Assessment Comments
Assessment is based on results of (1) routine water quality & fish tissue monitoring as part of the UI/ACOE Coralville Water Quality Study in 2000-01 and (2) statewide assessment of freshwater mussels in 1998-99.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "not supported," the Class B(WW) aquatic life uses are assessed (evaluated) as "partially supported," and the Class C (drinking water) remain assessed (monitored) as "fully supported/threatened." Fish consumption uses remain assessed as "fully supported." The sources of data for this assessment include (1) results of water quality monitoring conducted near the Iowa City water treatment plant by the University of Iowa (under contract with the U.S. Army Corps of Engineers) as part of the Coralville Reservoir Water Quality Study, (2) results of a statwide assessment of freshwater mussels in 1998-1999, (3) results of monitoring from the IDNR ambient city monitoring station upstream from Iowa City at the Dubuque Street bridge, and (4) results of fish tissue monitoring in 2000 and 2001 conducted for the University of Iowa/Army Corps of Engineers water quality project. EXPLANATION: Results of water quality at the long-term station near the University of Iowa water treatment plant suggest that the Class A uses should be assessed as "not supported." Of the 19 samples analyzed for indicator bacteria (fecal coliforms) during summers of 2000 and 1999, 3 were collected during conditions of high river flow. These high flows resulted in monitoring at river discharges that exceeded the long-term monthly average flow plus one standard deviation of this average (flow statistics from Fischer et al. 1990). 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." The geometric mean of fecal coliform bacteria in the 16 non-runoff-affected samples was 221 orgs/100 ml; five of the 16 samples (31%) exceeded the EPA-recommended single-sample maximum value of 400 orgs/100 ml. The geometric mean (221 orgs/100 ml) exceeds the state water quality criterion of 200 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if the geometric mean value exceeds 200 orgs/100 ml, the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). In contrast, results of IDNR's ambient city monitoring suggest full support of primary contact recreation uses. Of the 9 samples collected upstream from Iowa City at Dubuque Street, six were collected flows not materially affected by surface runoff. The geometric mean of these six samples was 33 fecal coliform orgs/100 ml; none of the six samples had levels of fecal coliforms that exceeded the U.S. EPA-recommended single-sample maximum value of 400 orgs/100 ml. Due, however, to the more complete data record for the UI/ACOE station at Iowa City, the Class A uses designated for this river segment will remain assessed as "not supported." Differences in the results from the UI/ACOE station and the IDNR city station are likely due to (1) lack of monitoring at the IDNR station during most of summer 2000 when levels of bacteria in this reach of the Iowa River tended to be relatively high, (2) relative lack of sources of these bacteria between the Coralville Dam and IDNR's Dubuque Street monitoring station, and (3) the location of the UI/ACOE station downstream from several potential sources of these bacteria, including Iowa River tributaries (e.g., Clear Creek, Muddy Creek) and runoff from the Coralville and Iowa City urban areas. The Class B(WW) aquatic life uses are assessed as "partially supported." Despite this assessments, results of monitoring at the Iowa/ACOE station suggest full support" of these uses: this monitoring showed no violations of Class B(WW) criteria for pH, ammonia-nitrogen, or dissolved oxygen in the 43 samples collected during the 2000-2001 biennial 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). In general, this comparison showed sharp declines in the numbers of mussel species ("species richness") from the mid-1980s to the late 1990s. For purposes of Section 305(b) reporting, 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. This assessment included the following factors: (1) the percent change in the number of species of freshwater mussels found in the 1984-85 survey versus the 1998-99 survey and (2) the number of mussel species found in the 1998-99 survey. Greater than a 50% decline in species richness from the 1984-84 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. For purposes of Section 305(b) assessment only, staff of the IDNR Water Quality Bureau used results from Arbuckle et al. (2000) to define categories of species richness for Iowa's mussel communities: less than three species indicates low species richness and "nonsupport" or "partial support" of aquatic life use; from four to seven species indicates moderate species richness and potential minor impacts (="fully supported / threatened"); more than seven species indicates high species richness and "full support" of aquatic life uses. Species richness of freshwater mussels at the 6 sample sites in this stream segment were 5, 5, 13, 10, 7, and 12 in the 1984-85 period and were 0, 7, 1, 2, 5, and 3, respectively, in the 1998-99 period for an average percent change of -56%. Based on these results, the full support of aquatic life uses suggested by results of IDNR and ACOE ambient monitoring is downgraded to "partial support." This study segment did, however, contain one sample site that showed an increase in species richness from 5 in the 1984-85 period to 7 species in the 1998-99 period. Despite this increase, the remaining five sample sites showed percent declines in species richness of from 29 to 100%. 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. 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. The Class C drinking water uses are assessed "fully supported / threatened" due to violations of the Class C criterion (MCL) for nitrate+nitrite. Five of 43 samples collected at the U.S. Army Corps station near the Iowa City water treatment plant (12%) exceeded the 10 mg/l MCL; all violations occurred during the months of June or July of 2000 and 2001; the maximum violation was 11.5 mg/l. The average nitrate level was 4.5 mg/l (standard deviation = 3.4 mg/l). One of 14 samples collected at the Iowa DNR station upstream from Iowa City at Dubuque Street (7%) exceeded the 10 mg/l MCL; this violation occurred in June 2001; the nitrate level in this sample was 13.0 mg/l. The average nitrate level was 4.6 mg/l (standard deviation = 4.1 mg/l); the median value was 3.6 mg/l. Based on DNR's Section 305(b) assessment methodology, if less than 15% of samples collected monthly or more frequently exceed the MCL for nitrate, drinking water uses should be assessed as "fully supporting / threatened." Fish consumption uses remain assessed as "fully supporting." Fish contaminant monitoring conducted down river from Coralville Reservoir in 2000 and 2001 as part of the Coralville Reservoir Water Quality Study showed that levels of organochlorine contaminants (chlordane, dieldrin, and heptachlor epoxide) in composite samples of whole-fish carp were well below ½ of the respective FDA action levels.
