Western Geo-Engineers

(530) 668-5300; Fax (530) 662-0273; wege@cal.net

A FULL SERVICE ENVIRONMENTAL CONSULTING COMPANY

Computer Modeling

Computer models are used to predict what will happen when a system is put under stress. The accuracy of the model is only as good as the completeness of the data and the underlying assumptions. There are two basic types of models: Analytical, where a set of equations is used to model the site's reaction to stress over the complete site; and numeric, where the site is broken into smaller elements and the interaction of these elements are calculated.

The choice of what models to use on a project depends on many factors:

• What you are modeling.
•  
  • Groundwater transport
  • Soil Migration
  • Air Transport
  • Bio-degradation
• How much data do you have or can afford.
• What you are trying to predict.
• Degree of accuracy needed.
•  
  • Simple worst case model sufficient, ie., Tier 1
  • More complex analytical model, ie., Tier 2
  • High degree of accuracy, numeric model, ie., Tier 3
• Complexity of site.

Analytical Modeling

Analytical modeling treats the entire system as a whole and uses one set of equations to calculate the system's reaction to stress. As such, these models are good for predicting the response of homogeneous systems. The greater the variability in the system modeled the greater the difficulty in finding the single set of equations that accurately represent the entire site.

Analytical models are often used when the amount of information available site is limited. When the data is limited, a number of worst case assumptions are made in order to determine highest possible risk levels.

Numeric Modeling

Numeric models break the site into smaller parts or cell, and the interaction of these parts is calculated. Breaking the site into smaller parts allows the model to represent a greater amount of variation in the system. Numeric modeling normally requires more information about the site. Additionally, tests must be performed on the system to put it under stress and the reactions to the stress measured. After the system's reaction to stress is known, the model is adjusted to fit the known result; this usually takes multiple runs and adjustments.

Once fitted to a site, the numeric models can be used to predict interactions of the site to further stress, i.e. groundwater pumping, vapor extraction, or air sparging.