Assessment Comments
Assessment based on: (1) results of IDNR/SHL biological sampling conducted in 2012, (2) results of biological sampling by the IDNR Fisheries Bureau from 1999-2002, and (3) results of water quality monitoring conducted from 2010-2012 on the South Fork Chariton River near Cambria (station RA-35) by Iowa State University under contract with the U.S. Army Corps of Engineers as part of the Rathbun Water Quality Project.
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 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 5b-t) based on (1) results of IDNR/SHL biological sampling conducted in 2012, (2) results of biological sampling by the IDNR Fisheries Bureau from 1999-2002 and (3) information in U.S. EPA's May 23, 2006 letter transmitting Iowa's final 2004 Section 303(d) list. This assessment is also based on results of water quality monitoring conducted from 2010-2012 on the South Fork Chariton River near Cambria (station RA-35) by Iowa State University under contract with the U.S. Army Corps of Engineers as part of the Rathbun Water Quality Project.
EXPLANATION: The presumptive Class A1 (primary contact recreation) are assessed as "not supported" based on levels of indicator bacteria that exceeded state water quality criteria. The geometric means of indicator bacteria (E. coli) in the 15 samples collected during the recreational seasons of 2010 through 2012 at the South Fork Chariton River near Cambria were as follows: the 2010 geometric mean was 679 orgs/100 ml, the 2011 geometric mean was 489 orgs/100 ml and the 2012 geometric mean was 208 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Ten of the 15 samples (67%) 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.”
The Class B(WW2) aquatic life uses are assessed (evaluated) as “partially supporting“ based on biological data collected in 2012 as part of the IDNR/SHL stream nutrient and follow-up sampling projects and on data collected in 1999-2002 as part of an IDNR Fisheries stream sampling project: Chariton research station. A series of biological metrics that 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 collected in the stream sampling reach. The biological metrics were combined to make a benthic macroinvertebrate index (BMIBI). The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 2012 FIBI score was 28 (fair) and the BMIBI scores were 20 adn 24 (both poor). The 1999-2002 evaluated FIBI scores were 3, 12, 24 (all poor) and 36 (fair). The aquatic life use support was assessed (evaluated) as partially supporting (=PS), based on a comparison of the FIBI scores with biological assessment criteria established for previous Section 305(b) reports. The biological assessment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2008. This FIBI BIC for this ecoregion is 33. This segment passed the FIBI BIC 1/5 times in the last 14 years and passed the BMIBI BIC 0/2 times in 2012.
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 FIBI samples collected in the previous 14 years (1999-2012) and had multiple BMIBI samples collected in a single year (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). 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).
Because the biological impairment of the Class B(WW2) uses is based on an “evaluated” (lower confidence) assessment, the IDNR assessment/listing methodology indicates that this assessment segment should be placed into IR Category 3b (potentially impaired; in need of further investigation). In their May 23, 2006 transmittal letter for Iowa’s 2004 Section 303(d) list, however, U.S. EPA Region 7 informed IDNR that biological impairments based on bioassessment data from the IDNR Fisheries Bureau, even though IDNR/UHL bioassessment protocols were not strictly followed, should nonetheless be placed into IR Category 5. Thus, this waterbody was placed into Category 5b of Iowa’s 2006 Integrated Report and will remain on Iowa’s future Section 303(d) lists.
Results of ambient physical/chemical water quality monitoring during the 2010-2012 period do not suggest water quality problems in this assessment segment. None of the 33 samples violated Class B(WW2) criteria for dissolved oxygen (minimum = 5.4 mg/L), pH (range = 6.8 - 8.3), or ammonia (maximum = 0.8 mg/L). These results suggest improving water quality conditions in this segment of the South Fork Chariton River. That is, previous assessment cycles have shown impairments related to low levels of dissolved oxygen (see assessments developed for the 2002 and 2004 assessment cycles). Although this stream segment has a history of potential problems with low levels of dissolved oxygen, levels of dissolved oxygen over the last 12 years of monitoring appear to have improved.
Although this river reach is not designated for Class C drinking water uses, the South Fork Chariton River does flow into Rathbun Reservoir which is used as a source of drinking water for a public water supply. Thus, the seasonal elevation of pesticide levels in this stream and other tributaries of Rathbun Reservoir presents a continuing concern for full support of the Class C (drinking water) uses designated for the Chariton River immediately downriver from Rathbun Reservoir. The levels of atrazine in this segment of the South Fork Chariton River during the 2010-12 period, however, do not suggest a serious threat to support of drinking water uses in Rathbun Reservoir. For example, the mean level of atrazine in the 12 samples collected during 2010-12 at Station RA-35 (2.4 ug/l) approaches but is below the atrazine MCL of 3 ug/l; the maximum atrazine level during this period was 14.0 ug/l of atrazine.
