Fourmile Creek IA 04-LDM-1112

The Class B(WW2) aquatic life uses are assessed (evaluated) as “partially supporting” based on biological data collected in 2006 as part of the DNR/SHL stream REMAP project and DNR/SHL biological sampling upstream and downstream of the Ankeny WWTP in 1999. 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 1999 FIBI scores were 37, 37, 30 (fair) and 23 (poor). The 1999 BMIBI scores were 48, 54 (fair), 68, 74 (good). The 2006 FIBI score was 53 (good) and the BMIBI score was not calculated due to very low benthic macroinvertebrate colonization. The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the FIBI score with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. The non-riffle habitat FIBI BIC for this ecoregion is 32 and the BMIBI BIC for this ecoregion is 62. This segment passed the FIBI BIC 3/5 times and passed the BMIBI BIC 2/4 times in the past 18 years.

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 recent five-year period to be considered “monitored”. This segment had multiple samples collected in the previous 18years (1999-2016); however, the multiple samples were not collected during a five-year period and were not collected in multiple years (BMIBI). Also, 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).