Variation characteristics of dissolved SiO2 in sea-land transition interface of shallow groundwater：Taking Leizhou Peninsula and Hainan Island coastline as examples
Due to the importance of silicon as a geochemical element, in coastal zone which is the transition zone of groundwater in coastal area, the study of the important chemical composition of groundwater SiO2 helps to further grasp the lithosphere rock process of sea-land surface. The continuous sampling and chemical tested of shallow groundwater were conducted in the typical volcanic caused region of south china, i.e., along shoreline of Leizhou Peinsula and Hainan island. The change, composition and dissolving balance of dissolved SiO2 in shallow underground water were analyzed. The results revealed that, the types of underground water was primarily Na-Cl type, alkaline, either weathered and leached, evaporation concentration or transitional type between the two. Ranging from 2.8 to 106.0 mg/L with significant spatial and time variation for different test wells, the concentration of dissolved SiO2 was related to nearby environment and derived from mineral dissolution, seawater intrusion and water-rock activity effect . The concentration series of dissolved SiO2 varied even for the single test well at the same period, also changed significantly in the different periods. H4SiO4 was the absolutely dominant among three chemical species of dissolved SiO2. Different silicate minerals had coexistence of dissolution, saturation and r balance states and mutualy transformed between different states of themselves. Overall, the proportion of saturated mineral species in groundwater of the study area was high. Therefore, coastal groundwater as the sea-land transition interface on the open system of high salinity, salt had a promoting effect on dissolve or saturation of different minerals. In general, the water - rock process of soluble SiO2 was affected by many factors, needed further strengthen the research.