Assessment Comments
Assessment is based on results of IDNR/UHL ambient monthly water quality monitoring at Columbus Junction from 2002-04 and IDNR/UHL biological (benthic macroinvertebrate) sampling data from 2000-2002.
Basis for Assessment
SUMMARY: Class A (primary contact recreation) uses are assessed (monitored) as "fully supported" based on results of ambient monitoring for indicator bacteria. Class B(WW) aquatic life uses are assessed (evaluated) as "partially supported" based on results of ambient water quality monitoring and IDNR/UHL biological (benthic macroinvertebrate) monitoring. Fish consumption uses remain "not assessed" due to the lack of 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 ambient station located at Columbus Junction (STORET station 10580001)
EXPLANATION: Results of water quality monitoring during the 2002-2004 assessment period for indicator bacteria (E. coli) at the IDNR ambient monthly station on the Iowa River at Columbus Junction show that the Class A (primary contact recreation) uses are "fully supported." 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 E. coli bacteria in the 24 samples collected during recreational seasons of 2002, 2003, and 2004 (69 orgs/100 ml) is well below the Iowa water quality criterion of 126 orgs/100 ml for protection of primary contact recreation uses. Four of the 24 samples had levels of E. coli greater than the Iowa single sample maximum value of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting (pgs 3-33 to 3-35of U.S. EPA 1997b), primary contact uses are "fully supported" if the geometric mean for E. coli is less than 126 organisms/100 ml. Also, according to U.S. EPA guidelines for Section 305(b) reporting, if levels of E. coli exceed the single-sample maximum value in more than 10% of the samples, the primary contact recreation uses should be assessed as “partially supported.” According to IDNR’s assessment/listing methodology, however, these results do not suggest that the violation frequency of Iowa’s single-sample maximum criterion is significantly greater than 10%, and thus these results do not suggest impairment of the Class A uses of this river segment. Monitoring results for indicator bacteria during the 2002-2004 assessment period are consistent with the generally low levels of indicator bacteria at the Columbus Junction station during recent biennial periods (see previous assessments for more information).
The results of IDNR ambient monthly water quality monitoring suggeste the Class B(WW) aquatic life uses be assessed (monitored) as "fully supported". Results from DNR ambient monthly monitoring at this station during the 2002-2004 assessment period show no violations of Class B(WW) water quality criteria in the 36 samples analyzed for dissolved oxygen or ammonia-nitrogen. One of 36 samples collected, however, exceeded the state standard for pH. According to U.S. EPA guidelines (U.S. EPA 1997b, page 3-17), a violation frequency of less than 10 % for conventional parameters such as pH nonetheless suggest "full support" of aquatic life uses. Thus, the percentages of violations of the pH criterion at this station (6%) does not suggest an impairment of aquatic life uses in this stream segment. Also, one the seven samples analyzed for pesticides violated the Class B(WW) chronic criterion for DDT. The level of DDT in the sample was 0.12 ug/l; the chronic criterion is 0.001 ug/l. This occurrence of a single violation of Iowa’s chronic water quality criterion for DDT during the assessment period, however, does not indicate impairment of the aquatic life uses. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-18), 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) does not indicate an impairment of aquatic life uses.
However, biological (benthic macroinvertebrate) monitoring from 2000-2002 suggest that the aquatic life uses be assessed (evaluated) as partially supporting. This evaluated biological assessment was based on data collected in 2000-2002 as part of the DNR/UHL stream ambient station benthic macroinvertebrate sampling project. A series of biological metrics that reflect stream water quality and habitat integrity were calculated from the biocriteria sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa collected in the stream sampling reach. The biological metrics were combined to benthic macroinvertebrate index of biotic integrity (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2000-2002 BMIBI scores were 32 (fair), 24 (poor) and 33 (fair). The BMIBI average was 30. The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the BMIBI score with biological impairment criteria (BIC) established from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The BMIBI BIC for this ecoregion is 51. This assessment is considered evaluated because the drainage area (12252 mi2) above this sampling site was greater than the maximum limit (500 mi2) that was used to calibrate the Iowa wadeable stream impairment criteria. Even though this site failed the BMIBI BIC, it is uncertain as to whether or not this segment is meeting the aquatic life criteria because it doesn’t fall in the calibrated watershed size.
Fish consumption uses remain "not assessed" due to lack of fish tissue monitoring in this river segment.
