For LT2, what value do I use to calculate my source water average E. coli if the laboratory reports a result as less than the detection limit? What value would I report if the sample result is non-detect and a dilution factor has been applied?
The LT2 requires that whole numbers be used to calculate the average of E. coli sample results collected from source water monitoring. The average of E. coli samples collected is compared to trigger levels specified in the LT2 Rule based on the source water type (i.e., flowing stream or lake/reservoir). Questions have arisen as to how systems should report E. coli results that are less than the detection limit of the method used, and how such results can and should be used to determine the average E. coli concentration. Laboratories are strongly encouraged to analyze at least 100 mL of sample or more, as appropriate. In situations where 100 mL can not be analyzed due to high loads of E. coli, the laboratory should perform a series of dilutions on the sample, and should analyze several of the dilutions, in order to capture the dilution that obtains the best countable range of E. coli in the sample. It is inappropriate for the laboratory to continuously analyze only a portion of the sample (i.e. 10 mL) if levels of E. coli are low and results less than detection limit are continuously obtained. EPA suggests the following reporting procedures if a water system receives E. coli samples results reported as less that the detection limit: ? If a laboratory analyzed 100 mL or more of water sample, and obtained no positive results (i.e. results were less than the detection limit (e.g., < 1 E. coli/100 mL), the water system may use zero "0" when reporting and determining the average E. coli concentration. ? Where the laboratory reports a sample result as less than the detection limit (e.g., < 50 E. coli/100 mL) and the sample was properly diluted based on historical information (i.e. E. coli loads were anticipated to be high) the water system may use zero "0" when reporting and determining the average. ? If only a portion of the water sample was analyzed (i.e. laboratory routinely only analyzes a portion of the sample and has no historical data on the loads of E.coli) the water system should use the final value reported by the laboratory. So, if the laboratory reported < 50 E. coli/100 mL and it has been learned the laboratory did not follow the appropriate dilution procedures the water system should report and use 50 E. coli/100 mL in their determination.
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