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 from 2000-02, (2) IDNR/UHL ambient water quality monitoring upstream from Iowa City, and (3) 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" based on high levels of indicator bacteria at the University of Iowa/Corps of Engineers (UI/ACOE) ambient monitoring station in Iowa City. The Class B(WW) aquatic life uses are assessed (monitored) as "partially supported" based on results of a statewide survey of freshwater mussels in 1998-99. The Class C (drinking water) remain assessed (monitored) as "fully supported/threatened" based on results of ambient monitoring by conducted by both IDNR/UHL and University of Iowa/Corps of Engineers. Fish consumption uses remain assessed as "fully supported" based on results of annual fish contaminant monitoring conducted by the University of Iowa/Corps of Engineers from 2000-2002. 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 statewide assessment of freshwater mussels in 1998-1999 conducted by Iowa State University (Arbuckle at al. 2000), (3) results of monitoring from the IDNR/UHL ambient city monitoring station upstream from Iowa City at the Dubuque Street bridge, and (4) results of fish tissue monitoring in 2000, 2001, and 2002 conducted for the UI/ACOE Coralville Reservoir Water Quality Study.
EXPLANATION: Results of water quality at the UI/ACOE 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 25 samples analyzed for indicator bacteria (fecal coliforms) during summers of 2000, 2001, and 2002, three samples 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 25 non-runoff-affected samples was 285 orgs/100 ml; ten of the 25 samples (40%) exceeded the EPA-recommended single-sample maximum value of 400 orgs/100 ml. The geometric mean (285 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 for indicator bacteria suggest “full support” of primary contact recreation uses. Of the 16 samples collected upstream from Iowa City at the IDNR/UHL station at Dubuque Street, 13 were collected at flows not materially affected by surface runoff. The geometric mean of these six samples was only 27 fecal coliform orgs/100 ml; none of the 13 samples had levels of fecal coliforms that exceeded the U.S. EPA-recommended single-sample maximum value of 400 orgs/100 ml. The low levels of indicator bacteria at the upriver IDNR/UHL station are consistent with monitoring results at other station located immediately downriver from Iowa’s federal flood control reservoirs. Typically, these stations show the lowest levels of indicator bacteria of any river monitoring stations in the state. The much higher levels of bacteria at the downriver ACOE monitoring stations likely reflects inputs of indicator bacteria potentially in the river’s intervening tributaries (e.g., Muddy Creek and Clear Creek) and/or from the Iowa City/Coralville urban area.
The Class B(WW) aquatic life uses are assessed (monitored) as "partially supported" based on results of the 1998-99 statewide survey of freshwater mussels; results of ambient monitoring at the IDNR/UHL and UI/ACOE stations, however, suggest full support" of these uses. Monitoring at these stations showed no violations of Class B(WW) criteria for pH, ammonia-nitrogen, or dissolved oxygen in the approximately (combined) 90 samples collected at both stations during the 2000-2002 assessment period. Despite these indications of good water quality, results from the 1998-99 statewide assessment of freshwater mussels in Iowa streams suggest a potential impairment to the aquatic life uses of this stream segment. 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. The confidence level of this assessment is relatively high; thus the assessment type is considered “monitored” in the context of Section 305(b) reporting. According to Iowa DNR’s assessment methodology, waterbodies identified as “impaired” based on a “monitored” assessment are candidates for Section 303(d) listing. 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 minus 56%. Based on these results, the “full support” of aquatic life uses suggested by results of IDNR and ACOE ambient water quality 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%. The nearly 60% decline in species richness suggest a significant modification of this aquatic community in the short timeframe of less than 15 years. Because results of chemical/physical monitoring suggests good water quality in this river segment, the impairment of aquatic life uses suggested by the apparent decline of freshwater mussels is likely related to problems with the quality of aquatic habitat as opposed to poor chemical/physical water quality. 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 (monitored) as "fully supported / threatened" due to violations of the Class C criterion (MCL) for nitrate+nitrite at both the IDNR/UHL and UI/ACOE ambient monitoring stations. Five of 64 samples collected at the UI/ACOE station near the Iowa City water treatment plant (8%) 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 4.2 mg/l. One of 28 samples collected at the Iowa DNR station upstream from Iowa City at Dubuque Street (4%) 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 one or more, but less than 10%, 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, 2001, and 2002 as part of the UI/ACOE 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.
