Assessment Comments
Assessment is based on results of monthly monitoring from 2004 through 2006 at the IDNR ambient city monitoring station located upstream from Ottumwa at the County Road T67 bridge at Chillicothe (STORET station 10900003).
Basis for Assessment
SUMMARY: The Class A1 (primary contact recreation) uses were assessed (monitored) as "partially supported" due to high levels of indicator bacteria (E. coli), and the Class B(WW1) aquatic life uses were assessed (monitored) as "fully supported"; both assessments are based on results of IDNR/UHL ambient water quality monitoring upstream from Ottumwa. The Class C (drinking water) uses were assessed (monitored) as "fully supported" based on results of the IDNR/UHL ambient monitoring. Fish consumption uses remain not assessed due to the lack of fish contaminant monitoring in this river reach. The source of data for this assessment is the results of monthly monitoring from 2004 through 2006 at the IDNR ambient city monitoring station located upstream from Ottumwa at the County Road T67 bridge at Chillicothe (STORET station 10900003).
EXPLANATION: The Class A1 uses are assessed (monitored) as "partially supported" based on results of ambient monitoring for indicator bacteria (E. coli). 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 collected during the recreational seasons of 2004 through 2006 (115 orgs/100ml) is below, and meets, the Iowa Class A1 water quality criterion of 126 orgs/100ml. Seven of the 24 samples (29%) 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 of E. coli is less than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "fully supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). These EPA guidelines state, however, that if more than 10% of the samples exceed the state’s single-sample maximum criterion, the primary contact recreation uses should be assessed as “partially supported”. According to IDNR’s assessment/listing methodology, these results suggest that significantly greater than 10% of the samples exceed IDNR’s single-sample maximum criterion, thus suggesting that the Class A1 uses should be assessed as “partially supported/impaired.” Thus, while the geometric mean is below the Class A1 standard and thus suggests “full support,” the percentage of violations of Iowa’s single-sample maximum criterion is significantly greater than 10 percent and thus suggests impairment (partial support) of the Class A1 uses of this river segment.
The Class B(WW1) aquatic life uses are assessed (monitored) as "fully supported" based on results of monitoring from the IDNR/UHL ambient station at Chillicothe from 2004 through 2006. Monitoring at this station showed no violations of Class B(WW1) water quality criteria for dissolved oxygen or ammonia-nitrogen in the 36 samples analyzed, for toxic metals in the 14 samples analyzed, or for pesticides and other toxic organic compounds in the nine samples analyzed during this assessment period. One of the 36 samples, however, exceeded the Class B(WW1) criterion for pH. The sample collected on June 1, 2004, contained a pH level of 10.4 pH units, thus greatly exceeding the water quality criterion of 9.0 pH units. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), however, the percentage of violations for pH at this station (3%) does not suggest a water quality impairment (the EPA guidelines allow up to 10% violations of these conventional parameters before impairment of water quality is indicated).
The Class C (drinking water) uses are assessed (monitored) as "fully supported." During the 2004-2006 assessment period, one of 35 samples violated the atrazine MCL of 3 ug/l (3.5 ug/l), and one of 36 samples also exceeded the nitrate MCL of 10 mg/l (11 mg/l). The average levels of both atrazine (0.36 ug/l) and nitrate (5.7 mg/l) are well below their respective MCLs. Based on IDNR’s assessment methodology, if the average level of nitrate is less than the respective MCLs, the Class C drinking water should be assessed as “fully supported.” Similarly, if the average level of a pesticide is less than the respective MCL, the Class C drinking water should be assessed as “fully supported.”
Fish consumption uses remain not assessed due to lack of recent fish tissue monitoring in this river segment.
