Assessment Comments
Assessment is based on results of monthly monitoring from 2006 through 2008 at the IDNR/UHL ambient city monitoring station located downstream from Marshalltown at the county road E35 bridge (STORET station 10640002).
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to high levels of indicator bacteria. The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient water quality monitoring downstream of Marshalltown. Fish consumption uses remain 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 January 2006 through September 2008 at the IDNR/UHL ambient city monitoring station located downstream from Marshalltown at the county road E35 bridge (STORET station 10640002).
EXPLANATION: The Class A1 uses are assessed (monitored) as "not supported" due to levels of indicator bacteria (E. coli) that exceed Iowa water quality criteria. The geometric mean level of indicator bacteria (E. coli) in the 22 samples analyzed during the recreational seasons of 2006 through 2008 (248 orgs/100ml) exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml. Ten of the 22 samples (45%) 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." IDNR/UHL ambient monitoring at the Co. Rd. E35 station during the 2006-2008 assessment period showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 34 samples analyzed, for toxic metals in the five samples analyzed, or for pesticides in the approximately three samples analyzed.
Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
