Sixmile Creek IA 06-BSR-1533
mouth (S28T94N R48W Sioux Co.) to confluence with unnamed tributary in S19 T95N R46W Sioux Co.
- Cycle
- 2016
- Release Status
- Final
- Overall IR
- 5 - Water is impaired or threatened and a TMDL is needed.
- Trend
- Unknown
- Created
- 9/2/2016 2:22:51 PM
- Updated
- 9/21/2016 1:09:17 PM
The presumptive Class A1 (primary contact recreation) uses remain assessed (evaluated) as "not supported" (IR 5p) due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW-2) aquatic life uses remain assessed (evaluated) as “partially supported” (IR 5b-t) due to (1) results of biological sampling for fish and aquatic macroinvertebrates in 2000 and (2) results of chemical monitoring that shows levels of ammonia occasionally violate chronic aquatic life criteria. Note: the description for the ammonia impairment of aquatic life uses was omitted from the narrative of the 2006 assessment; this impairment was, however, included on Iowa's 2006 final/approved Section 303(d) list. The sources of data for this assessment remain (1) results of IDNR/SHL biological sampling conducted in 2000 as part of the stream biocriteria project and (2) results of IDNR/SHL ambient monitoring conducted near Chatsworth at Station 8 during the 2002-2004 assessment period in support of TMDL development (STORET station 11840003). Note: the 2008 impairment of the Class A1 uses was identified as an IR-5a impairment. Due to the presumptive designation of the Class A1 uses, the correct impairment category is IR-5p, and this correction was made for the 2010 assessment. Because the data upon which these assessment are based are now more than five years old, and because as data age beyond five years they are less able to represent current water quality conditions, the assessment type is changed from “monitored” (a higher confidence assessment) to “evaluated” (lower confidence 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 (see http://www.iowadnr.gov/Portals/idnr/uploads/water/standards/files/swcdoc2.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 assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW2) water quality criteria.] The presumptive Class A1 uses remain assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E. coli). The geometric mean level of indicator bacteria (E. coli) in the 21 samples collected at the IDNR/SHL TMDL monitoring station near Chatsworth during summer recreational seasons of 2002-2003 (1,212 orgs/100ml) far exceeds the Iowa Class A1 water quality criterion of 126 orgs/100ml. Sixteen of the 21 samples (76%) 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 3-35 of U.S. EPA 1997b).
Results of biological monitoring conducted in 2000 as part of the IDNR/SHL stream biocriteria project suggest impairment (evaluated) of the Class B(WW-2) aquatic life uses. 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 scores were 2 (poor) and 10 (poor); the BMIBI scores were 58 (good) and 46 (fair). The aquatic life use support was assessed (evaluated) 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 43 and the BMIBI BIC for this ecoregion is 54. This segment passed the FIBI BIC 0/2 times and passed the BMIBI BIC 1/2 times in 2000.
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 recent five-year period to be considered “monitored”. This segment had multiple samples collected in 2000; however, the multiple samples were not collected during a five-year period and were not collected in multiple years. Additionally, 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). 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 previously on Iowa’s Section 303(d) list and thus remains on Iowa’s Section 303(d) list of impaired waters (IR Category 5b-t).
Similarly, results of IDNR/SHL ambient water quality monitoring conducted from March 2002 through December 2003 in support of TMDL development suggest (evaluated) “partial support” of the Class B(WW-2) aquatic life uses. Although levels of dissolved oxygen and pH in the 23 samples analyzed met their respective Class B(WW-2) water quality criteria, this monitoring suggested that levels of ammonia occasionally violate aquatic life criteria. Levels of ammonia-nitrogen violated Class B chronic criteria in two of the 26 samples collected from March 2002 through December 2003. These samples were collected on June 9, 2002 (2.6 mg/l) and on June 12, 2002 (1.2 mg/l). Ammonia levels in June 2003 approached, but did not exceed, the respective chronic aquatic life criteria (e.g., the sample level of ammonia on June 26, 2006 (2.1 mg/l) approached the Class B chronic criterion of 2.24 mg/l). Based on IDNR’s 2008 Section 303(d) listing methodology, however, these results do not suggest that significantly more than 10 percent of the samples exceed Iowa’s chronic criterion for ammonia and thus do not suggest an impairment of the Class B(WW2) aquatic life uses. Because, however, more than one violation of the chronic criterion for ammonia occurred, this potential impairment will be added to Iowa's list of waters in need of further investigation (IR 3b).