Assessment Comments
Assessment is based on: (1) the results of monthly water quality monitoring from 2004 through 2006 at the IDNR/UHL ambient station located at Columbus Junction (STORET station 10580001) and (2) results of IDNR/UHL benthic macroinvertebrate monitoring from 2001-2002.
Basis for Assessment
SUMMARY: Class A1 (primary contact recreation) uses are assessed (monitored) as "partially supported" based on results of ambient monitoring for indicator bacteria. Class B(WW1) aquatic life uses remain assessed (evaluated) as "partially supported" based on results of 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 sources of data for this assessment are (1) the results of monthly water quality monitoring from 2004 through 2006 at the IDNR/UHL ambient station located at Columbus Junction (STORET station 10580001) and (2) results of IDNR/UHL benthic macroinvertebrate monitoring from 2001-2002. [Note: monitoring at the IDNR/UHL ambient station at Columbus Junction was terminated in August 2006 due to ongoing concerns about station placement relative to the confluence of the Iowa and Cedar rivers immediately upstream from the monitoring station. This station was relocated upriver on the Iowa River at Lone Tree.]
EXPLANATION: Results of water quality monitoring during the 2004-2006 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 "partially supported." 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 E. coli bacteria in the 21 samples collected during recreational seasons of 2004, 2005, and 2006 (93 orgs/100 ml) is below the Iowa water quality criterion of 126 orgs/100 ml for protection of primary contact recreation uses. Six of the 21 samples (29%), however, had levels of E. coli greater than the Iowa single sample maximum criterion 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 criterion 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, the results of bacterial monitoring suggest that the violation frequency of Iowa’s single-sample maximum criterion is significantly greater than 10% and thus suggest impairment of the Class A1 uses of this river segment. Despite the indication of impairment, monitoring results for indicator bacteria during the 2004-2006 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 chemical/physical water quality monitoring suggest that the Class B(WW1) aquatic life uses should be assessed (monitored) as "fully supported." Results from DNR ambient monthly monitoring at this station during the 2004-2006 assessment period show no violations of Class B(WW1) water quality criteria in the 32 samples analyzed for dissolved oxygen or ammonia-nitrogen. One of the 32 samples collected, however, exceeded the state standard for pH; the sample collected on September 1, 2005 contained a pH of 9.4, thus exceeding the Class B(WW1)/A1 criterion of 9.0 pH units. 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 percentage of violations of the pH criterion at this station (3%) does not suggest an impairment of aquatic life uses in this stream segment.
Results of biological (benthic macroinvertebrate) monitoring from 2001-2002, however, suggest that the aquatic life uses should remain assessed (evaluated) as partially supporting. This evaluated biological assessment was based on data collected from 2001-2002 as part of the IDNR/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 2001-2002 BMIBI scores were 24 (poor) and 33 (fair). The BMIBI average was 28.5. 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 (12,252 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.
