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Uncertainty in Simple Transport Models
Subsurface transport models require various input parameters. These parameters are both variable and uncertain. The variability originates with subsurface heterogeneity. Uncertainty results from inadequacies in measurements.
The Concentration Uncertainty model can be used to assess uncertainty in parameter estimates by entering ranges of values for the transport parameters and source defintion. The applet then runs the model for all combinations of the variable quantities. The table lists each parameter of the one-dimensional model and indicates which inputs can be treated as variable.
| Parameter | Variable ? |
|---|---|
| Hydraulic Conductivity | yes |
| Porosity | yes |
| Gradient | yes |
| Dispersivity | yes |
| Fraction Organic Carbon | yes |
| Organic Carbon Partition Coefficient (Koc) | yes |
| Half Life | yes |
| Scenario Definition | Variable ? |
|---|---|
| Source Concentration | yes |
| Source Duration | yes |
| Distance to Receptor | no |
| Chemical Contaminant | no |
| Threshold Concentration | no |
Parameters that are are variable can be fixed simply by entering the same values for both the low and high values of the input range.
With nine potentially variable parameters, the model is run 512 times (2number of variable parameters). The applet tabulates the results and finds the minimum and maximum of three quantities:
- contaminant first arrival time
- maximum contaminant concentration
- duration of contamination
Each of these quantities is illustrated in the figure:
The applet draws seven graphs: the minimum and maximum of each of the three output quantities, and the breakthrough curve for the mean of the parameter ranges. 
Run the "ConcentrationUncertainty" applet with this link
A paper (PDF, 8 pp., 0.05 MB, About PDF), describing the ConcentrationUncertainty applet gives additional background information and presents preliminary results in defining generic worst case parameter sets.
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