Also, it is assumed that contaminants move at the same rate as water in the subsurface, where actual contaminants may move slower or faster than water. This assumption is also typically conservative because the soil matrix, biological process, and chemical processes tend to retard the transport of contaminants in the subsurface.
Several methods exist for delineating wellhead protection areas.
These methods range in complexity and cost of implementation. The following table summarizes the relative costs and complexity of the common delineation models. Analytical models make use of established groundwater flow equations to provide a more accurate delineation of WHPAs. WHPAs delineated with analytical models tend to be elongated in shape with the majority of the WHPA lying upgradient of the well in the direction of groundwater flow. Numerical models are generally accepted as the superior method for the delineation of WHPAs.