Assessment Comments
Assessment is based on results of IDNR/UHL ambient city monitoring upstream from Marshalltown from 2002-04.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses were assessed (monitored) as "not supported" due to levels of indicator bacteria that exceed state water quality criteria. The Class B(WW) aquatic life uses were assessed (monitored) as "fully supported" based on results of ambient water quality monitoring upstream from Marshalltown. Fish consumption uses were 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 2002 through December 2004 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 A uses were assessed as "not supported" due to levels of indicator bacteria (E. coli) that exceed state water quality criteria. Due to recent changes in Iowa’s Water Quality Standards, Iowa’s 2006 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 the 2006 Section 305(b) assessments and Section 303(d) listings.
The geometric mean level of indicator bacteria (E. coli) in the 24 samples collected during recreational seasons of 2002 through 2004 (184 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 value 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(WW) aquatic life uses were assessed as "fully supported." Monitoring at the IDNR ambient station at Highway 330 during 2002-2004 showed no violations of Class B(WW) water quality criteria in the 36 samples analyzed for dissolved oxygen and pH, in the 32 samples analyzed for toxic metals, or in the approximately 7 samples analyzed for pesticides. The level of ammonia-nitrogen in one of the 36 samples analyzed, however, exceeded the Class B(WW) chronic criterion. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-18), however, one violation of a water quality criterion for a toxic pollutant in an abundant data set (at least 10 samples over at three-year period) set does not indicate an impairment of aquatic life uses.
Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
