Assessment Comments
Assessment is based on: (1) results of IDNR/UHL biological (biocriteria) monitoring in 2004 and (2) results of IDNR/UHL ambient chemical/physical monitoring upstream from Sac City from 2004-2006 at STORET station 10810002.
Basis for Assessment
[Note: Prior to the current (2008) Section 305(b) cycle, this stream segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008 (see http://www.iowadnr.com/water/standards/files/06mar_swc.pdf), this segment is also now presumptively designated for Class A1 (primary contact recreation) uses. This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW1) aquatic life uses remain assessed (monitored) as "partially supported” based on results of IDNR/UHL biological (biocriteria) sampling in 2004. Results of IDNR/UHL ambient monitoring 2004 through 2006, however, suggest good chemical/physical water quality. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment. The sources of data for this assessment include (1) results of IDNR/UHL biological (biocriteria) monitoring in 2004 and (2) results of IDNR/UHL ambient chemical/physical monitoring upstream from Sac City from 2004-2006 at STORET station 10810002.
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as "not 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 (136 orgs/100ml) slightly exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml. Seven of the 24 samples (29%) exceed 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 level of E. coli is greater than the state criterion of 126 orgs/100 ml., the primary contact recreation uses should be assessed as "not supported" (see pgs 3-33 to 3-35of U.S. EPA 1997b). Also, these EPA guidelines state 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, the results from the IDNR/UHL ambient monitoring station 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 level is only slightly greater than the water quality criterion, the percentage of violations of Iowa’s single-sample maximum criterion (29%) also suggests impairment of the Class A1 uses of this river segment.
The assessment of the Class B(WW1) aquatic life uses (monitored; partially supported) remains based on data collected in 2004 as part of the DNR/UHL stream biocriteria project. The 2004 FIBI score was 56 (good) and the BMIBI score was 45 (fair). The aquatic life use support was assessed as partially supporting (= PS), based on a comparison of the FIBI and BMIBI scores with biological impairment criteria (BIC) established for previous Section 305(b) reports. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004. The riffle habitat FIBI BIC for this ecoregion is 53 and the BMIBI BIC for this ecoregion is 62.
In contrast, results of IDNR/UHL ambient chemical/physical water quality monitoring from 2004 through 2006 suggest relatively good water quality in this river segment. Results from this monitoring show no violations of Class B(WW1) criteria for dissolved oxygen, pH, or ammonia in the 36 samples collected, for toxic metals in the ten samples collected, or in the seven samples analyzed for the pesticide chlorpyrifos. These results suggest relatively good water quality and do not suggest impairment of the Class B(WW1) aquatic life uses.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.
