and run into a tributary of the North Skunk River. Several hundred dead fish accumulated at the low-head dam on the North Skunk River near Lynnville. Both dead carp and game fish were found. The party responsible for the kill was identified, and restitution for the value of the fish killed and costs of investigation was sought and received.

According to IDNR's assessment methodology for Section 305(b) reporting, occurrence of a single pollution-caused fish kill during an assessment period indicates "partial support" of the aquatic life uses. If, however, a consent order has been issued to the party responsible for the kill and monetary restitution has been sought for the fish killed, the affected waterbody should be placed in IR Category 4d (impaired but TMDL not required). Thus, this assessment segment was moved from Category 5b of Iowa's 2008 Integrated Report to Category 4d of Iowa’s 2010 Integrated Report based on updated information regarding restitution for this fish kill. IDNR considers IR 4d fish kill waters with no subsequent kills in at least the five years subsequent to the kill as appropriate for movement to IR categories 2b or 3b (i.e., Iowa’s list of waters in need of further investigation). Thus, because this kill occurred more than five years ago and because no subsequent kills have been reported, this impairment was moved to Category 3b of the 2014 Integrated Report and thus was added to Iowa’s list of waters in need of further investigation.

The assessment of the Class B(WW-2) aquatic life uses is also based on data collected in 2001 as part of the IDNR/SHL stream biocriteria project and on 2002 Manchester Fisheries stream sampling data also suggests "impairment" of the aquatic life uses. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2001 FIBI scores were 16, 17, 19 (all poor); the BMIBI scores were 65, 71, 72 (all good). The 2002 Fisheries FIBI score was 24 (poor). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. The FIBI BIC for this ecoregion is 36 and the BMIBI BIC for this ecoregion is 51. This segment passed the FIBI BIC 0/4 times and passed the BMIBI BIC 3/3 times in the last 14 years. This impairment was attributed to a non-pollutant stressor (other habitat alterations) and thus appropriate for Category 4c of Iowa's 2006 Integrated Report. This impairment remains in IR Category 4c.

This aquatic life assessment is now considered "evaluated" based on a change in the 2010 IDNR assessment methodology. IDNR now requires a segment have two or more biological samples collected from the segment in multiple years over a five-year period to be considered “monitored”. This segment had multiple samples collected in the previous 14 years (2001-2014); however, the multiple samples were not collected during over a five-year period. Additionally, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). 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).  However, despite this change in assessment methodology and type, this waterbody remains in IR Category 4c.