Assessment Comments
Assessment is based on results of monthly monitoring from 2008 through 2010 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 2008 through December 2010 at the IDNR/UHL ambient city monitoring station located downstream from Marshalltown at the county road E35 bridge (STORET station 10640002). Note: USGS conducted monitoring in this assessment segment at station 05451500 at Marshalltown in June 2008. Too few data were produced from this monitoring during the 2008-2010 assessment period (one sample) to be of use for assessment for purposes of either Section 305(b) reporting or Section 303(d) listing.
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 means of indicator bacteria (E. coli) in the 22 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10640002 downstream from Marshalltown were as follows: the 2008 geometric mean was 93 orgs/100 ml, the 2009 geometric mean was 383 orgs/100 ml and the 2010 geometric mean was 667 orgs/100 ml. The 2009 and 2010 geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Eleven of the 22 samples (50%) 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." IDNR/UHL ambient monitoring at the Co. Rd. E35 station during the 2008-2010 assessment period showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 33 samples analyzed or for toxic metals and pesticides in the three samples analyzed.
Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
