Assessment Comments
Assessment is based on results of IDNR/UHL monthly ambient chemical/physical monitoring conducted during the 2006-2008 assessment period near Larrabee (STORET station 10180001 (formerly station 911060)) and 2005 IDNR/UHL stream REMAP biological sampling near Larrabee.
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as “partially supported.” The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient physical/chemical monitoring and assessed as "fully supported" (evaluated) based on the 2005 IDNR/UHL stream REMAP biological sampling near Larrabee. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment. This assessment is based on results of IDNR/UHL monthly ambient chemical/physical monitoring conducted during the 2006-2008 assessment period near Larrabee (STORET station 10180001 (formerly station 911060)) and 2005 IDNR/UHL stream REMAP biological sampling near Larrabee.
EXPLANATION: The Class A1 uses were assessed (monitored) as "partially supported." The geometric mean level of indicator bacteria (E. coli) in the 21 samples collected during the recreational seasons of 2006 through 2008 (114 orgs/100ml) is less than the Iowa Class A1 water quality criterion of 126 orgs/100ml. Five of the 21 samples (24%) exceeded Iowa’s single-sample maximum value of 235 orgs/100 ml. The results of monitoring for E. coli during the 2002-2004, 2004-2006, and 2006-2008 periods suggest that levels of indicator bacteria are relatively low in this river segment. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if the geometric mean of E. coli is less than the applicable state criteria, the contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). Thus, these data suggest that the Class A1 uses should be assessed as “fully supported.” These EPA guidelines, however, also suggest that if more than 10% of samples exceed the single-sample maximum criterion for indicator bacteria, the primary contact recreation uses should be assessed as "partially supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). According to IDNR’s assessment/listing methodology, the monitoring data from 2006-08 suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum criterion, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired.”
The Class B(WW1) aquatic life uses remain assessed (monitored) as “fully supported.” Monitoring at the IDNR/UHL station near Larrabee showed no violations of Class B(WW1) water quality criteria in the 32 samples collected during the 2006-2008 assessment period for dissolved oxygen (minimum value = 8.1 mg/l), pH (range = 7.9 to 8.8 units), or ammonia-nitrogen (maximum value = 0.84 mg/l). In addition, levels of toxic metals in the two samples analyzed were all below their respective Class B(WW1) criteria. (Note: pesticides were not monitored at this location during the 2006 through 2008 period.) These results suggest "full support" of the Class B(WW1) aquatic life uses.
Supporting the WQ results and assessment, the results of the 2005 biological monitoring also indicate "full support" (evaluated) of the Class B(WW1) aquatic life uses. This evaluated biological assessment was based on data collected in 2005 as part of the DNR/UHL stream REMAP project. A series of biological metrics that 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 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 2005 FIBI score was 51 (good) and the BMIBI score was 57 (good). The aquatic life use support was assessed (evaluated) as fully supporting (=FS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The FIBI BIC for this ecoregion is 43 and the BMIBI BIC for this ecoregion is 54. This assessment is considered evaluated because the drainage area (1833 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site passed both the FIBI and BMIBI BICs, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size.
Fish consumption uses remain "not assessed due to the lack of recent fish contaminant monitoring in this river reach.
