Assessment Comments
Assessment is based on results of monthly monitoring from 2004 through 2006 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 2004 through 2006 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. Due to recent changes in Iowa’s Water Quality Standards, Iowa’s assessment methodology for indicator bacteria has changed. Prior to 2003, the Iowa WQ Standards contained a high-flow exemption for the Class A criterion for indicator bacteria (fecal coliforms) designed to protect primary contact recreation uses: the water quality criterion for fecal coliform bacteria (200 orgs/100 ml) did not apply "when the waters [were] materially affected by surface runoff." Due to a change in the Standards in July 2003, E. coli is now the indicator bacterium, and the high flow exemption was eliminated and replaced with language stating that the Class A criteria for E. coli apply when Class A1, A2, or A3 uses “can reasonably be expected to occur.” Because the IDNR Technical Advisory Committee on WQ Standards could not agree on what flow conditions would define periods when uses would not be reasonably expected to occur, all monitoring data generated for E. coli during the assessment period, regardless of flow conditions during sample collection, will be considered for determining support of Class A uses for purposes of Section 305(b) assessments and Section 303(d) listings.
The geometric mean level of indicator bacteria (E. coli) in the 24 samples analyzed during the recreational seasons of 2004 through 2006 (177 orgs/100ml) exceeds the Iowa Class A water quality criterion of 126 orgs/100ml. Ten of the 24 samples (42%) 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 2004-2006 assessment period showed no violations of Class B(WW1) water quality criteria for dissolved oxygen, pH, or ammonia-nitrogen in the 36 samples analyzed, for toxic metals in the 14 samples analyzed, or for pesticides in the approximately nine samples analyzed.
Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
