Assessment Comments
Assessment is based on results of IDNR/UHL monthly ambient monitoring near Larrabee from 2000-02.
Basis for Assessment
[Note: For the 2002 report, the previous waterbody segment for the Little Sioux River (IA 06-LSR-0030-1), which extended 38 miles from the Highway 3 at Cherokee (Cherokee Co.) (approximately equal to confluence with Mill Creek north of Cherokee) to the Linn Grove Dam (Buena Vista Co.), was split into two subsegments: (1) Highway 3 at Cherokee to confluence with Waterman Creek (O'Brien Co.) (IA 06-LSR-0030-1 - this one) and (2) Waterman Creek to the Linn Grove Dam (Buena Vista Co.) (IA 06-LSR-0030-2). See previous Section 305(b) assessments from this subsegment for the original 38-mile river reach.]
SUMMARY: The Class A (primary contact recreation) uses are assessed as “partially supported” due to high levels of indicator bacteria. The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient physical/chemical monitoring. 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 monitoring conducted during the 2000-2002 assessment period near Larrabee (STORET station 10180001 (formerly station 911060)).
EXPLANATION: The Class A uses were assessed (monitored) as "partially supported" due to high levels of indicator bacteria (fecal coliforms). The geometric mean of indicator bacteria (fecal coliforms) in the 19 samples not materially affected by surface runoff during the recreational seasons of 2000, 2001, and 2002 at the Larrabee monitoring station was below the Iowa water quality criterion (200 fecal coliform orgs/100ml) to protect primary contact recreation uses; however, the percentage of samples that exceeded the U.S. EPA-recommended single-sample maximum value suggests only "partial support" of the Class A uses. For purposes of Section 305(b) assessments, DNR uses the long-term average monthly flow plus one standard deviation of this average to identify river flows that are materially affected by surface runoff. According to the Iowa Water Quality Standards (IAC 1990:8), the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) does not apply "when the waters are materially affected by surface runoff." The geometric mean of fecal coliform bacteria in the 19 non-runoff-affected samples was 146 orgs/100 ml, with four samples (21%) exceeding the EPA-recommended single-sample maximum value of 400 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of the samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are "partially supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
The Class B(WW) aquatic life uses remain assessed as “fully supported.” Monitoring at the IDNR/UHL station near Larrabee showed no violations of Class B(WW) water quality criteria during the 2000-2002 assessment period for dissolved oxygen (minimum value = 6.7 mg/l), pH (range = 7.7 to 8.9 units), or ammonia-nitrogen (maximum value = 0.40 mg/l) in the 24 samples analyzed. In addition, levels of toxic metals in the four samples analyzed, and levels of pesticides in the six samples analyzed, were all below the analytical level of detection. These results suggest "full support" of the Class B(WW) aquatic life uses.
Fish consumption uses remain "not assessed due to the lack of recent fish contaminant monitoring in this river reach.
