Assessment Comments
Assessment is based on results of IDNR/UHL ambient monthly monitoring downstream from Marshalltown from 2002-04.
Basis for Assessment
SUMMARY: The Class A (primary contact recreation) uses are assessed (monitored) as "partially supported" due to high levels of indicator bacteria. The Class B(WW) aquatic life uses are assessed (monitored) as "fully supported" based on results of ambient water quality monitoring downstream of Marshalltown. Fish consumption uses were 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 2002 through 2004 at the IDNR/UHL ambient city monitoring station located downstream from Marshalltown at the county road E35 bridge (STORET station 10640002).
EXPLANATION: The Class A uses were assessed as "partially 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 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 analyzed during the recreational seasons of 2002 through 2004 (113 orgs/100ml) is below the Iowa Class A water quality criterion of 126 orgs/100ml. However, nine of the 24 samples (38%) exceeded Iowa’s single-sample maximum value of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of the samples exceed the state single-sample maximum criterion value of 235 orgs/100 ml, the primary contact recreation uses should be assessed as "partially supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). According to IDNR’s assessment/listing methodology, these results suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum value, thus suggesting that the Class A uses should be assessed as “partially supported/impaired”.
The Class B(WW) aquatic life uses were assessed (monitored) as "fully supported." IDNR/UHL ambient monitoring at the Co. Rd. E35 station during the 2002-2004 assessment period showed no violations of Class B(WW) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 36 samples analyzed, for toxic metals in the 32 samples analyzed, or for pesticides in the approximately seven samples analyzed.
Fish consumption uses remain "not assessed" due to lack of recent fish tissue monitoring in this river segment.
