A Coupled Approach to Modeling Vadose Zone and Ground Water Flow and Solute Transport at Different Scales.

Incorporation of variably saturated water flow and solute transport processes into groundwater flow models has posed many challenges for ground water modelers in the past. A simple representation of vadose zone processes in ground water models is a very attractive approach. However, it leads to poor modeling efficiency due to computational constraints. A balance between modeling efficiency and computational demand of the adopted approach is thus needed to reasonably represent the effects of vadose zone processes. Recently, the HYDRUS package for MODFLOW has been developed that has been shown to represent vadose zone flow processes in ground water models at different spatial and temporal scales. In this research, we extend the capabilities of the HYDRUS package by additionally also including the solute transport in the vadose zone. Along with modeling water flow, the new HYDRUS package simulates solute transport such that the MODFLOW-HYDRUS code produces concentrations as a function of time that can be incorporated into the source function for MT3D. Since the original HYDRUS-1D software package, on which the coupled approach presented here is based, can model complex solute reaction and transport processes, we have dramatically simplified the solute transport module to facilitate the usability of the coupled approach. The module simulates solute transport in the variably-saturated zone using the third-type (Cauchy) boundary condition for the upper boundary and free solute drainage for the bottom boundary. The new HYDRUS package assumes that the solute can exist in all three phases (liquid, solid, and air) in the vadose zone while assuming instantaneous and linear interactions between phases. A simple first-order decay term has been used in the package to account for degradation in the subsurface.