SOIL WATER FLOW AND CONTAMINANT TRANSPORT MODELING THROUGH VADOSE ZONE IN DOON VALLEY, INDIA

Abstract

The sub-surface solute and moisture flow at any particular site is governed by the prevailing hydrogeological conditions of that area. In the present study an effort has been made to depict these conditions using a mechanistic model to study the behavior of moisture movement and * solute migration to the underlying shallow groundwater resources. The area of study is the Doon valley which is an intermontane watershed in northern India between the Lesser Himalayas and Shiwalik ranges. Simulation has been done for moisture and solute flow at twelve different point locations in the valley. The simulation employs the widely used Richards equation for moisture flow in the porous formation and fickian based adjective-dispersive equation for solute transport. The assigned physical or hydraulic parameters like moisture content, hydraulic conductivity, curve fitting parameters etc. should be representative of actual subsurface conditions so as to replicate the natural process as closely as possible. Based on the results of numerical simulations intrinsic vulnerability of the study area is quantified. The role of topography including soil type, slope and land cover which are generally avoided in such mechanistic models is also considered here using the Rational method of rainfall and runoff relation. The simulation is performed for constant flux as well as for transient flux boundary condition. In latter case daily precipitation data of the study area along with evapotranspiration has been used. The time required by solute to travel from surface to the underground water resources until its peak concentration is taken as an indicator of vulnerability. The results show the progressive moisture contents along the soil profile, top and bottom fluxes etc. during the simulation period at different point locations in the study area. The vulnerability results show a higher risk at northern-eastern and northern region of the study area while a lower vulnerability at southern. The study may assist in decision making and planning of Industrial location and sustainable water resources development in the valley.