Numerical Simulation of Solute Transport in Groundwater Flow System

Abstract:

Abstract: The paper aims at development of coupled numerical model i.e. Galerkin finite element for groundwater flow simulation (FEFLOW) and modified method of characteristics for solute transport (MMOCSOLUTE). This model is applied to simulate Total Dissolved Solid (TDS) from irrigation return flow in Mahi Right Bank Canal (MRBC) aquifer of 0.31 Mha in India. This aquifer is water table aquifer with transmissivity from 866 to 3539 m²/d and the specific yield of 0.15. The model run period is of 10 years. The initial conditions for simulation are obtained from 24 observation wells. The boundary conditions are river boundary. The aquifer is discretized into 2260 triangular elements , 1280 nodes and 10 separate zones. The net regional recharge is assigned to each node using recharge distribution coefficient technique. The FEFLOW simulations showed that the difference between average change in computed and the observed groundwater head is 1.1 % . The change in concentration under advection alone is worked out by tracking the solute particles backward in time along characteristic curves. At the base of the characteristic curve the concentration is interpolated from the known concentrations of the adjacent nodes depending upon the quadrantal location of the solute particle at previous time level. Dirichlet boundary conditions are considered with known concentrations along Mahi and Shedi rivers and Alang drain. The source TDS concentration of 370 ppm is assigned to each node. The base values of longitudinal dispersivity, transverse dispersivity and effective porosity are considered as 30.5 m, 9.1 m and 0.28 respectively and the effect of variation of these parameters on model results are also analyzed. The stability of the transport model simulations based on the Peclet number criterion is also carried out by proper choice of the time step size as 7.0 days. The results showed that the proposed model is adequate as the computed TDS concentration values are quite comparable to observed values for the considered simulation period.

Keywords: FEFLOW-MMOCSOLUTE model, MRBC aquifer, Recharge distribution coefficient, TDS of irrigation return flow