Candlelight Creek IA 01-NEM-6372

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 Candlelight Creek; thus, this stream remains Section 303(d)-impaired and in Category 5p of Iowa's Integrated Report. 

EXPLANATION:  Results of monitoring for indicator bacteria conducted weekly from April through October 2008 at Station CLC1 suggest that the presumptive Class A1 uses of this segment should be assessed (monitored) as "not supported."  A total of 28 samples were collected at station CLC1 during this period; these data allowed calculation of twenty-four, 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 947 and 6,919 orgs/100 ml, with 27 of the 28 samples (96%) exceeding Iowa’s Class A1 single-sample maximum criterion of 235 orgs/100 ml.  According to U.S. EPA guidelines and IDNR’s assessment/listing methodology, these results suggest non-support of the presumptive Class A1 (primary contact recreation) uses due to geometric means for E. coli that exceed Iowa’s water quality criterion of 126 E. coli organisms/100. 

Results of monthly monitoring for E. coli between May and November 2013 showed continued high levels of indicator bacteria in this stream segment.  The geometric mean of E. coli in the six samples was 1,463 orgs/100 ml.  Five of the six of the samples (83%) exceeded the Class A1 single-sample maximum criterion of 235 orgs/100 ml.  These results suggest that this stream segment is appropriately considered impaired due to high levels of indicator bacteria.  

The presumptive Class B(WW1) aquatic life uses remain assessed (evaluated) as “partially supported” due to violations of criteria for dissolved oxygen.  Four of the 29 samples (14%) collected in 2008 at station CLC1 violated the Class B(WW1) criterion of 5 mg/l (monitoring for dissolved oxygen was not conducted in 2013).  The minimum level of dissolved oxygen was 3.3 mg/l.  According to U.S. EPA guidelines for Section 305(b) reporting, if more than 10% of samples exceed state criteria for conventional parameters such as dissolved oxygen, the aquatic life uses should be assessed as "impaired" (see pgs 3-33 to 3-35of U.S. EPA 1997b).  According to IDNR’s assessment/listing methodology, however, the results from station CLC1, however, do not indicate that significantly greater than 10% of the samples exceed the Class B(WW1) criterion for dissolved oxygen.  Thus, these results suggest that the presumptive Class B(WW1) uses should be assessed (monitored) as “fully supported.”  Despite this indication of full support, the occurrence of four violations of the dissolved oxygen criterion in a single year is unusual and suggests a potential adverse water quality impact.  Thus, this assessment segment will be added to Iowa’s list of waters in need of further investigation.  Additional monitoring is needed to better determine any potential water quality impacts on the aquatic life of this stream.