Assessment Comments
Assessment is based on results of monthly monitoring from 2006 through 2008 at the IDNR/UHL ambient city monitoring station located upstream from Fort Dodge (STORET station 10940002).
Basis for Assessment
[Note: Prior to the 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), this segment is also now presumptively 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 presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on results of ambient monitoring for indicator bacteria (E. coli). This assessment represents a new Section 303(d) impairment for this river segment. The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring from 2006-2008. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this river segment. The source of data for this assessment is the results of monthly monitoring from 2006 through 2008 at the IDNR/UHL ambient city monitoring station located upstream from Fort Dodge (STORET station 10940002).
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on results of monitoring for indicator bacteria (E. coli). The geometric mean level of indicator bacteria (E. coli) in the 22 samples collected (112 orgs/100ml) during the recreational seasons of 2006 through 2008 is below, and meets, Iowa’s Class A1 water quality criterion of 126 orgs/100ml. Seven of the 22 samples (32%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and according to IDNR’s assessment/listing methodology, if the geometric mean level of E. coli is less than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). These guidelines, however, also suggest that if more than 10% of samples exceed the single-sample maximum criterion for indicator bacteria, the primary contact recreation uses should be assessed as "partially supported." According to IDNR’s assessment/listing methodology, the monitoring data from 2006-08 suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum criterion, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired.”
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported." Monitoring at the IDNR city station upstream from Fort Dodge showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 34 samples collected, for toxic metals in the five samples collected, and for pesticides in the three samples analyzed, during the 2006-2008 assessment period. These results suggest "full support" of the Class B(WW1) aquatic life uses.
This segment of the Des Moines River was sampled as part of the 1998-99 statewide study of freshwater mussels in Iowa streams and rivers (Arbuckle et al. 2000). As part of this study, sampling results from 1998 and 1999 (Arbuckle et al. 2000) were compared to results from stream sites surveyed in 1984 and 1985 by Frest (1987). On a statewide basis, this comparison showed sharp declines in the numbers of mussel species ("species richness") in Iowa streams and rivers from the mid-1980s to the late 1990s. For stream segments having four or more species reported for the 1984-95 survey, results of this comparison were used by staff of the Iowa DNR Water Quality Bureau to assess the degree to which the aquatic life uses of the sampled stream segments are supported. The results of this sampling on the Des Moines River, however, do not meet IDNR guidelines for developing an assessment of support for the aquatic life uses. Species richness of freshwater mussels at the five sample sites in this segment was 0, 0, 1, 1, and 2 in the 1984-85 period and was 0, 0, 0, 0, and 3 in the 1998-99 period, respectively, for an average 1984-85 species richness of approximately 1 and an average percent change of minus 30%. Based on these results, the aquatic life uses are considered “not assessed” due (1) to IDNR’s assessment methodology in which assessment decisions are developed only for those stream segments having an average of four or more species reported in the 1984-85 (Frest) survey and (2) the difficulty of interpreting status of mussel communities showing relatively low species richness during the both the historical (1984-85) and current (1998-99) surveys.
Fish consumption uses remain "not assessed" due to the lack of recent fish contaminant monitoring in this river segment.
