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RMA4 Decay Coefficient


MattT
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All,

I am attempting to model the transport and decay of a bacterial pollutant in a small waterbody. I have calculated the velocity solution and ran a few preliminary RMA4 model runs. I am using SMS 10.1 and injecting a pollutant at a point source (node) at a concentration of 50,000 ppm. After the initial model run, (assuming no decay.....fully conservative), I ran a second model run to determine the effect of a decay coefficient. Upon a literature review, that bacterial pollutant shouldd be reduced to 90% within 1 day. However, when I run the RMA4 model with a decay coefficient of 0.90/day, the model results are almost exactly the same as my initial run with no decay. Can you please provide some guidance regarding the units of this decay coefficient? Since I am injecting 50,000 ppm and theoretically 90% should be gone in 1 day, will the decay coefficient be 5,000/day??? I need to know if the "value"/day is represented as a percentage or as an actual magnitude (with the same units as my injected pollutant). Seems as though it shoudl be %, but that doesn't apprear to have any effect.

Also, upon a further review of the RMA4 manual, what is the calculation routine if the diffusion coefficient method is selected, but the DX and DY coefiecients are both 1.0? For my case, it looks as though the DX and DY coefficients should be around 0.1 (marshy estuary). Is the model more conservative with the 1.0 designation?

Many Thanks in advance!

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I need to know if the "value"/day is represented as a percentage or as an actual magnitude (with the same units as my injected pollutant). Seems as though it shoudl be %, but that doesn't apprear to have any effect.

First order decay in RMA4 is given by

c=c0e-βt giving β=-ln(c/c0)/t, where t is time in days and β decay coefficient

so, a 10% reduction of concentration in 1 day gives β=-ln(0.9)/1 = 0.105

heimo

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