Assessment Comments
Assessment remains based on (1) results of IDNR/SHL biological sampling in 2000 & 2006 [FIBI = 41 (fair), 19 (poor); BMIBI = 26 (poor), 45 (fair). FIBI BIC = 52; BMIBI BIC = 61] and (2) IDNR/SHL TMDL-related water quality monitoring from 2011 and 2012 near Monona (station SC04/SiCr1).
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 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 (monitored) as “not supported” due to levels of indicator bacteria that continue to exceed state water quality criteria (IR 5p). The Class B(WW2) aquatic life uses remain assessed (evaluated) as "partially supported" based on results of IDNR/SHL biological sampling in 2000 & 2006 (IR 5b-t). Results of chemical water quality monitoring suggest a new impairment of the Class B(WW2) aquatic life uses due to low dissolved oxygen (IR 5a). The sources of data used for this assessment are (1) IDNR/SHL TMDL-related monitoring conducted from June 2011 through November 2012 near Monona (STORET station 11220014; TMDL site No. SC04/SiCr1) and (2) biological sampling conducted on Silver Creek in 2000 & 2006 as part of the INDR/SHL stream biocriteria project. Note: this stream segment contains impairments appropriate for subcategory 5p (bacterial impairment), subcategory 5a (dissolved oxygen impairment) and subcategory 5b-t (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 are assessed (monitored) as "not supported" due to continued violations of Iowa’s water quality criteria for indicator bacteria. The geometric means of indicator bacteria (E. coli) in the six monthly samples collected during the recreational season of 2011 (1,839 orgs/100 ml), and the geometric mean of the eight samples collected during 2012 (1,559 orgs/100 ml) both far exceeded the Class A1 criterion of 126 orgs/100 ml. Eleven of the combined 14 samples (79%) 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 is greater than 126 orgs/100 ml., the primary contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b).
The assessment of the Class B(WW2) aquatic life uses is, in part, based on data collected in 2000 & 2006 as part of the IDNR/SHL 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 1994-2008. 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 13 years. Also, because these data are now considered too old (greater than five years) to accurately characterize current water quality conditions, the assessment category is considered “evaluated” (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).
This aquatic life assessment is now also 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 13 years (2000-2012); 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 Section 303(d) list of impaired waters.
Water quality data collected during the 2011-2012 period also suggest impairment of the aquatic life uses of this stream. Although no violations of Class B(WW2) criteria for ammonia, pH, or temperature occurred in the approximately 13 samples analyzed, four of the 13 samples analyzed violated the Class B(WW2) criterion for dissolved oxygen. According to Iowa DNR’s listing methodology, if significantly more than 10% of the samples exceed water quality criteria for conventional parameters such as dissolved oxygen, the aquatic life uses are assessed as impaired. The percentage of samples during the 2011-2012 period that violated the Class B(WW2) dissolved oxygen criterion (31%) is significantly greater than 10% and thus suggests impairment of the aquatic life uses. This is a new impairment for this assessment segment.
