Assessment Comments
Assessment based on results of IDNR/SHL ambient monthly monitoring near Hornick from 2010-12 and on IDNR/SHL biological sampling conducted in 2012 and 2013.
Basis for Assessment
[Note 1: For the 2002 report, the previous waterbody segment for the West Fork Little Sioux River (IA 06-LSR-0120-0), which extended 32 miles from its mouth to confluence with Mud Creek near Moville in Woodbury Co., was split into two subsegments: (1) mouth to confluence with an unnamed tributary 0.5 miles north of Climbing Hill (Woodbury Co. - this one) (IA 06-LSR-0120-1) and (2) unnamed tributary 0.5 miles north of Climbing Hill to Mud Creek near Moville, Woodbury Co. (IA 06-LSR-0120-2). See previous Section 305(b) from this subsegment for the assessments of the original 32-mile river reach.]
[Note 2: 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.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" (IR 5p) due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW2) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-c) based on results of IDNR/SHL biological sampling conducted in 2012 and 2013. The extensive channel modifications on this river segment, however, remain a concern regarding the full support of aquatic life uses. This assessment is based on results of IDNR/SHL monthly ambient monitoring conducted during the 2010-2012 assessment period on West Fork Little Sioux River at the Highway 141 bridge approximately 1 mile east of Hornick in (STORET station 10970002) and on 2012 and 2013 IDNR/SHL biological sampling conducted near Hornick.
EXPLANATION: The presumptive Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at the West Fork of the Little Sioux River at Hornick were as follows: the 2010 geometric mean was 386 orgs/100 ml, the 2011 geometric mean was 129 orgs/100 ml and the 2012 geometric mean was 157 orgs/100 ml. All three geometric means slightly exceed the Class A1 criterion of 126 orgs/100 ml. Eleven of the 24 samples (46%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
Monitoring at the IDNR station east of Hornick showed no violations of Class B(WW2) water quality criteria in the 36 samples collected during the 2010-2012 assessment period for dissolved oxygen (minimum value = 7.5 mg/l), pH (range = 7.6 to 8.4 units), temperature (maximum = 29.2C), ammonia-nitrogen (maximum value = 1.5), chloride, or sulfate. These results suggest "full support" of the Class B(WW2) aquatic life uses. The extensive habitat alterations due to channel straightening in this stream system suggest a concern regarding full support of the aquatic life uses. This conclusion is based on a stream use assessment conducted by IDNR staff in 1995 (see assessments developed for the 1996, 1998, and 2000 reporting cycles for more information).
In contrast to the water quality aquatic life assessment, the aquatic life assessment based on biological sampling suggests the aquatic life uses are "partially supporting". This evaluated biological assessment was based on data collected in 2012 and 2013 as part of the IDNR/SHL large river sampling project. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach. The biological metrics were combined a benthic macroinvertebrate index (BMIBI). The index rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2012 BMIBI score was 29 (poor) and the 2013 BMIBI score was 61 (good). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI scores with biological impairment criteria (BIC) established from a statistical analysis of biological data collected at stream ecoregion reference sites from 1994-2008. The BMIBI BIC for this ecoregion is 54. This assessment is considered evaluated because the biological data did not provide conclusive evidence to justify impairing this segment.
