Estimating Contaminant Mass using RockWorks Solid Models

One of the great benefits of RockWorks is the quick volume calculations available through the RockPlot3D viewer.  As the user adjusts the isosurface or filter settings (for example, to isolate material above a contamination threshold), the program returns a volume in cubic meters or cubic feet, depending on the project units.  Let’s say that you then want to calculate the mass of contamination within the project?  This takes a little bit more thought on the user’s part, but can be done easily using the recently improved Solid Math program.

First, we’ll talk about soil contamination.  Soil contamination models are often based on concentration data provided by the lab in mg/kg, or mg of contaminant per kg of soil.  The Solid Math tool can be used to convert the estimated concentration in each model voxel to the mass of contamination.  This calculation requires two bits of information:

  1. The density of the soil
  2. The volume of each model voxel (in cubic feet or cubic meters)

The density of the soil should be in kg/ft3 or kg/m3, depending on your project units.  If soil density is given to you in lb/ft3, multiply by 16.0185 to convert to kg/ft3.

The volume of each model voxel is constant within the model and can be found by looking in the project summary at the top of the program.  The spacing in each direction can by multiplied to determine the volume of each cell in cubic feet or cubic meters.  In the example below, the spacing is 5ft in the X and Y directions and 1ft in the Z direction, so the volume of each model voxel is 25 cubic feet.

Recent improvements to the Solid Math tool in RockWorks2021 allow the user to string together multiple operations.  In the example below, the program is set up to multiply the values in a model composed of GRO in soil concentrations by 54 (soil density) and then 25 (cell volume).  This converts the value assigned to each voxel to the mg of contaminant within the voxel.

Once the Solid Math step is completed, you can use the Solid|Statistics|Report tool to determine the sum of all of the values within the new GRO Mass model.

Calculating the amount of dissolved contamination in groundwater requires slightly different information.  In the following example, we’ll assume that the concentrations have been modeled in mg/l.  In addition to this information, you’ll need to determine:

  1. The porosity of the saturated material
  2. The density of water (28.32 l/ft3 or 1000 l/m3)
  3. The volume of each model voxel (in cubic feet or cubic meters).

In the image below, you can see how the solid math program is set up.  The final model is composed of the amount of contamination in mg.

Keep in mind that this approach assumes a constant porosity across the model extent.  If you have created a porosity model (perhaps a lithology model converted to a porosity model) you can multiply the concentration model by the porosity model using the Solid Math tool.

If you have any questions, please reach out to us at tech@rockware.com. Thanks for reading!

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