Robin Creek IA 01-NEM-6373

[Note: A TMDL for the bacterial impairment for streams in the Duck Creek basin was prepared by Iowa DNR in March 2010. This TMDL was approved by U.S. EPA in 2011. According to U.S. EPA, however, the TMDL for the Duck Creek watershed does not assign the required elements to Robin Creek; thus, this stream remains Section 303(d) impaired and in Category 5p of Iowa's Integrated Report.]

Results of monitoring for indicator bacteria conducted weekly from April through October 2008 at Station RC1A suggest that the presumptive Class A1 uses of this segment should be assessed (monitored) as "not supported." A total of 29 samples were collected at station RC1A during this period; these data allowed calculation of twenty-five, 30-day/five-sample geometric means. All of the 30-day geometric means exceeded Iowa’s Class A1 criterion of 126 organisms / 100 ml. The minimum and maximum geometric means were 309 and 6,665 orgs/100 ml, with 26 of 29 samples (90%) exceeding Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml. According to U.S. EPA guidelines and DNR’s assessment/listing methodology, these results suggest non-support of the presumptive Class A1 (primary contact recreation) uses due to geometric means of E. coli that exceed Iowa’s water quality criterion of 126 E. coli organisms/100.

Results of monthly monitoring between May and November 2013 showed continued levels of indicator bacteria in this stream segment that exceed the Class A1 criterion. The geometric mean of E. coli in the six samples was 245 orgs/100 ml. Three of the six of the samples (50%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml. Although these results suggest that the presumptive Class A1 uses of this stream segment are appropriately considered impaired due to high levels of indicator bacteria, the 2013 geometric mean is well below the Class A2 (secondary contact recreation) criterion of 630 orgs/100 ml. Thus, if the final designation of this stream segment was Class A2, the 2013 monitoring results suggest the potential for de-listing of this impairment.

The presumptive Class B(WW1) aquatic life uses remain “not assessed” due to the lack of information upon which to base an assessment. Levels of dissolved oxygen, however, were measured during the TMDL-related sampling in 2008 at station RC1A (monitoring for dissolved oxygen was not conducted during 2013). One of the 29 samples (3.4%) violated the Class B(WW1) criterion of 5 mg/l. The minimum level of dissolved oxygen was 3.2 mg/l. According to U.S. EPA guidelines for Section 305(b) reporting, if less than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen, the aquatic life uses should be assessed as "fully supported" (see pgs 3-33 to 3-35 of U.S. EPA 1997b). Although these results suggest “full support” of the presumptive aquatic life uses, the lack of additional data for ammonia, toxic metals, and/or pesticides, and the lack of biological data for this segment, prevents development of a complete assessment of the aquatic life uses. Thus, the presumptive aquatic life uses remain “not assessed”.