Assessment Comments
Assessment remains based on statewide assessment of freshwater mussels in Iowa (Arbuckle et al. 2000). Potential causes & sources of impairment were applied to all sites in study. Assessment is also based on results of IDNR bacteria monitoring during summer 2008.
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, 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 remain assessed (monitored) as “not supported” due to levels of indicator bacteria (E. coli) that exceed Iowa’s geometric mean criterion. The source of data for this assessment is IDNR bacterial monitoring during the summer 2008 recreation season at STORET station 15310001. The Class B(WW1) aquatic life uses remain assessed (evaluated) as "not supported" based on information from the report "Statewide Assessment of Freshwater Mussels (Bivalva, Unionidae) in Iowa Streams" by 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. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.
EXPLANATION: The Class A1 (primary contact recreation) uses remain assessed (monitored) as “not supported” based on results of IDNR bacteria monitoring in summer 2008. Seven samples were collected from June through August 2008 as part of IDNR’s county/city beach monitoring project. The geometric means of E. coli in these seven samples was 2,407 orgs/100 ml; all seven samples exceeded Iowa’s single sample maximum criterion of 235 orgs/100 ml. Although IDNR’s listing methodology requires that a minimum of ten E. coli samples be collected over a three-year period to support a Section 303(d) listing, this segment is being added to Iowa’s 2010 Section 303(d) list based on overwhelming evidence of impairment. That is, even if three additional samples had been collected, and each of these three samples had less than 10 orgs/100 ml (the INDR detection level), the resulting geometric mean of E. coli (465 orgs/100 ml) would still have exceeded Iowa’s Class A1 criterion of 126 orgs/100 ml. Thus, the Class A1 uses of this segment are assessed (“monitored”) as “not supported” and were added to Category 5a of IDNR’s 2010 Integrated Report (Iowa’s Section 303(d) list).
For purposes of Section 303(d) listing, the assessment of the Class B(WW1) aquatic life uses 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. 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. 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. Species richness of freshwater mussels at the two sample sites in this segment of Whitewater Creek was 8 and 3, respectively, in the 1984-85 period (average of 5 species) and was zero at both sites in the 1998-99 period for an average percent change of minus 100%. Based on these results, the aquatic life uses are assessed (monitored) as “not 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 instream levels of nutrients (phosphorus and nitrogen). Their study also suggested the importance of stream shading provided by riparian vegetation to mussel species richness.
Because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence). Despite this change in assessment type, this waterbody remains in IR Category 5b.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.
