Study On The Chemical Characteristics Of Groundwater And Its Permeability Dissolution In Coral Reef Island

Underground fresh water is an important resource in the South China sea reef area,which is of great significance to the harmony and stability of the island animal and plant ecological environment.In the past,the research on groundwater chemistry and groundwater movement in China was mostly carried out in the mainland,while the research on the chemical characteristics and water quality analysis of the island groundwater was rare.Because of the great differences in the occurrence environment,state and influencing factors of groundwater in coral reef island,the theory and method of groundwater chemistry in continental area can not be completely copied and applied to the research of groundwater chemistry in coral reef island.There are two difficulties.On the one hand,the occurrence of the underground water in the reef is in the dynamic evolution,which is a vulnerable system susceptible to various natural factors and human activities.Because the coral reef island is surrounded by high salinity sea water around and at the bottom,the recharge of atmospheric rainfall,the leakage and loss of fresh water,the tide and dispersion of sea water all affect the underground fresh water area On the other hand,the permeability of reef sand stratum is strong,the organic acid and carbon dioxide produced by plant root secretion and microorganism,as well as the acid substance produced by human activities are infiltrated with groundwater,the dissolution and dissolution of carbonate rock occur,the pore and structure of reef sand stratum are changed,and the content of related ions and water quality of reef groundwater are changed.Therefore,this paper first grasps the general situation of the underground water environment of a coral reef artificial island in the South China Sea through hydrometeorological observation and geological investigation;on this basis,it uses a variety of hydrochemistry methods and statistical methods to analyze the main ion components of the underground water,and reveals the formation mechanism of the chemical field of the underground water of the reef;finally,combined with the indoor test of coral sand,it studies the dissolution The law of influence on the island reef aquifer.The main research results are as follows:?1?In addition to HCO₃^-,the contents of five main ions(Na⁺,Mg²⁺,Ca²⁺,Cl^-,SO₄^2-)and TDS values increase from the center to the outer edge of the reef,and the ion concentration increases linearly with the depth;HCO₃^-ion concentration decreases linearly with the depth,but the correlation is not high.There are two different chemical types of groundwater.The Cl-Na type water accounts for 92.4% of the water samples taken,and the rest is HCO₃-Na type water.?2?Based on the analysis of the relationship between the main ions and the characteristic coefficient of hydrochemistry,Na⁺,Mg²⁺,Ca²⁺,SO₄^2-are all linearly positively correlated with Cl^-,and the concentration of Mg²⁺,Ca²⁺ deviates from the theoretical ratio line to a certain extent,which indicates that groundwater is not only a simple mixture of local rainwater and seawater,but also a certain amount of reactive carbonate minerals are dissolved in the mixing process.?3?In the aquifer medium of the study area,the main reactive minerals are aragonite and magnesia calcite,dolomite,gypsum and rock salt are the secondary non reactive minerals.Factor analysis shows that the contribution rate of common factor f₁ is 53.874%,the contribution of common factor f₂ to carbonate mineral dissolution is 21.362%,and the contribution of common factor f₃ to natural environment is 10.471%.?4?The results show that the change process of permeability coefficient of single graded coral sand column can be divided into four stages: rapid increase,slow increase,gradual decrease and basic stability.However,the permeability coefficient of coral sand column with continuous gradation does not decrease gradually,but increases rapidly,slowly and then keeps stable.The permeability coefficient basically accords with the quadratic polynomial fitting relationship with time.?5?The change of Ca²⁺ concentration in the exudate with time is more in line with the linear fitting relationship under the double logarithmic coordinate.Parameters a and B are related to the p H value of the osmotic solution and the initial concentration of Ca²⁺ in the solution.After the test,the particle size distribution range is increased compared with that before the dissolution,and the peak value of the particle size distribution curve wave is decreased,and the content proportion of each carbonate mineral component before and after the test also changes slightly.