Assessment Comments
Assessment is based on results of IDNR/UHL ambient city monitoring upstream from Marshalltown from 2000-02.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses were assessed (monitored) as "not supported" due to levels of indicator bacteria that exceed state water quality standards. The Class B(WW) aquatic life uses were assessed (monitored) as "fully supported" based on results of ambient water quality monitoring upstream from Marshalltown. Fish consumption uses were not assessed due to the lack of recent fish contaminant monitoring. 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 upstream from Marshalltown at the state highway 330 bridge northwest of Marshalltown.
EXPLANATION: The Class A uses were assessed as "not supported" due to levels of indicator bacteria (fecal coliforms) that exceed state water quality criteria. 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." Eleven of the 16 samples collected from the Highway 330 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 (210 orgs/100ml) is slightly above the Iowa Class A water quality criterion of 200 orgs/100ml. Three of the 11 samples (27%) 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 the geometric mean is greater than 200 orgs/100ml, the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
The Class B(WW) aquatic life uses were assessed as "fully supported." Monitoring at the Highway 330 station 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 9 samples analyzed during the 2000-2002 assessment period. Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
