Algorithm Of LNAPLs Migration And Its Numerical Simulation

The vadose zone, constituted by the unsaturated porous media, is the part ofEarth between the land surface and the top of the phreatic zone. LNAPLs can pollutethe vadose zone and the groundwater after it leaking into soil. In order to discover,survey and control the contamination as early as possible, It is necessary to researchits characteristics and its migration mechanism in soil. Giving the derivative processof the control equation of LNAPLs migration in vadose zone, this paper uses thefinite element collocation method to solve it. Based on those, this paper researchesthe LNAPLs migration mechanism in soil after its leaking, including analyzes theinfluences of leakage volume, leakage rate, leakage form and the soil inherentpermeability coefficient on LNAPLs migration in vadose zone. Finally, this paperanalyzes the influences to soil relative dielectric constant in the process of LNAPLsmigration in soil.In the beginning, this paper studied the basal characteristics about LNAPLs andthe underground environment and the its migration mechanism. There are twoexistence forms that is free state and dissolved state or volatile state when LNAPLsleak into soil. There are three free fluids in soil porous: water phase, air phase andLNAPLs phase. The influence factors of its migration in soil mainly include:volatilization, dissolution, adsorption, desorption, degradation and soil properties.LNAPLs migration in unsaturation soil media is a variable saturation multiphaseflow problem.Based on the above information, this paper gives the derivative process of thecontrol equations of LNAPLs migration in vadose zone which is free state’s flowequation and dissolved state’s (or volatile state’s）convection-diffusion equation.And the finite element collocation method is used to solve the control equations.First, the author reduces the control equation to pressure based differential equation.Because the terms of relative coefficient of permeability and saturation which are thefunctions of the pressure are included in the equations, the control equations hasstrong nonlinear characteristics. Secondly, section the region with regularquadrilateral grid and the implicit difference method is presented to approximate thetemporal derivative. Then, this paper adopts piecewise bi-cubic Hermite interpolating basis function to approximate the pressure terms and adopts piecewisebilinear Lagrange interpolating basis function to approximate the relative coefficientof permeability term and the saturation term. Calculate collocation node coordinatesof each grid. Lastly, substitute all the basis functions and their order derivatives intothe implicit equation which can educe the collocation equation. This paper solve thecollocation equation with the conjugate gradient method.Based on the above mathematical model and the solution method, this papersets up a physical model as an example and analyzes the influences of leakagevolume, leakage rate, leakage form and the soil intrinsic permeability coefficient onLNAPLs migration in vadose zone. Then, this paper also analyzes the influences tosoil relative dielectric constant in the process of LNAPLs migration in soil.The LNAPLs migration mechanism in soil after its leaking is obtained by theabove simulation and calculation. And the results show that, LNAPLs will firstlymigrate downward by gravity, when LNAPLs leaking into soil. It will obviouslymigrate laterally by the capillary force when it reaches the capillary water zone. But,the density of LNAPLs is lighter than that of water, so LNAPLs will float on thewater when it reaches the groundwater table. And with the increasing of the LNAPLsaggregation, the water table will be concave-down. The scope of the migration ofLNAPLs in soil is affected by the leakage volume: the more the leakage volume, thewider the diffusion range. And the scope of migration is affected by the leakage rate:the lesser leak rate is with the more obvious migration scope in short term, and themore leak rate is with the mote obvious migration scope in long term. For leakageform, in the case of a certain amount of total leakage in per unite time, it is obviousthat the diffusion ranges are affected by the non-uniform leak rate in short termwhile it is unobvious in long term. The lesser the inherent soil permeabilitycoefficient the little the diffusion range. The the soil relative dielectric constant willdecrease because of LNAPLs’ leakage. And because of the diffusion of LNAPLs,the residual water in soil is pushed to both sides of the area, thereby forming a clear“mountain”.