Assessment Comments
Assessment is based on: (1) results of water quality monitoring conducted from 2008-2010 on the South Fork Chariton River near Promise City (station RA-12) by Iowa State University under contract with the U.S. Army Corps of Engineers as part of the Rathbun Water Quality Project, and (2) results of biological monitoring by the IDNR Fisheries Bureau from 1999-2002.
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 (2012) 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 violate state water quality criteria. The Class B(WW2) aquatic life uses remain assessed (evaluated) as "partially supported" based on (1) results of biological monitoring by the IDNR Fisheries Bureau from 1999-2002 that suggest impairment of these uses and (2) 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 2008-2010 on the South Fork Chariton River near Promise City (station RA-12) 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 28 samples collected during the recreational seasons of 2008 through 2010 at the South Fork of the Chariton River near Promise City were as follows: the 2008 geometric mean was 318 orgs/100 ml, the 2009 geometric mean was 953 orgs/100 ml and the 2010 geometric mean was 794 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Twenty-one of the 28 samples (75%) 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 remain assessed as “partially supporting” based on biological data collected in 1999-2002 as part of an IDNR Fisheries stream sampling project: Chariton research station. A series of biological metrics which reflect stream water quality and habitat integrity were calculated from the Fisheries sampling data. The biological metrics are based on the numbers and types of 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). The index ranks the biological integrity of a stream sampling reach on a rising scale from 0 (minimum) to 100 (maximum). The 1999-2002 evaluated FIBI scores were 15, 21, 22, 22, 22, 22, 24, 25 (all poor), 27, 28, 28, 33 and 35 (all 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 1994-2004. The FIBI BIC for this ecoregion is 33. This segment passed the FIBI BIC 2/13 times in the last 12 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 recent five-year period to be considered “monitored”. This segment had multiple samples collected in the previous 12 years (1999-2010); 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-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 chemical/physical water quality monitoring do not suggest impairments of the Class B(WW2) aquatic life uses of this stream segment. Results of monitoring during the 2008-2010 assessment period showed that none of the 30 samples violated Class B(WW2) criteria for pH (range = 6.8-8.0), ammonia-nitrogen (maximum level = 0.7 mg/l), or dissolved oxygen (minimum = 5.3 mg/L). Although this stream segment had a history of potential problems with low levels of dissolved oxygen, levels of dissolved oxygen over the last ten years of monitoring appear to have improved. Based on results of recent monitoring, this stream segment does not currently show a dissolved oxygen-related impairment of its Class B(WW2) aquatic life uses.
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 the full support of the Class C (drinking water) uses designated for the Chariton River immediately downriver from Rathbun Reservoir. Fortunately, however, the levels of atrazine in the South Fork Chariton River during the 2004-06 period do not suggest a serious threat to support of drinking water uses in Rathbun Reservoir. The mean level of atrazine in the 11 samples collected in 2004 at Station RA-12 (1.1 ug/l) was approximately one-third of the MCL for atrazine of 3 ug/l. The maximum levels of 5.6 ug/l of atrazine occurred in the sample collected on May 26, 2004. Atrazine was not monitored at Station RA-12 during the 2006-10 period.
