Assessment Comments

Assessment is based on results of monthly monitoring from 2004 through 2006 at the IDNR ambient station located at the County Road G28 bridge NE of Conesville (IDNR station 10700001).

Basis for Assessment

SUMMARY:  The Class A1 (primary contact recreation) uses are assessed (monitored) as “fully supported” based on results of ambient monitoring for indicator bacteria.   The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported” based on results of ambient chemical/physical water quality monitoring.   Fish consumption uses are not assessed due to the lack of recent fish contaminant monitoring in this assessment segment.   The source of data for this assessment is the results of monthly monitoring from 2004 through 2006 at the IDNR ambient station located at the County Road G28 bridge NE of Conesville (IDNR station 10700001).  

EXPLANATION:  The Class A1 uses were assessed as "fully supported" based on results of IDNR ambient monitoring near Conesville.   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 (59 orgs/100ml) is well below the Iowa Class A1 water quality criterion of 126 orgs/100ml.   Three of the 24 samples 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.”  According to IDNR’s assessment/listing methodology, however, these results do not suggest that the violation frequency of Iowa’s single-sample maximum criterion is significantly greater than 10%, and thus these results do not suggest impairment of the Class A1 uses of this river segment.  

The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of monitoring from the IDNR ambient station near Conesville from 2004 through 2006.   Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen or ammonia in the 36 samples collected or for pesticides or toxic metals in the 10 samples analyzed during this assessment period.   During the assessment period, levels of pH occasionally violated the Class B(WW1) criterion of 9.0 pH units.   Six of the 36 samples had pH values greater than 9.0 units, with a maximum value of 9.3 units.   According to U.S.  EPA assessment guidelines, if more than 10% of samples exceed state criteria for pH, the primary contact (Class A) and aquatic life (Class B) uses should be assessed as "impaired" (see pgs 3-17 of U.S.  EPA 1997b).   Based on IDNR’s assessment methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s pH criteria and thus do not suggest an impairment of the Class A1 or Class B(WW1) uses of this river segment.   Violations of pH in ambient waters tend to reflect high levels of primary productivity and typically do not reflect the addition of pollutants to surface waters.  

Fish consumption uses are not assessed due to the lack of recent fish contaminant monitoring in this river segment.   Previous assessments were based on results of fish contaminant monitoring conducted by USGS in 1995 as part of the NAWQA project.   These data are now considered too old (greater than 10 years) to accurately characterize current water quality conditions.