A modeling study on vadose zone leaching, saturated zone mixing, and groundwater flow in heterogeneous aquifer

A vadose zone leaching, saturated zone mixing, and groundwater flow (VG) model was developed as a "user friendly" model with extensive use of Graphic User Interface and the capability of simulating vertical heterogeneity aquifer. A 1-D finite difference scheme was employed for solving the leaching equation in a vertically heterogeneous vadose zone. The transport processes, including liquid-phase advection, liquid- and vapor-phase dispersion, sorption, and decay of contaminant, were taken into account. The transport processes in all the phases were combined together in one equation under the assumption of the instantaneous linear equilibrium partitioning relationships. This not only provided more consistent formation, but also was computationally more efficient since only one differential equation needed to be solved. The mass-balance principle was used for the mixing calculation within the top portion of the aquifer at the bottom of vadose zone. A 2-D finite difference method was employed for developing a 2-D heterogeneous groundwater flow sub-model. This sub-model provides determination of the. contaminant track in the heterogeneous saturated zone, and evaluation of the natural rate of groundwater flow and the risk of contaminant. Soil column tests were performed to test the validity of the model. Three different soil sample sizes of Ottawa quartz sand and 480 ppm saline water as groundwater contamination were used to be validated successfully in a lab steady state soil column study. The important input parameters of soil properties in these simulations were effective porosity, water filled porosity, and bulk density. In the field study at Wood River Junction, RI, the developed model demonstrated its useful performance for evaluating a regional aquifer contaminant site by a good match between simulated data and historical data.