A wellhead protection area (WHPA) is the surface and subsurface area surrounding a well that supplies a public water system through which contaminants are likely to pass and eventually reach the well.
In Washington, WHPAs are based on time-of-travel criteria, or the theoretical distance a particle of water travels in a prescribed period of time.
At a minimum, the DOH requires communities to look at the following WHPAs:
- 6-month time-of-travel WHPA
- 1-year time-of-travel WHPA
- 5-year time-of-travel WHPA
- 10-year time-of-travel WHPA
The time-of-travel WHPAs are determined by estimating the travel distance of a hypothetical particle of water traveling through the aquifer to a pumping well for a selected travel time, (e.g., 1 year). Time-of-travel WHPAs are based on several conservative assumptions. First, time-of-travel criteria do not consider vertical movement of water or contaminants from the land surface to the screened interval of the well.
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.
Methods & Models
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