Assessment Comments

Assessment is based on results of (1) IDNR ambient monthly monitoring at Cedar Bluff and (2) statewide survey of freshwater mussels in 1998-99.

Basis for Assessment

[Note:  For the 2002 report, the previous waterbody segment IA 02-CED-0020-0, which extended 52 miles from Mud Creek (aka Sugar Creek) to its confluence with Prairie Creek at Cedar Rapids, was split into three subsegments:  (1) Mud Creek to Rock Run Creek in Cedar Co.  (IA 02-CED-0020-1), (2) Rock Run Creek to Highway 30 at Cedar Rapids (IA 02-CED-0020-2), and (3) Highway 30 to Prairie Creek at Cedar Rapids (IA 02-CED-0020-3).   See assessment information for segment IA 02-CED-0020-1 (Mud Creek to Rock Run Creek) for previous Section 305(b) assessments for this river reach.]

SUMMARY:  The Class A (primary contact recreation) uses were assessed (monitored) as "partially supported," and the Class B(WW) aquatic life uses were assessed (evaluated) as "partially supported."  Fish consumption uses were not assessed.   The sources of data for this assessment are (1) the results of monthly monitoring from October 1999 through September 2001 at the IDNR ambient station located at the County Road F28 bridge near Cedar Bluff and (2) results of a statewide survey of freshwater mussels conducted by Iowa State University in 1998 and 1999.   EXPLANATION: The Class A uses were assessed as "partially supported."  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 15 samples collected from the Cedar Bluff station during the 2000 and 2001 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 (68 orgs/100ml) is well below the Iowa Class A water quality criterion of 200 orgs/100ml.   However, two of the 13 samples (15%) 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% or less 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 3-35of U.S.  EPA 1997b).   The Class B(WW) aquatic life uses were assessed as "partially supported" based on results of the survey of freshwater mussels in 1998-99.   Resulst of monitoring from the IDNR ambient station near Cedar Bluff in 2000 and 2001, however, suggest "full support" of these uses.   Monitoring at this station showed no violations of Class B(WW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 24 samples collected or for toxic metals and toxic organic compounds in the four samples analyzed during this biennial period.   Levels of pH, however, occasionally violated the Class B(WW) criterion of 9.0 pH units.   Four of the 24 samples had pH values greater than 9.0 units, with a maximum value of 9.3 units.   All violations occurred on days when dissolved oxygen levels were well above 100% saturation:  November 1, 1999 (pH=9.1; % DO saturation=120%), April 6, 2000 (pH=9.3; DO saturation>140%), May 3, 2000, (pH=9.2; DO saturation>140%); November 1, 2000 (pH=9.2; DO saturation>140%).   These conditions suggest that high levels of primary productivity resulted in the high levels of pH.   Because these violations are more related to natural conditions than to pollution, the occurrenc of high levels of pH in this river segment is not seen as a water quality impairment.   Despite the results of water quality monitoring that suggest "full support" of the Class B(WW) uses, 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.   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 five sample site in this stream segment were 4, 5, 5, 9, and 5 in the 1984-85 period and were 0, 0, 0, 2, and 0, respectively, in the 1998-99 period for an averge percent change of -92%.   Based on these results, the full support of aquatic life uses suggested by results of IDNR ambient monitoring is downgraded to "partial support."  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 were not assessed due to lack of recent fish tissue monitoring in this river segment.  The most recent fish tissue monitoring in this segment was conducted for the U.S.  EPA/IDNR fish tissue (RAFT) monitoring program in 1996 (see assessment for the 2000 report above).   These data are now more than five years old and are considered too old for characterizing current water quality conditions.