Calculated Fixed Radius (CFR) Model
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
The Model Used
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 (PDF) to see the WHPAs for the wells delineated in this wellhead protection plan.