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Wellhead Protection Area (WHPA) User's Guide: Implementation of Hydraulic Head Computation and Display into the WHPA Code

This document is an addendum to the Version 2.0 WHPA Code User’s Guide (Blandford and Huyakorn, 1991). WHPA is a user-friendly microcomputer model developed by the U.S. Environmental Protection agency (EPA) to assist State and local technical staff with the delineation of wellhead protection areas (WHPAs) as mandated by the 1986 Amendments to the Safe Drinking Water Act. Previous versions of the WHPA code (2.1 or less) have the capability to delineate time-related, steady-state, or hybrid capture zones using ground-water flow pathline delineation techniques. To determine pathlines and times of travel, ground-water flow velocities and some physical aquifer properties are required. Since ground-water flow velocities are a function of the difference in hydraulic head (or potentials), rather than the head values themselves, it is not necessary to determine hydraulic heads in order to determine pathlines and capture zones.

Calculation of the hydraulic head values, however, can be useful for visualization and model  calibration purposes, and in some cases may serve as a “reality check” for a simulation scenario. For these reasons, options to compute hydraulic head fields were implemented into the RESSQC, MWCAP, and semi-analytical GPTRAC modules of the WHPA code. The purpose of this addendum is to document the implementation and proper application of the new options.
Chapter 2 outlines the implemented changes in the preprocessor and postprocessor modules. It presents several demonstration and verification examples and provides some rules of thumb for model calibration. For interested readers, the mathematical formulations are presented in Chapter 3.
This latest version of the WHPA code (2.2) is entirely compatible with previous versions (2.0 or 2.1) in that existing input and output files may be utilized without modification.
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