Research On The Regularity Of Leaching And Desalting Of Calcareous Sand From Islands In The South China Sea

Groundwater is an important factor in the ecological environment construction of coral islands,and a key link to affect the ecological process of coral islands.The formation of down-ground desalination water bodies,formed by coral sand filling island depends not only on the geological and stratigraphic structure of coral islands,but also on the meteorological conditions of rainfall evaporation.It is also closely related to the texture and permeability of reef sand,the displacement of pore water by rain w ater,and the desalination process of reef sand.The desalination process of reef sand directly affects and even determines the desalination level of underground water body.The study of desalination process and regularity of coral reef sand is helpful to analyze the desalination process of groundwater and reveal the formation law and mechanism of underground fresh water.Coral sand is a special marine lithosoil medium with calcium carbonate content exceeding 95%.The grain is irregular in shape and has high void ratio.In addition to the presence of high salinity seawater in the pores,the reef sand grain itself also absorbs salt ions.The newly filled artificial island has a high salt content in the reef sand.After a long process of desalination and desalination under the action of natural rainfall,the exploration of artificial and rapid desalination of reef sand will play an important role in promoting the formation of underground fresh water.In this paper,a large number of laboratory tests on desalination of reef sand have been carried out.The following conclusions have been obtained:1.The indoor test of artificial rainfall desalination shows that the salt accumulates gradually down the seepage path during each leaching of fine sand,and with the increase of seepage path,the peak value gradually moves backward along the flow path;However,the conductivity of coarse sand decreases at the same time under the action of leaching,although the amount of salt ions adsorbed on the surface of coarse sand is much more,but the adsorption capacity is relatively weak,the salt in coarse sand moves downward with seepage during the process of leaching.So the effect of water content on the conductivity of coarse sand is more obvious.After 4 times of leaching,coarse sand can be basically desalted,and fine sand needs at least 9 times.In general,coarse sand is easier to desalinate than fine sand.2.Under the condition of keeping the temperature constant,the conductivity of calcareous sand increases with the increase of volume water content,and the conductivity increases with the increase of volume water content.The electrical conductivity of calcareous sand is exponentially related to the cumulative infiltration amount.With the continuous accumulation of the infiltration amount,the conductivity at different depths decreases sharply and then slows down,and finally tends to be stable.Through the curve fitting,the fitting formula of the change of infiltration quantity and electrical conductivity is obtained.According to the formula,the time limit for the growth of animals and plants can be calculated,thus providing a certain reference for the data analysis of the island reef site.3.In the laboratory experiment of reef-sand desalination with additional electric field,when the rain intensity is 0.0455ml/min/cm2 and the potential gradient is 0.804V/cm,the concentration of chlorine ion in each outlet decreases the most than that of the control group without electric field.The results show that the additional electric field can promote the desalination of calcareous sand.