Study On The Migration Characteristics Of Dense Non-Aqueous Phase Liquid In Saturated Zone

With the development of industrial society,groundwater is polluted by a variety of different pollutants,among which Dense Non-Aqueous Phase Liquid(DNAPL)is the most difficult to treat among organic pollutants.The key to controlling groundwater pollution lies in accurately locating the polluted area and determining the degree of pollution.The traditional borehole sampling method spends a lot of manpower and material resources while obtaining limited data,and it is easy to form a superior channel for pollutants to invade deep aquifers.Using non-invasive geophysical methods to study the migration and occurrence of DNAPL under the influence of underground media,water flow and other conditions can provide ideas and theoretical basis for the prevention and control of groundwater pollution.In this paper,there were five laboratory physical models,including two-dimensional homogeneous static water,homogeneous dynamic water,single heterogeneous dynamic water,compound heterogeneous dynamic water and three-dimensional heterogeneous dynamic water,constructed.High-density Electrical Resistivity Tomography(ERT)was used to obtain timelapse Resistivity data.From the perspective of resistivity,a comprehensive geophysical parameter,the electrical characteristics distribution under different hydraulic and medium conditions were analyzed and discussed,and the migration and occurrence characteristics of DNAPL under different media and hydrodynamic conditions were analyzed.The research results and conclusions are as follows:(1)Under homogeneous static water conditions,the migration of DNAPL was affected by gravity and capillary force.It mainly migrates downwards,with symmetrical migration as the supplement.The migration rate was less than the injection rate,and enrichment occurs at the injection point.The spatial distribution in the aquifer was characterized by overall symmetrical distribution around the axis where the injection point was located.The average rate of horizontal migration on the 10% change rate isoline was 0.82 cm/h,and the average rate of vertical migration was 2.98 cm/h.After 270 min,the horizontal migration distance was basically stable and there was no significant increase,indicating that the capillary force acts on the migration in the horizontal direction has the farthest distance.(2)Under homogeneous dynamic water conditions,DNAPL was obviously affected by hydrodynamic action,and its vertical and downstream migration was promoted.The geometric center of the region with the maximum change rate was shifted to the lower right of the aquifer section with the horizontal average rate of 3.53cm/h and the vertical average rate of 4.29cm/h.The hydrodynamic force promotes its vertical and downstream migration,and its spatial position moves to the depth and downstream of the aquifer under the combined influence of gravity and hydrodynamic force.(3)In the single heterogeneous dynamic water experiment,the vertical migration of DNAPL was blocked due to the strong capillary force of horizontal low permeability body.Under the influence of gravity,hydrodynamic and capillary force,the main body of DNAPL concentrates above the low-permeability body near the downstream side,and mainly migrates around the low-permeability body to the depth and downstream of the aquifer(4)Composite heterogeneous dynamic water experiment,due to the combined action of gravity,hydrodynamics and the strong capillary force of the low permeability body,when the main body of DNAPL migrated to the low permeability body,it was blocked to migrate downward and downstream due to the strong capillary force of the low permeability body,accumulated above and upstream of the low permeability body,and migrated upstream along the low-permeability body until invading the deep aquifer.(5)In the three-dimensional heterogeneous dynamic water experiment,the migration characteristics of DNAPL with Y-0.2 section as the boundary were basically symmetrical.The upstream migration of DNAPL was inhibited and downstream migration was promoted,and the profile represented by Y-0.2 without vertical low permeability mirror body was most obviously affected by the flow due to the smooth flow.Under the action of gravity and hydrodynamic force,the injected DNPAL migrated to the bottom,upper and lower reaches of the aquifer,and migrated to the vertical low-permeability body,which was enriched in the upstream side of the low-permeability body and flowed around the non-low-permeability pass to the downstream side.