North Skunk River IA 03-NSK-863

The assessment of the Class B(WW2) aquatic life uses is also based on data collected in 2001 as part of the DNR/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 DNR assessment methodology. DNR 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 DNR’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). DNR 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.

Note: The Class B(WW2) aquatic life uses were historically 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 DNR'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 have been placed in Category 4d of Iowa’s 2010 Integrated Report. DNR 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 had 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.

According to Iowa DNR’s assessment/listing methodology, any IR 2b or 3b fish kills that occurred more than 10 years ago will be moved to IR Category 3a (not assessed) for the current IR cycle.