Assessment Comments
Assessment is based on (1) results of IDNR/UHL biological (biocriteria) sampling in 2006 [FIBI = 19 (poor); BMIBI = 26 (poor). FIBI BIC = 52; BMIBI BIC = 61] and (2) IDNR/UHL TMDL-related water quality monitoring in 2006 near Monona.
Basis for Assessment
[Note: Prior to the current (2008) Section 305(b) cycle, this stream segment was designated only for Class B(LR) aquatic life 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 now presumptively designated for Class A1 (primary contact recreation) uses. The stream remains designated for aquatic life uses (now termed Class B(WW2) aquatic life uses). Thus, for the current (2008) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (evaluated) as “not supported” due to levels of indicator bacteria that exceed state water quality criteria. The Class B(WW2) aquatic life uses are assessed (monitored) as "not supported" based on results of IDNR/UHL biological (biocriteria) sampling in 2006. The sources of data used for this assessment are (1) IDNR/UHL TMDL-related monitoring conducted from June-September 2006 near Monona (STORET station 11220014; TMDL site No. SiCr1) and (2) biological monitoring conducted on Silver Creek in 2006 as part of the INDR/UHL stream biocriteria project. Note: this stream segment contains impairments appropriate for subcategory 5a (bacterial impairment) and subcategory 5b (biological impairment) of Iowa's Integrated Report.
EXPLANATION: The Class A1 (primary contact recreation) uses were assessed (evaluated) as "not supported" due to violations of Iowa’s water quality criteria for indicator bacteria. 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 A1 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 of E. coli in the six samples collected at the monitoring station on Silver Creek was 49,258 orgs/100 ml. All six of the samples collected (100%) exceeded Iowa’s single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines for Section 305(b) reporting and IDNR’s assessment/listing methodology, if the geometric mean 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). Although these results indicate impairment of the primary contact recreation uses designated for this stream segment, the number of samples is relatively low and does not meet IDNR guidelines for developing a “monitored” (i.e., higher confidence) assessment. Thus, the assessment type is considered “evaluated” (lower confidence). While lower confidence assessments are not appropriate for addition to the state’s Section 303(d) list of impaired waters (IR Category 5), this assessment is appropriate for addition to the state list of waters in need of further investigation (IR Categories 2b or 3b). Because, however, the geometric mean is two orders of magnitude greater than the state water quality criterion, this segment will be place in IR Category 5a.
The assessment of the Class B(WW2) aquatic life uses is based on data collected in 2006 as part of the DNR/UHL stream biocriteria project. A series of biological metrics which 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 and fish species that were collected in the stream sampling reach. The biological metrics were combined to make a fish community index of biotic integrity (FIBI) and a benthic macroinvertebrate index (BMIBI). The indexes rank the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2006 FIBI score was 19 (poor) and the BMIBI score was 26 (poor). The aquatic life use support was assessed as not supporting (=NS), 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 FIBI BIC for this ecoregion is 52 and the BMIBI BIC for this ecoregion is 61.
