Assessment Comments
Assessment remains based on: (1) results of IDNR/UHL biological (biocriteria) monitoring in 1998 & 2004 and (2) results of IDNR/UHL ambient chemical/physical monitoring upstream from Sac City from 2008-2010 at STORET station 10810002.
Basis for Assessment
Note: Prior to the 2008 Section 305(b) cycle, this stream segment was designated only for Class B(WW) aquatic life uses, including fish consumption uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is also now presumptively designated for Class A1 (primary contact recreation) uses. This segment remains designated for warmwater aquatic life use (now termed Class B(WW1) uses), and for fish consumption uses (now termed Class HH (human health/fish consumption uses).]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses are assessed (monitored) as "not supported" due to levels of indicator bacteria that violate state water quality criteria. The Class B(WW1) aquatic life uses remain assessed (evaluated) as "partially supported” based on results of IDNR/UHL biological (biocriteria) sampling in 1998 and 2004. Results of IDNR/UHL ambient monitoring 2008 through 2010, however, suggest good chemical/physical water quality. Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment. The sources of data for this assessment include (1) results of IDNR/UHL biological (biocriteria) monitoring in 1998 and 2004 and (2) results of IDNR/UHL ambient chemical/physical monitoring upstream from Sac City from 2008-2010 at STORET station 10810002 upstream from Sac City.
Note: A TMDL for pathogen indicator (E. coli) impairments in the Raccoon River basin was prepared by IDNR and approved by EPA in June 2008. Because this TMDL covers bacterial impairments, this impairment was moved from IR Category 5a from the 2008 assessment/listing cycle to IR Category 4a (impaired; TMDL approved) for the 2010 cycle.
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 23 samples collected during the recreational seasons of 2008 through 2010 at IDNR station 10810002 upstream from Sac City were as follows: the 2008 geometric mean was 201 orgs/100 ml, the 2009 geometric mean was 157 orgs/100 ml, and the 2010 geometric mean was 159 orgs/100 ml. All three geometric means very slightly exceed the Class A1 criterion of 126 orgs/100 ml. Ten of the 23 samples (43%) 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.” Note: Results of ambient monitoring at IDNR Station 10810002 have shown low levels of indicator bacteria (E. coli) over the last three Integrated Reporting cycles. The recreation season geometric means for the 2004-06 assessment period was 136 orgs/100 ml, and the geometric mean for the 2006-08 period as 152 orgs/100 ml. This pattern continued during the current (2008-10) assessment period with recreation season geometric means ranging from 157 to 201 orgs/100 ml.
The assessment of the Class B(WW1) aquatic life uses (evaluated; partially supported) was based on data collected in 1998 and 2004 as part of the IDNR/UHL stream biocriteria project. The 1998 FIBI score was 55 (good) and the BMIBI score was 61 (good). The 2004 FIBI score was 56 (good) and the BMIBI score was 44 (fair). 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 riffle habitat FIBI BIC for this ecoregion is 53 and the BMIBI BIC for this ecoregion is 62. This segment passed the FIBI BIC 2/2 times and passed the BMIBI BIC 0/2 times in the past 13 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 13 years (1998-2010); however, the multiple samples were not collected during a five-year period. 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).
In contrast, results of IDNR/UHL ambient chemical/physical water quality monitoring from 2008 through 2010 suggest relatively good water quality in this river segment. Results from this monitoring show no violations of Class B(WW1) criteria for dissolved oxygen, pH, or ammonia in the approximately 30 samples collected or for toxic metals and pesticides in the three samples analyzed. These results suggest relatively good water quality and do not suggest impairment of the Class B(WW1) aquatic life uses.
Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring in this stream segment.
