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TSRIA Document 5: 

PPL Plume Behavior Does Not Affect High Coburn Readings 

During the February 27 meeting with EPA, EPA speculated that a possible reason for 

the higher readings at Coburn Road might be the plume behavior at the PPL stack. EPA 

conjectured that some high Coburn Road readings might have occurred due to, for 

example, a low flow exhaust rate or a lower-than-normal temperature at the stack. The 

reasons for such conditions at the stack could include startup, shutdown, low production 

and other related causes. 

The result of a low flow (and/or temperature) condition could, theoretically, yield a plume 

that would not have the same buoyancy as normal. This low-buoyancy plume might, 

theoretically, impact the ground near Coburn more than during normal buoyancy. 

To examine this hypothesis, we compiled data relating to this issue. Under the SIP 

monitoring program, PPL measures a parameter that specifically addresses the 

buoyancy of the plume on an hourly basis. This hourly measurement is required by 

DEQ, and it is provided to the agency on a quarterly basis. 

The parameter in question is known as “buoyancy flux.” This term is in common use for 

purposes of dispersion modeling. In fact, the dispersion models use this specific 

parameter in order to estimate how far a plume will rise above the top of the stack as it 

moves in a downwind direction from its origin. For purposes of this discussion, we will 

not engage in the mathematics or derivation of the formula that describes buoyancy 

flux. Suffice it to say that it is an hourly variable than takes into account stack plume exit 

velocity, volume flow rate, stack exit temperature, ambient temperature differential, etc. 

Using that parameter, two data sets were obtained. The first is a listing of all hourly 

buoyancy flux data collected at PPL but excluding those hours in which PPL was not 

operating. The second data set was a list of hourly buoyancy flux data at any time that 

Coburn road exceeded 60 ppb. 

If one assumes that higher Coburn readings are related to unusual plume-rise stack 

conditions (reflected as low buoyancy flux), then there should be a difference between 

the two data sets described above. We tested this hypothesis using a standard 

statistical technique: an “Unpaired T-Test” assuming the variances between the two 

means are not equal. The results of that investigation are found in the table below: 
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Buoyancy Flux Comparison and Statistics 

Normal Operations vs. Periods of Peak Coburn Data 

Buoyancy Flux 

(PPL Exhaust)

Descriptor 2009 – 2013 > 60 PPB Readings at Coburn 

Mean, BFlux 324 309 

Variance 5,584 4,681 

Count 

(operating 

hours) 

34,665 57 

Unpaired T-Test 

(assuming unequal variance)

Hypothesis: The unpaired difference between the two means is the same. 

(There is no difference between ‘normal’ PPL buoyancy flux and the buoyancy flux 

when Coburn readings are high.) 

Degrees of 

Freedom 56 

t-statistic 1.66 

Critical tstatistic (95%) 2.00 

Conclusion: 

There is not enough evidence to reject the (null) hypothesis. It is 

reasonable to conclude that the two data sets are, in effect, the same. 

Thus, conditions that lead to high ppb readings at Coburn are not 

directly associated with buoyancy flux data from PPL. 

The results of the table indicate that there is no statistical difference in PPL exhaust 

plume-rise characteristics (buoyancy flux) during periods of high Coburn readings 

versus the rest of the five-year period. Plume rise variation also does not explain the 

high readings associated with the QA failures identified in the data-set.