Assessment Comments
Assessment is based on results of (1) IDNR ambient city monitoring in 2000-02, (2) ISU statewide assessment of freshwater mussels in 1998-99, and (3) U.S. EPA/IDNR fish tissue (RAFT) monitoring at Iowa City in 2001.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" based on results of ambient monitoring for indicator bacteria. 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 fish consumption uses are assessed as "fully supported" based on results of fish contaminant monitoring in 2001. The sources of data for this assessment are (1) the results of monthly monitoring from May 2000 through December 2002 at the IDNR ambient city monitoring station located downstream from Iowa City at the county road F62 bridge east of Hills, IA (station 10520003), (2) results of the statewide assessment of freshwater mussels in Iowa (Arbuckle et al. 2000), and (3) results of EPA/IDNR fish tissue (RAFT) monitoring at Iowa City in 2001.
EXPLANATION: The Class A uses were assessed as "partially supported" due to levels of indicator bacteria (fecal coliforms) that violate state water quality standards. 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." Thirteen of the 16 samples collected from the County Road F62 bridge station during the 2000, 2001, and 2002 recreational seasons were collected at flows not materially affected by surface runoff. The geometric mean level of indicator bacteria (fecal coliforms) in these 13 non-runoff-affected samples (148 orgs/100ml) is below the Iowa Class A water quality criterion of 200 orgs/100ml. However, three of the 13 samples (23%) 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 more than 10% of the samples exceed the single-sample maximum value of 400 orgs/100 ml, the primary contact recreation uses are "partially supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
The Class B(WW) aquatic life uses were assessed as "partially supported" based on results of the statewide survey of freshwater mussels; results of monitoring from the IDNR ambient station, however, suggest full support of the Class B(WW) uses. Monitoring at the IDNR ambient station near Hills, however, showed no violations of Class B(WW) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 28 samples collected or for pesticides in the nine samples analyzed during this assessment period.
Despite results of IDNR/UHL ambient water quality monitoring that suggest “full support” of aquatic life uses, results of the 1998-99 statewide survey of freshwater mussels suggest a potential impairment to these uses. 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 9 sample sites in this stream segment were 22, 17, 11, 10, 14, 18, 7, 16, and 6 in the 1984-85 period and were 10, 4, 3, 2, 2, 8, 6, 8, and 4, respectively, in the 1998-99 period for an average percent change of minus 59%. Despite the moderate diversity of freshwater mussels found in this river segment during the 1998-99 survey, the nearly 60% decline in species richness suggest a significant modification of this aquatic community in the short timeframe of less than 15 years. Based on these results, and based on IDNR’s assessment methodology, the “full support” of aquatic life uses suggested by results of IDNR/UHL ambient monitoring is downgraded to "partial support." 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.
Fish consumption uses are assessed as fully supported based on results of EPA/IDNR fish tissue (RAFT) follow-up monitoring in 2001 that showed levels of all contaminants were well below U.S. FDA action levels and IDNR levels of concern. This river segment has a history of relatively high levels of chlordane, and RAFT follow-up monitoring has thus been conducted on an every-other-year basis since 1994 (see assessments for the 1994 through 2000 reports). The level of chlordane in the sample of channel catfish fillets collected in 2001 was 0.049 mg/kg; this level is much lower that chlordane levels in previous samples (1997: 0.17 mg/kg; 1999: 0.16 mg/kg). This site was again sampled as a “follow-up” site in the 2003 RAFT program; the results from this monitoring are not yet available.
