Numerical Simulations Of Typical DNAPLs Migration In Saturated Heterogeneous Porous Media

The rapid development and application of petroleum organic chemical products bring us great convenience,but also cause the contamination of soil and groundwater.Organic contaminants,with high toxicity,low solubility and low viscosity,mainly exist as non-aqueous phase liquids(NAPLs)in subsurface,which can lead to a long-term contamination source and is difficult to remediate.In particular,the contamination caused by dense non-aqueous phase liquids(DNAPLs),which is heavier than water,is most serious.The migration behavior and distribution of DNAPLs in subsurface can be influenced by many factors,including its own physicochemical properties and the spatial structure of heterogeneous media.The DNAPLs contamination is generally caused by multicomponent DNAPLs rather than single component DNAPLs.Horizontal spatial continuity and spatial dimensionality are the most important factors of the structure of heterogeneous media.Thus,it is very important to investigate the effects of horizontal spatial continuity,reduced dimensionality and multicomponent DNAPLs on the migration and distribution of DNAPLs,which will facilitate to simulate the migration process of DNAPLs and identify exact position of the source zones.Based on previous studies,TCE and PCE were selected as representatives of DNAPLs.TMVOC-MP software was used to simulate the migration and distribution of DNAPLs in saturated heterogeneous porous media.Based on Markov Chain transition probability model,we built stochastic permeability fields and took fully account of spatial continuity,asymmetry and anisotropy of heterogeneous media.By simulating the migration and distribution of DNAPLs in heterogeneous media with TMVOC-MP,we investigated the effects of horizontal spatial continuity,reduced dimensionality and multicomponent DNAPLs on the migration behavior and distribution of DNAPLs.The main conclusions were drawn as follows:(1)As an important factor of the spatial structure of heterogeneous media,horizontal spatial continuity significantly influenced the migration and distribution of DNAPLs.Increasing horizontal spatial continuity of heterogeneous media resulted in a wider range of lenses in horizontal direction,which led to more accumulation of DNAPLs on lenses and greater migration distance of DNAPLs.Thus,the occurrence state of DNAPLs changed correspondingly.(2)The change in the spatial dimension of heterogeneous media could significantly influenced the migration and distribution of DNAPLs.Compared with the simulation in two-dimensional model,both the migration distance of DNAPLs in horizontal and vertical directions and the amount of DNAPLs reached the bottom decreased in three-dimension model.While the amount of DNAPLs in discrete state significantly increased in three-dimension model compared to the two-dimension simulation.Therefore,the three-dimensional simulation could obtain better performance in forecasting and remediating the DNAPLs contamination when compared with the two-dimensional simulation.(3)Lateral migration process would undermine the influence of heterogeneity of a single plane on DNAPLs migration.When the lateral migration distance was shorter than the length of lenses,DNAPLs preferentially moved downward through lateral migration and the total amount of DNAPLs in a single plane decreased correspondingly.Thus,it changed the occurrence state of DNAPLs and undermined the influence of the heterogeneity of a single plane on the migration of DNAPLs finally.(4)The physicochemical properties of DNAPLs has an influence on the migration behavior and distribution of DNAPLs in heterogeneous media.Numerical simulation results suggested that the migration behavior of DNAPLs was primarily under the control of the spatial structure of heterogeneous media,while the occurrence state of DNAPLs was affected by the components of DNAPLs.It is necessary to identify the components of DNAPLs in order to improve accuracy of forecast and remediation for DNAPLs contamination.