Assessment Comments
Assessment is based on results of (1) IDNR monthly monitoring near Olin in Jones Co. from 2000-02 and (2) results of a statewide survey of freshwater mussels from 1998-99.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses were assessed (monitored) as "fully supported" based on results of ambient chemical/physical water quality monitoring from 2000-02 The Class B(WW) aquatic life were assessed (monitored) as "partially supported" due to the apparent decimation of freshwater mussel species in this river segment. Fish consumption uses were not assessed due to the lack of fish contaminant monitoring in this river segment. The sources of data for this assessment include (1) the results of monthly monitoring at the IDNR fixed ambient station at the Highway 38 bridge north of Olin in from 2000 through 2002 (STORET station 10530001) and (2) information from the report "Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams" by Arbuckle et al. (2000).
EXPLANATION: The Class A uses were assessed (monitored) as "fully supported" based on low levels of indicator bacteria. 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." Eleven of the 21 samples were collected at flows greater than the monthly mean plus one standard deviation of this average and were thus considered "materially affected by surface runoff." Results of IDNR monitoring in the recreational seasons of 2000, 2001, and 2002 showed that the geometric mean level of indicator bacteria (fecal coliforms) in the 10 non-runoff-affected samples (108 orgs/100ml) was below the Iowa Class A water quality criterion of 200 orgs/100ml. One of the 10 samples (10%) exceeded the U.S. EPA-recommended single-sample maximum value of 400 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if no more than 10% of samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are assessed as "fully supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
The results of ambient water quality also suggest “full support” of the Class B(WW) aquatic life uses due to (1) the lack of violations of Iowa Class B(WW) water quality criteria for pH, dissolved oxygen, and ammonia-nitrogen in the 36 samples collected from January 2000 through December 2002 at the IDNR fixed ambient station near Olin and (2) the lack of violations of Class B(WW) water quality criteria in the 9 samples from this station analyzed for pesticides during the 2000-2002 period.
Results from the 1998-99 statewide assessment of freshwater mussels in Iowa streams, however, suggest a potential impairment to the aquatic life uses of this stream segment. 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’s rivers and streams from the mid-1980s to the late 1990s. 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. For purposes of Section 303(d) listing, this assessment was based on the percent change in the number of species of freshwater mussels found in the 1984-85 survey versus the 1998-99 survey. Greater than a 50% decline in species richness from the 1984-85 to the 1998-99 period suggests an impairment of the aquatic life uses. In addition, low species richness in the 1998-99 survey suggests potential impairment. Species richness of freshwater mussels at the four sample sites in this stream segment were 6, 2, 1, and 8 in the 1984-85 period and were 1, 0, 0, and 4, respectively, in the 1998-99 period. Although species richness declined at all four sites, the average decline at the two sample sites with four or more species in the 1984-85 survey (from 6 to 1 species, and from 8 to 4 species) was 67%. Based on IDNR’s assessment approach, this degree of decline suggests “partial support” of the Class B(WW) aquatic life uses for purposes of both Section 303(d) listing and 303(d) reporting. Based on these results, the full support of aquatic life uses suggested by results of IDNR ambient monitoring is downgraded to "partially supported." As presented by Arbuckle et al. (2000), the potential causes of declines in species richness of Iowa's freshwater mussels include siltation, destabilization of stream substrate, stream flow instability, and high in-stream levels of nutrients (phosphorus and nitrogen). Their study also suggested the importance of stream shading provided by riparian vegetation to mussel species richness. Additional monitoring is needed to better define the biological status of this stream segment as well as the site-specific causes and sources of impairments of these uses that may exist.
Fish consumption uses were not assessed due to the lack of fish contaminant monitoring information for this river reach.
