Assessment Comments
Assessment remains based on results of IDNR/UHL TMDL-related, chemical/physical monitoring conducted from July to October 2004.
Basis for Assessment
[Note: Prior to the 2008 Section 305(b) cycle, this stream segment was classified only for general uses. Due to changes in Iowa’s surface water classification that were approved by U.S. EPA in February 2008, this segment is now presumptively designated for Class A1 (primary contact recreation) uses and for Class B(WW1) aquatic life uses. According to the Iowa Water Quality Standards, all perennial rivers and streams and all intermittent streams with perennial pools that are not specifically listed in the Iowa surface water classification are designated as Class A1 and Class B(WW1) waters. Thus, for the current (2012) assessment, the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW1) water quality criteria. This waterbody was not assessed for Section 305(b)/303(d) purposes prior to the 2008 assessment/listing cycle.]
SUMMARY: The presumptive Class A1 primary contact recreation uses remain not assessed due to the lack of information upon which to base an assessment. The Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supported” based on results of limited chemical/physical water quality monitoring conducted at two stations in 2004. The source of data for this assessment is IDNR/UHL TMDL-related, chemical/physical monitoring conducted from July to October 2004. This amount of data (five samples each from two stations) does not meet IDNR guidelines for Clean Water Act assessment and listing that require at least 10 samples be collected per site over a three-year period. Assessments based on data that do not meet IDNR assessment/listing guidelines are considered “evaluated” (i.e., of lower confidence); these assessments are not appropriate for adding waters to Iowa’s Section 303(d) list of impaired waters. Such assessments are, however, appropriate for addition of waters to IR subcategories 2b or 3b and to Iowa’s list of waters in need of further investigation.
EXPLANATION: The presumptive Class A1 (primary contact recreation) uses remain not assessed due to the lack of information upon which to base an assessment. The monitoring conducted from July to October 2004 did not include analysis for indicator bacteria (e.g., E. coli).
The presumptive Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supported” based on results of IDNR-UHL TMDL-related monitoring from July to October of 2004. Results of this monitoring show no violations of Class B(WW1) water quality criteria for conventional parameters dissolved oxygen or pH in the combined ten samples collected at the two stations on Powell Creek. One of the five samples from each station, however, exceeded the Class B(WW1) criterion for ammonia-nitrogen. These samples contained ammonia nitrogen concentrations of 16.0 and 6.9 mg/l, thus exceeding state Class B(WW1) criteria. According to U.S. EPA guidelines for Section 305(b) reporting (U.S. EPA 1997b, page 3-18), this single violations of chronic criteria for ammonia do not suggest an impairment of aquatic life uses. These guidelines state that uses are “fully supported” if chronic criteria for a toxic pollutant (for example, ammonia) are exceeded in up to one sample from a dataset based on monthly monitoring. Although these results suggest that the Class B(WW1) aquatic life uses of this river segment should be assessed as “fully supported,” the extremely high levels of ammonia, combined with the limited amount of monitoring, suggest both a potential impairment of the aquatic life uses as well as the need for additional monitoring. Such assessments are appropriate for IR Categories 2b or 3b and are appropriate for addition to Iowa’s list of waters in need of further investigation.
