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If the bottom boundary condition is Fixed Outflow, then the contaminant concentration at any depth will reach a maximum value at a determinable time. After reaching this maximum value the concentration will decrease if the contaminant source is finite (i.e., the Reference Height of Leachate is finite), or the concentration will remain at the maximum value if the contaminant source is infinite (i.e., Constant Concentration top boundary condition). See Example 4.
When the output type is maximum concentrations then the Maximum Concentration tab below will appear.
The following parameters can be specified on this tab.
Search Depth: This is the depth for which to search for the maximum concentration, any depth between 0 and the maximum thickness of the deposit above the base boundary. The default depth is the depth of the base boundary. In searching for the maximum concentration the depth used will be the depth closest to the nearest sublayer interface. It is recommended that the user be sure to have a sublayer interface at the depth requested, or specify a large number of sublayers for the layer.
Accuracy: This is the accuracy to which the maximum base concentration is to be calculated, typically a value of 0.1% is used.
Number of Iterations: This is the maximum number of iterations to try to obtain the maximum base concentration to the required accuracy, typically a value of 25 is used. If the lower and upper time limits are well selected, convergence to an accuracy of 0.1% can usually be obtained within 10 iterations.
Upper and Lower Time Limits: The user needs to specify lower and upper time limits in which the maximum is expected to occur. If the lower and upper time limits do not bracket the time of the maximum, the program will usually adjust the time limits to include the time of the maximum. However, if both the specified limits are at times when there is negligible concentrations at the depth being considered, then the program may not be able to find a meaningful maximum.
If the top boundary condition is Constant Concentration and the lower time limit is close to or above the time of the maximum, the program will adjust the lower time limit until it is lower than the time of the maximum. When the top boundary condition is Constant concentration, the user is advised to check the results by using different values for the lower and upper time limits.
All Depths: This allows the user to select whether to calculate the concentrations at all depths (Yes), or at selected depths (No). If the user chooses all depths the concentrations will be calculated at the boundary between all sublayers. If the user wishes to calculate at selected depths then the user will be asked to specify the Number of Depths. After specifying the number of depths the user will be asked to specify the depths to calculate the concentrations.