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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:

Each of these quantities is illustrated in the figure:

Graphic that illustrates the first arrival time, maximum concentration and duration above threshold

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. Graphic that illustrates breakthrough curves for all three outputs and the mean parameter values

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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