Assessment Comments
Assessment is based on results of water quality monitoring conducted from 2010 through 2012 on Honey Creek southwest of Melrose (station RA-40) 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 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 (monitored) as "fully supported" (IR 2a). The source of data for this assessment is the results of water quality monitoring conducted from 2010 through 2012 on Honey Creek southwest of Melrose (station RA-40) 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 11 samples collected during the recreational seasons of 2010 through 2012 at Honey Creek were as follows: the 2010 geometric mean was 533 orgs/100 ml, the 2011 geometric mean was 607 orgs/100 ml, and the 2012 geometric mean was 1,692 orgs/100 ml. All three geometric means exceed the Class A1 criterion of 126 orgs/100 ml. Eight of the 11 samples (73%) 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.”
Results of ambient chemical/physical water quality monitoring do not suggest impairments of the Class B(WW2) aquatic life uses of this stream segment. None of the 28 samples collected during the 2010-12 period at station RA-40 violated Class B(WW2) criteria for pH (range = 6.9-8.2), and none of the samples collected violated the Class B(WW2) criterion for ammonia (maximum value = 0.7 mg/l). Two of 28 samples (7%), however, violated the Class B(WW2) water quality criterion for dissolved oxygen. According to U.S. EPA guidelines for Section 305(b) water quality assessments (U.S. EPA 1997b, page 3-17), if criteria for conventional parameters (e.g., pH, dissolved oxygen and temperature) are not exceeded in significantly more than 10% of the samples, the aquatic life uses should be assessed as "fully supporting." Thus, these results suggest relatively good water quality and “full support” of the Class B(WW2) aquatic life uses.
These results are in contrast to the violations for dissolved oxygen criteria during the 2000-2002 assessment period when monitoring results for dissolved oxygen suggested a potentially significant water quality problem. Three of the 24 samples (12.5%) collected at station RA-40 from 2000 through 2002 violated the Class B water quality criterion for dissolved oxygen; all of these violations occurred in the relatively dry year of 2000. The relative lack of violations for dissolved oxygen in the samples collected at Station RA-40 since 2000 suggest improved water quality in this stream segment and the absence of any aquatic life impairments due to low levels of dissolved oxygen.
Although this stream is not designated for Class C drinking water uses, Honey Creek 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. Fortunately, however, the levels of atrazine in Honey Creek during the 2010-2012 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 10 samples collected during the 2010-12 period at station RA-40 (1.5 ug/l) was well below the atrazine MCL of 3 ug/l. The maximum level was 8.4 ug/l.
