Assessment Comments

Assessment is based on results of IDNR/UHL monthly ambient monitoring from 2004 through 2006 approximately two miles south of Humboldt (STORET station 10460001 (formerly station 444061)).

Basis for Assessment

[Note:  Prior to the current (2008) Section 305(b) cycle, this stream segment was designated only for Class B(WW) aquatic life uses, including fish consumption 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), and due to the completion of a Use Attainability Analysis in 2007, this segment is also now designated for Class A1 (primary contact recreation) uses.   This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on results of ambient monitoring for indicator bacteria (E.  coli) during recreational seasons of 2004 through 2006.   The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring from 2004-2006.   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 from 2004 through 2006 approximately two miles south of Humboldt (STORET station 10460001 (formerly station 444061)).  

EXPLANATION:  The Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on results of monitoring for indicator bacteria (E.  coli).   Due to recent changes in Iowa’s Water Quality Standards, Iowa’s 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 Section 305(b) assessments and Section 303(d) listings.  

The geometric mean level of indicator bacteria (E.  coli) in the 24 samples collected (103 orgs/100ml) during the recreational seasons of 2004 through 2006 is below, and meets, Iowa’s Class A1 water quality criterion of 126 orgs/100ml.   Six of the 24 samples (25%), however, exceeded Iowa’s single-sample maximum criterion 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 criterion 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 the violation frequency of Iowa’s single-sample maximum criterion in this assessment segment is, by two violations, significantly greater than 10%, and thus these results suggest an impairment of the Class A1 uses of this river segment.  

Results of bacterial monitoring from 2004-2006 at monitoring stations upriver from Saylorville Reservoir, suggest low levels of E.  coli in the upper Des Moines River.   For example, the three-year geometric means (N=24-36 samples) for stations near Boone (127 orgs/100 ml), downstream from Fort Dodge (99 orgs/100 ml), upstream from Fort Dodge (82 orgs/100 ml), and on the West Fork Des Moines River near Humboldt (103 orgs/100 ml) all met the Iowa Class A1 criterion of 126 E.  coli orgs/100 ml.   Although the percentage of samples greater than Iowa’s single-sample maximum criterion suggests impairment (“partial support”) at several of these stations, the difference between impairment and “full support” is typically one sample violation.   These results suggest generally low levels of indicator bacteria in the Des Moines River upriver from Saylorville Reservoir.  

Monitoring at the IDNR/UHL station south of Humboldt showed no violations of Class B(WW1) water quality criteria during the 2004-2006 assessment period for dissolved oxygen, pH, or ammonia-nitrogen in the 36 samples analyzed, for toxic metals in the ten samples analyzed, or for pesticides in the seven samples analyzed.   These results suggest "full support" of the Class B(WW1) aquatic life uses.  

Fish consumption uses were "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.