Groundwater Models: Can You Believe What They Are Saying?

the past and to predict the future. Groundwater models can tell you when and where a decades-old chemical spill occurred, that the groundwater supply for a city will run out in a certain number of years based on projected usage, or how best to clean up groundwater contamination at a site. Groundwater modeling using computers became a significant part of groundwater investigations in the late 1960s. For decades, groundwater modeling had an aura about it, partly because it was the realm of a few experts and partly because people were in awe of computers. Today, though, anyone can purchase and/or download sophisticated groundwater models for a desktop computer. Using preand post-processors (computer software that facilitates model development and information output), today's modeler can achieve in hours what early groundwater modelers took days or weeks to achieve, if they could do it at all. This advancement in groundwater modeling has been both a blessing and a curse. While groundwater models can help decision-makers with important legal, environmental, water supply, and geotechnical questions, groundwater models are notorious for their "garbage in, garbage out" character. Although consultants, regulators, and industry often rely on groundwater models to answer questions that involve millions of dollars, why should you believe a groundwater model? This article has two main objectives. First, to explain what groundwater models are and what they can do. Second, to explain the steps involved in creating a groundwater model, the steps groundwater modelers normally take to demonstrate that their model accurately represents reality, and what additional steps can be taken to support the model. The groundwater modeling literature is large, varied, and complex. By necessity, in this article we focus on the most commonly used types of models: groundwater flow models and advective transport models, and at that, we can provide only a brief overview. What is a groundwater model? A model is any sim-