Assessment Comments
Assessment is based on results of IDNR ambient monthly monitoring downstream from Marshalltown from 2000-02.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" due to high levels of indicator bacteria. The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient water quality monitoring downstream of Marshalltown. Fish consumption uses were not assessed due to the lack of recent fish contaminant monitoring in this river segment. The source of data for this assessment is the results of monthly monitoring from April 2000 through December 2002 at the IDNR/UHL ambient city monitoring station located downstream from Marshalltown at the county road E35 bridge (STORET station 10640002).
EXPLANATION: The Class A uses were assessed as "partially supported" due to levels of indicator bacteria (fecal coliforms) that exceed U.S. EPA and IDNR assessment guidelines. For purposes of Section 305(b) assessments, IDNR 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." Eleven of the 16 samples collected from the Co.Rd. E35 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 11 non-runoff-affected samples (85 orgs/100ml) is well below the Iowa Class A water quality criterion of 200 orgs/100ml. However, two of the 11 samples (18%) 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 (monitored) as "fully supported." IDNR/UHL ambient monitoring at the Co.Rd. E35 station showed no violations of Class B(WW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 28 samples collected, or in the nine samples analyzed for pesticides, during the 2000-2002 assessment period. The level of pH, however, violated the Class B(WW) criterion of 9.0 pH units in one sample. The sample collected on August 21, 2000, had a pH level of 9.1 units which is greater than the Iowa water quality standard of 9.0 pH units. This violation occurred on a day with a relatively high level of dissolved oxygen (12.4 mg/l) and a percent DO saturation of 140%. These conditions suggest that a high level of primary productivity resulted in the high level of pH. Because this violation is more related to natural conditions than to pollution, the occurrence of the high level of pH in this river segment is not seen as a water quality impairment. Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
