Assessment Comments
Assessment is based on results of monthly monitoring from January 2006 through December 2008 at the IDNR/UHL ambient city monitoring station located upstream from Marshalltown at the state highway 330 bridge northwest of Marshalltown (station 10640003).
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that exceed state water quality criteria. The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient water quality monitoring upstream from Marshalltown. Fish consumption uses remain 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 January 2006 through December 2008 at the IDNR/UHL ambient city monitoring station located upstream from Marshalltown at the state highway 330 bridge northwest of Marshalltown (station 10640003).
EXPLANATION: The Class A1 uses are assessed (monitored) as "not supported" due to levels of indicator bacteria (E. coli) that exceed state water quality criteria. The geometric mean level of indicator bacteria (E. coli) in the 22 samples collected during recreational seasons of 2006 through 2008 (337 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml. Thirteen of the 22 samples (59%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, and according to IDNR’s assessment/listing methodology, if the geometric mean level of E. coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b).
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported." Monitoring at the IDNR ambient station at Highway 330 during 2006-2008 showed no violations of Class B(WW1) water quality criteria in the 34 samples analyzed for dissolved oxygen, pH and ammonia; in the five samples analyzed for toxic metals, or in the approximately three samples analyzed for pesticides.
Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
