Assessment Comments
Assessment is based on the results of monitoring at STORET station 13880005 (Site 12m-5) conducted as part of a Clean Water Act Section 319 water quality project from May to August 2012.
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 presumptively designated as Class A1 and Class B(WW1) waters. Thus, for the current assessment, perennial flow is presumed, and the available water quality monitoring data will be compared to the applicable Class A1 and Class B(WW1) water quality criteria.]
SUMMARY: The presumptive Class A1 (primary contact recreation) uses remain “not assessed” (IR 3a) due to lack of information upon which to base an assessment. The presumptive Class B(WW1) aquatic life uses are assessed (evaluated) as “partially supporting” (IR 3b) due to violations of the criterion for dissolved oxygen. The source of data for this assessment is the results of monitoring at STORET station 13880005 (Site 12m-5) conducted as part of a Clean Water Act Section 319 water quality project from May to August 2012.
EXPLANATION: The presumptive Class A1 uses remain “not assessed” due to lack of monitoring for indicator bacteria (E. coli) in this assessment segment.
The presumptive Class B(WW1) aquatic life uses are assessed (evaluated) as “partially supporting” based on violations of the Class B(WW1) criterion for dissolved oxygen. Five samples were analyzed for dissolved oxygen at Site 12m-5 in summer 2012 with two samples (40%) violating the Class B(WW1) criterion for dissolved oxygen of 5 mg/l. This assessment is considered evaluated (of lower confidence)—and thus not appropriate for Section 303(d) listing—for two reasons: (1) the Iowa DNR assessment/listing methodology requires that at least 10 samples be collected over the three-year assessment period (in this case, from 2010 through 2012) in order to develop a “monitored” (i.e., higher confidence) assessment that is appropriate for Section 303(d) listing and (2) all violations of dissolved oxygen criteria occurred in this headwater stream segment in the exceptionally warm and dry mid-to late summer period of 2012; that is, violations of Class B(WW1) water quality criteria for dissolved oxygen would be expected in headwater streams during the warm and low-flow conditions that occurred throughout Iowa during summer of 2012. Levels of pH also occasionally violated the Class A/B(WW1) criterion of 6.5 units with pH values occurring below this level. Five samples were analyzed for pH with one sample (20%) violating the Class A/B(WW1) criterion of 6.5 pH units (no samples collected as part of the Twelvemile water quality project exceeded the upper pH criterion of 9.0 pH units). This type of violation for pH is not uncommon in headwater streams in southern Iowa during low-flow or no-flow conditions of mid to late summer. As with the case for dissolved oxygen, too few samples were analyzed for pH during the 2010-12 period to meet requirements of Iowa’s assessment/listing methodology for a “monitored” (higher confidence) assessment that would be appropriate for Section 303(d) listing. The only other parameter monitored at this station that has corresponding Class B(WW1) criteria is ammonia. Five samples were analyzed for ammonia at site 12m-5 with not samples violating Class B(WW1) chronic criteria (early life stages presumed present). In general, violations of the temperature/pH-dependent Class B(WW1) chronic criteria (early life states presumed present) were very rare during the Twelvemile project.
