CENTRIFUGE MODELING OF BROMIDE MIGRATION IN UNSATURATED SANDWICH LAYER SOILS

In order to effectively control and eliminate soil pollution, the migration of pollutants is one of the most important research concepts in environmental geotechnical engineering. Infiltration and migration of solutes in the unsaturated zone is a very complex process, and research advances in this field represented a breakthrough in the problem of groundwater pollution. Geotechnical centrifuge model experiments can simulate the flow and solute transport processes in a gravity field within a short time period without changing the material and hydrogeological conditions. They can also simulate important physical phenomena for pollutant migration in soil prototypes. Thus, geotechnical centrifuge modeling has been widely used in solute migration research for the unsaturated zone.

The experiment material was quartz sand. Two kinds of soil sample were used: the saturated moisture content and permeability coefficient of sample 1 were 19.59% and 64.99 cm/day, respectively, and 21.46% and 32.04 cm/day for sample 2. The TLJ-500gt centrifuge in the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, which has a radius of 4.5 m, was used in this study. When the centrifuge spun with 50 g acceleration, the sodium bromide solution was released to a model box through the storage tank. Two columns were used. Column 1 consisted of layers of soil sample 1. Column 2 consisted of soil sample 1 except for a sandwich layer of soil sample 2 between 10 and 15 cm, where the top is 0 cm and the bottom is 50 cm.

Observation shows that the water content increased with increasing depth (0-1500 cm) in column 1. However, in column 2, the water content increased at depth of 0-700 cm and suddenly decreased an depth of 700-1500 cm. The water content of soil above the sandwich layer in column 2 was higher than that of soil in column 1. The curves of bromide migration show the same trends with water transport.

Solute migration in soils is mainly based on the penetration ability in different soils. The permeability of sample 1 was larger, so the solute transport in sample 1 was more active. However, the sandwich layer had a good solute holding capacity. The consequence was that the water contents and Br concentrations of the soil above the sandwich layer in column 2 was higher than that of the homogeneous soil in column 1 at the same depth.