Fourmile Creek IA 04-LDM-1112

Assessment Comments

Assessment is based on results of IDNR/UHL biological REMAP sampling in 2006 and IDNR/UHL biological sampling in 1999.

Basis for Assessment

Note:  Although USGS water quality monitoring was conducted at three locations in this segment during the 2004-2006 assessment period, only one sample per site was collected and analyzed.   These 3 samples, collected on March 8, 2005, did not violate the respective Class B(WW2) criteria for ammonia, dissolved oxygen, or pH.   This amount of data is insufficient for developing an updated assessment of support of designated uses.    

SUMMARY:  The Class A1 (primary contact recreation) uses remain "not assessed" due to the lack of information upon which to base an assessment.   The Class B(WW2) aquatic life uses are assessed (evaluated) as "fully supported" based on results of IDNR/UHL biological REMAP sampling in 2006.   Even though this segment is considered fully supporting for aquatic life, it is a candidate for follow-up sampling due to the following reasons:

1) The results of the 2006 biological sampling suggest very low numbers of benthic macroinvertebrates at the time of sampling.   IDNR protocol requires at least one of the semi-quantitative samples to contain a minimum of 85 organisms for the sample to be considered valid and a BMIBI calculated.   The semi-quantitative samples from the 2006 sample contained 3, 16, and 21 organisms, respectively.   Low benthic macroinvertebrate colonization should not be construed as an impairment; however, it is an indication of possible water quality and/or habitat problems.

2) The original samples in 1999 were collected above and below the Ankeny WWTP, the IBI scores were averaged following IDNR protocol because the sites were in the same segment and the assessment was based on those averages.   In 2008, corrections were made to the drainage areas of the sites and to the BMIBI metric calculations.   The corrected 1999 FIBI average was 31.8 and the 1999 BMIBI average score was 61.   The non-riffle habitat FIBI BIC for this ecoregion is 32 and the BMIBI BIC is 62.   Therefore, this segment barely failed using the 1999 data.  

3) In the next several years the Ankeny WWTP is set to go offline and the waste will be pumped to the Des Moines WWTP for processing.

4) This segment is long (15.8 miles) and the characteristics of the stream change considerably from the top of the segment (where the three sites are located) to where the stream enters the Des Moines River.    

EXPLANATION:  The Class A1 (primary contact recreation) uses remain "not assessed" due to the lack of information upon which to base an assessment.

The assessment of the Class B(WW2) aquatic life uses was based on data collected in 2006 as part of the DNR/UHL stream REMAP project.   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 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 Fully Supporting (=FS), 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 1994-2004.   The non-riffle habitat FIBI BIC for this ecoregion is 32.