MODULE-2 Wellhead Protection Areas

What is a Wellhead Protection Area?

The State of Washington Wellhead Protection Program Guidance Document defines a Wellhead Protection Areas (WHPA) as the surface and subsurface area surrounding a water well or well field supplying a public water supply system through which contaminants are reasonably likely to move toward and reach such well or well field within one (1), five (5), and ten (10) years. WHPAs have a critical recharging effect on groundwater used for drinking water supplies, making them highly susceptible and vulnerable to groundwater contamination from land use activities.

How are Wellhead Protection Areas Determined?

In Washington, a WHPA is based on established times of travel. Each management zone in the WHPA is an area that corresponds to a certain established time of travel in the aquifer. Thus, each of the zones represents a certain time interval between a particle of water at the zone boundary and its eventual arrival at the well.

Wellhead Management Zones:

Again, it should be noted that these aquifer travel time zones do not consider vertical movement of a water particle or contaminant from the land surface down to the aquifer. The rate of vertical movement or infiltration rate for an aquifer can be highly variable. Within any given aquifer setting, the infiltration rate will depend on topography, soils, geology, and the nature of land surface activities (relative percent impervious surfaces vs. open space). It may vary significantly over a region and even within an individual wellhead protection zone.

Because it may be difficult to estimate and predict infiltration rates for all settings, the WHPA delineation methods recommended for the State program do not include vertical movement as a part of the base models. This creates a conservative (protective) estimate of travel time. Where infiltration characteristics are known, WHPA zone management plans can and should consider the implication of vertical movement to the aquifer.

next page - The Sanitary Control Area