Experimental Studies Of Reactive Solute Transport Through Capillary Transition Zone

Vadose zone is geological body, above the phreatic water level in groundwater system and below the surface, and is a positive and key belt for matter and energy exchanges between groundwater system and surface water environment, which has a crucial role in constitution of chemical composition of groundwater and groundwater pollution prevention and control.Capillary transition zone that forms vadose zone with the soil water zone and the intermediate zone, is a zone formed when water rises to a certain height from the water table along the small pores in the loose rocks. The distinguish between the capillary transition zone and the other two belts is that it has a closely hydraulic connection with phreatic table, and its lower portion is saturated, while water zone in the capillary saturated couldn’t reach the groundwater under the negative pressure of the capillary water.Capillary transition zone is a positive and key belt for matter and energy exchanges between unsaturated zone and vadose zone; on one hand, the unsaturated zone accepts the material supply entering to vadose zone by capillary Transition zone; on the other hand it transports to two other belts of vadose zone with substance through the capillary water. Capillary water is the last bulwark to hinder exogenous contaminants into unsaturated zone through the vadose zone.Despite of it, the blocking effect of vadose zone on pollutants has been recognized by the academia; but the understanding of the blocking mechanism of vadose zone on pollutants is still quite limited. To carry out capillary transition zone with reactive solute transport studies will contribute to in-depth understanding of the issue. The difficulties are as follows:(1) how to build and observe the capillary transition zone;(2) how to choose a representative contaminant;(3) how to observe the pollutants changes in quality in the capillary transition zone.In this paper, pointed to the scientific issues and research difficulties, the following studies have been carried out by comprehensive application of soil column experiments and numerical simulation:(1) To design capillary water-soil interaction experiments in soil columns with different lithologies under the conditions of different salinity groundwater recharges, to observe the wetting front, access to the regulation of capillary water rising processes, and to research the influences of lithology and salinity on heights and speed of capillary rise;(2) To select a change valence of heavy metal as the target pollutant, which is toxic, widely distributed, strongly migrated, and with strongly adsorbed redox in the environment, inert component Br as a contrast, to research the chromium changes in quality before and after though capillary transition zone, to confirm the residual amount of chromium and the possible functions under the different salinity conditions (Og/L, lOg/L and60g/L).(3) Based on the Hydrus-1D simulation software, to quantitative evaluate the time, the quality and the forms ratio in the process of blocking effect of vadose zone on pollutants, compared with the vadose zone.According to experiments and software simulation, we got the main conclusions as follows:(1) The height of capillary rise was related with capillary diamerter. With the decreased of the capillary diameter, the height of capillary increased, and the rate of capillary rise decreased. When the soil lithology changed, the relationship of capillary rise height and time and the total groundwater recharge were different. The clay loam presented a power function while the sand presented a log function.The capillary rise velocity was related with the degree of mineralization. During the capillarity of clay loam, the capillary rise velocity of lower degree of mineralization (lOg/L) was faster than60g/L. But they were all faster than Og/L. It was because crystal was precipitation and then attached on the pipe wall during the capillary rise. It could lead to decreasing capillary diameter, increasing the capillary force and increasing the capillary rise velocity. On the other side, the density wound increase which lead the gravity and capillary force increase with the increase of the degree of mineralization. So the capillary rise velocity of lower degree of mineralization was faster than high degree of mineralization. During the capillarity of sandy soil, the capillary rise velocity of high degree of mineralization was faster at the beginning, but it would be opposite law at last.(2) The quality of Cr(VI) would change cause of part of Cr(VI) was stayed when the flow through the capillary water. This was mainly a result of an adsorption in the capillary water. Because of the characteristics of capillary transition zone are different, the breakthrough time was shown as:no capillary water<being suspended capillary<being support capillary water;10g/L>60g/L>0g/L. It was same with residue in capillary transition zone. Because the solute would mainly suffer downward gravity and capillary force and the upward friction when there was no capillary water. And the solute would suffer downward gravity and the upward frication when there was suspended capillary water, so the infiltration velocity was slower. Also, the solute would suffer downward gravity and the upward frication and capillary force when there was support capillary water, so the infiltration velocity was slowest. What’s more, the breakthrough curve of Cr(VI) was deviation to right compare to Br-. It is because that there was absorption in the capillary water zone. TDS impacted it because that:①when the value of groundwater TDS was increased, the crystal was precipitation and then attached on the pipe wall during the capillary rise. It could lead to decreasing capillary diameter which would slow the infiltration velocity.②when the value of groundwater TDS was increased to10g/L, the sodium adsorption ratio (SAR) in capillary water zone was increase. It would destroy the soil structure which would slow infiltration ability and velocity. When the value of groundwater TDS was increased to60g/L, the gravity was larger, so the breakthrough time was faster. (3)We used Hydrus-1D to evaluate the retardation of capillary transition zone. We could know that the capillary water had a blocking effect on the infiltration of pollutants into groundwater, and the effect would strengthen when the flow closing the groundwater. When the sustained release of pollutants was720min, the quality of Cr(VI) would reduce58.8%through the column with capillary water, and50%Cr(VI) would reduce without capillary water. Therefore, the capillary water had the ability to blocking the pollutants. However, Cr(VI) was still easier to penetrate the vadose zone into groundwater and polluted groundwater in the shallow water table areas. This would pose a threat to safety of drinking water, so environmental protection departments should pay more attention.The innovation of this paper was:carried out the migration of Cr(VI) in the capillary water firstly, advanced the research of vadose zone solute transport to the depth of capillary water.