Assessment Comments
Assessment is based on: (1) results of chemical/physical water quality monitoring conducted from 2008 through 2010 on Wolf Creek near Chariton (station RA-41; lower segment) and near Humeston (station RA-44; upper segment) 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 IDNR/UHL biological monitoring in 2005 and 2011.
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 (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 are assessed (evaluated) as "partially supported" based on results of biological monitoring. The sources of data for this assessment include (1) results of chemical/physical water quality monitoring conducted from 2008 through 2010 on Wolf Creek near Chariton (station RA-41; lower segment) and near Humeston (station RA-44; upper segment) 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 IDNR/UHL biological monitoring in 2005 and 2011.
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 site RA-41 (downstream) were as follows: the 2008 geometric mean was 281 orgs/100 ml, the 2009 geometric mean was 1013 orgs/100 ml and the 2010 geometric mean was 959 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Twenty-two of the 28 samples (79%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. The geometric means of indicator bacteria (E. coli) in the 28 samples collected during the recreational seasons of 2008 through 2010 at site RA-44 (upstream) were as follows: the 2008 geometric mean was 263 orgs/100 ml, the 2009 geometric mean was 608 orgs/100 ml and the 2010 geometric mean was 470 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Sixteen of the 28 samples (57%) 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.”
Regarding support of the Class B(WW2) aquatic life uses, none of the 27 samples collected from 2008-10 at (downstream) station RA-41 violated Class B(WW2) criteria for ammonia-nitrogen (maximum level = 1.0 mg/l). Similarly, none of the 27 samples collected from 2008-10 from the upstream station (RA-44) violated ammonia criteria (maximum level = 1.0 mg/l). One of the 29 samples (3%) collected at station RA-41, and none of the 29 samples collected (at station RA-44 during the 2008 through 2010 assessment period violated the Class B(WW2) water quality criterion for dissolved oxygen. One of the 29 samples (3%) collected at station RA-41, and none of the 29 samples collected (at station RA-44 during the 2008 through 2010 assessment period violated the Class B(WW2) water quality criterion for pH. The single violation of the criteria for dissolved oxygen and the single violation of the criteria of pH that occurred during the 2008-2010 period does not suggest an impairment: according to U.S. EPA assessment guidelines, if 10% or less of the samples exceed state criteria for conventional parameters such as dissolved oxygen and pH, the aquatic life uses should be assessed as "fully supported" Water quality conditions in Wolf Creek, as in other tributaries in the Rathbun watershed, is strongly and adversely influenced by low flow periods when levels of dissolved oxygen are most likely to violate criteria designed to protect aquatic life.
Results of biological monitoring from IDNR/UHL biological monitoring suggest that the Class B(WW2) uses should be assessed (evaluated) as "partially supported." This biological assessment was based on data collected in 2005 and 2011 as part of the IDNR/UHL stream biocriteria project. 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 The 2005 FIBI scores were 18 and 18 (poor) and the single biocriteria BMIBI score was 31 (fair). The 2011 FIBI scores were 24 (poor) and 34 (fair) and the BMIBI scores were 33 (fair) and 56 (good). 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 FIBI BIC for this ecoregion is 33 and the BMIBI BIC for this ecoregion is 41. This segment passed the FIBI BIC 1/3 times and passed the BMIBI BIC 1/3 times in the last six years. The biological impairment criteria were determined from a statistical analysis of data collected at stream ecoregion reference sites from 1994-2004.
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 six years (2005-2011); however, the multiple samples were not collected during a five-year period. 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).
Although this stream reach is not designated for Class C drinking water uses, Wolf Creek does flow into Rathbun Reservoir which is used as a source of drinking water for a public water supply. The seasonal elevation of pesticide levels in the Chariton River 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. Fortunately, however, the levels of atrazine in Wolf Creek during the 2004-2006 period do not suggest a serious threat to support of drinking water uses in Rathbun Reservoir. For example, the mean level of atrazine in the 11 samples collected in 2004 at Station RA-41 (1.2 ug/l) is well-below the atrazine MCL of 3 ug/l. Similarly, at station RA-44, the mean level of atrazine in the 11 samples collected in 2004 (1.3 ug/l) was well-below the atrazine MCL of 3 ug/l. Atrazine was not monitored at either station RA-41 or RA-44 during the 2008-10 period.
