Assessment Comments
Assessment based on: (1) results of IDNR/SHL monthly ambient monitoring conducted from 2010 through 2012 on Indian Creek at a bridge 4.8 miles north of Colfax and 1.8 miles east of Green Castle in Jasper County (station 10500001) and (2) IDNR/SHL biological sampling near Mingo in 2003 and 2012.
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 (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 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" (IR 5p) due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW2) aquatic life uses are assessed (evaluated) as "partially supported" (IR 3b-c) based on results of 2003 and 2012 biological sampling and on results of ambient chemical/physical water quality monitoring. The sources of data for this assessment are (1) results of IDNR/SHL monthly ambient monitoring conducted from 2010 through 2012 on Indian Creek at a bridge 4.8 miles north of Colfax and 1.8 miles east of Green Castle in Jasper County (station 10500001) and (2) IDNR/SHL biological sampling near Mingo in 2003 and 2012.
EXPLANATION: The presumptive Class A1 uses are assessed (monitored) as "not supported" based on results of ambient monitoring for indicator bacteria (E. coli). The geometric means of indicator bacteria (E. coli) in the 24 samples collected during the recreational seasons of 2010 through 2012 at IDNR station 10500001 near Colfax were as follows: the 2010 geometric mean was 1,868 orgs/100 ml, the 2011 geometric mean was 989 orgs/100 ml and the 2012 geometric mean was 270 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Seventeen of the 24 samples (71%) exceeded the Class A1 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 a recreation season geometric mean exceeds the respective water quality criterion, the contact recreation uses are "not supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Thus, because at least one recreation season geometric mean exceeded criteria for Class A1 uses, these uses are assessed as “impaired.”
Based exclusively on the results of ambient chemical/physical water quality monitoring, the Class B(WW2) aquatic life uses should be assessed (monitored) as "fully supported". Results of chemical/physical monitoring at the IDNR/SHL station show no violations of Class B(WW2) criteria for dissolved oxygen, pH, ammonia nitrogen (maximum value = 0.22 mg/l), temperature, chloride, or sulfate in the 36 samples analyzed during the 2010-2012 assessment period. However, the biological data collected in this segment indicate "partial support."
In contrast to the water quality aquatic life use assessment, the biological data collected in 2003 and 2012 indicate that the Class B(WW2) aquatic life uses be assessed as “partially supporting." A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the biological sampling data. The biological metrics are based on the numbers and types of benthic macroinvertebrate taxa and fish species 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 2003 FIBI score was 30 (fair) and the BMIBI score was 58 (good). The 2012 BMIBI score was 18 (poor). 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 non-riffle habitat FIBI BIC for this ecoregion is 32 and the BMIBI BIC for this ecoregion is 62. The segment passed the FIBI BIC 1/1 times in 2003 (using the FIBI UAV of 7 points applied to single samples) and passed the BMIBI BIC 0/2 times in the last 10 years.
This assessment is considered "evaluated" because there were not two or more samples collected in multiple years from 2008-2012. 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).
