Assessment Comments
Assessment remains based on (1) results of IDNR/UHL biological (biocriteria) sampling in 2000 & 2006 [FIBI = 41 (fair), 19 (poor); BMIBI = 26 (poor), 45 (fair). FIBI BIC = 52; BMIBI BIC = 61] and (2) IDNR/UHL TMDL-related water quality monitoring from 2006 through 2008 near Monona.
Basis for Assessment
[Note: Prior to the 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, 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 (2012) 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 remain assessed (monitored) as “not supported” due to levels of indicator bacteria that exceed state water quality criteria. The Class B(WW2) aquatic life uses remain assessed (monitored) as "not supported" based on results of IDNR/UHL biological (biocriteria) sampling in 2000 & 2006. The sources of data used for this assessment are (1) IDNR/UHL TMDL-related monitoring conducted from June 2006 through October 2008 near Monona (STORET station 11220014; TMDL site No. SiCr1) and (2) biological monitoring conducted on Silver Creek in 2000 & 2006 as part of the INDR/UHL stream biocriteria project. Note: this stream segment contains impairments appropriate for both subcategory 5p (bacterial impairment) and subcategory 5b (biological impairment) of Iowa's Integrated Report.
Note: The Iowa DNR TMDL for sediment and ammonia for this segment of Silver Creek was approved by U.S. EPA in July 2012. Thus, these impairments are moved from IR Category 5a (impaired; TMDL required) to IR Category 4a (impaired; TMDL approved). Because not all the impairments identified for this assessment segment were covered by the TMDL (indicator bacteria; biological impairment with unknown cause), this segment remains impaired and in IR Category 5.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses remain assessed (monitored) as "not supported" due to violations of Iowa’s water quality criteria for indicator bacteria. The geometric mean of E. coli in the 18 samples collected during the recreation seasons of 2006 through 2008 at the monitoring station on Silver Creek (SiCr1) was 7,906 orgs/100 ml (note: duplicate samples collected in 2008 were averaged before calculating the overall geometric mean). All 18 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).
The assessment of the Class B(WW2) aquatic life uses is based on data collected in 2000 & 2006 as part of the IDNR/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 2000 FIBI score was 41 (fair) and the BMIBI score was 45 (fair). The 2006 FIBI score was 19 (poor) and the BMIBI score was 26 (poor). 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 FIBI BIC for this ecoregion is 52 and the BMIBI BIC for this ecoregion is 61. This segment passed the FIBI BIC 0/2 times and the BMIBI BIC 0/2 times in the last 11 years.
This aquatic life assessment is now considered "evaluated" based on a change in the 2010 IDNR assessment methodology. IDNR now requires a segment have two or more biological samples collected from the segment in multiple years over a five-year period to be considered “monitored”. This segment had multiple samples collected in the previous 11 years (2000-2010); however, the samples were not collected over at five-year period. According to IDNR’s assessment/listing methodology, impairments based on “evaluated” assessments are of lesser confidence and are thus not appropriate for Section 303(d) listing (Category 5 of the Integrated Report). IDNR does, however, consider these impairments as appropriate for listing under either Category 2b or 3b of the Integrated Report (waters potentially impaired and in need of further investigation). However, despite this change in assessment methodology and type, this waterbody was added to IR Category 5b for the 2010 IR cycle and remains on Iowa’s 2012 Section 303(d) list of impaired waters.
Water quality data collected during the 2006-2008 period also suggest a potential impairment of the aquatic life uses of this stream. Two of the 29 samples collected violated the chronic aquatic life criteria for ammonia. According to Iowa DNR’s listing methodology, if more than 10% of the samples exceed chronic criteria for ammonia, the aquatic life uses are assessed as impaired. Although the frequency of violations of the ammonia criteria (7%) does not suggest an ammonia impairment, the occurrence of these violations suggest a water quality concern at this stream. Also, two of the 29 samples analyzed violated criteria for dissolved oxygen; the frequency of violation (7%) does not suggest impairment of aquatic life uses.
