Assessment Comments
Assessment is based on results of IDNR/UHL monthly ambient monitoring near Larrabee from 2002-04.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as “partially supported” due to high levels of indicator bacteria (E. coli). 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 chemical/physical monitoring conducted during the 2002-2004 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 (E. coli). Due to recent changes in Iowa’s Water Quality Standards, Iowa’s 2006 assessment methodology for indicator bacteria has changed. Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses: the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff." Due to a change in the Standards in July 2003, E. coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E. coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.” Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E. coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of the 2006 Section 305(b) assessments and Section 303(d) listings.
The geometric mean level of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2002 through 2004 (73 orgs/100ml) is well below the Iowa Class A water quality criterion of 126 orgs/100ml. Six of the 24 samples (25%), however, exceeded Iowa’s single-sample maximum value of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if levels of E. coli exceed the single-sample maximum value in more than 10% of the samples, the primary contact recreation uses should be assessed as “partially supported” (see pgs 3-33 to 3-35of U.S. EPA 1997b). According to IDNR’s assessment/listing methodology, these results suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum value, thus suggesting that the Class A uses should be assessed as “partially supported/impaired”. Despite the impairment, the results of monitoring for E. coli during the 2002-2004 period suggest that levels of indicator bacteria are relatively low in this river segment.
The Class B(WW) aquatic life uses remain assessed (monitored) as “fully supported”. Monitoring at the IDNR/UHL station near Larrabee showed no violations of Class B(WW) water quality criteria in the 36 samples collected during the 2002-2004 assessment period for dissolved oxygen (minimum value = 5.0 mg/l), pH (range = 7.8 to 9.0 units), or ammonia-nitrogen (maximum value = 0.53 mg/l). In addition, levels of toxic metals in the ten samples analyzed, and levels of pesticides in the seven samples analyzed, were all below their respective Class B(WW) criteria. 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.
