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Model

Utilities
Wellhead Protection

Joe Stump, P.E., Utilities Manager
Joe.Stump@co.yakima.wa.us

Office Hours: M - F, 8:00 AM - 4:00 PM
Accounting Office: 574-2290
Engineering Office: 574-2300

 

WELLHEAD PROTECTION DELINEATIONS

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. 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.

Delineation Method

Relative Cost

Relative
Complexity

Calculated Fixed Radius Model (CFR)

Low

Low

Analytical Model

Moderate

High

Numerical Model

High

High

The CFR model is a simple volumetric flow model used to calculate WHPAs by drawing a circular boundary around the well for a specified time-of-travel (i.e., 1-year).   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. Numerical models require a significant amount of field information and are based on the formation of a grid that simulates the aquifer in question. At each node in the grid, data on water surface elevation, hydraulic conductivity, and aquifer thickness are input, forming a matrix of equations that simulate the behavior of the aquifer under varying pumping conditions. For the Upper Yakima Valley Regional Wellhead Protection Plan, an analytical model was determined to be the most accurate and cost effective delineation method.  Input parameters for the analytical models included regional groundwater gradient, local groundwater gradients, site hydrogeology, well construction information, and pumping data.

The figures below shows two typical WHPAs derived from an analytical model approach in comparison to the CFR method. The elongated WHPA shown in blue might be generated where the local hydraulic gradient controls, whereas the elliptical WHPA shown in red might occur where the pumping rate of the well controls.   Both of these characteristic analytical model shapes were identified in the wells present in the Upper Yakima Valley.  Follow the link to the Inventory Site to see the WHPAs for the wells delineated in this wellhead protection plan.

Analytical Model - Figure 1

Analytical Models - fig. 2

Analytical Model - fig. 3