Assessment Comments
Assessment is based on results of monthly monitoring from January 2008 through December 2010 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 2008 through December 2010 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 means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10640003 upstream from Marshalltown were as follows: the 2008 geometric mean was 225 orgs/100 ml, the 2009 geometric mean was 713 orgs/100 ml and the 2010 geometric mean was 912 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Fifteen of the 22 samples (68%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported." Monitoring at the IDNR ambient station at Highway 330 during 2008-2010 showed no violations of Class B(WW1) water quality criteria in the 33 samples analyzed for dissolved oxygen, pH and ammonia or in the three samples analyzed for toxic metals and pesticides.
Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
