| Basalts are widely distributed in the world.The Cenozoic basalts and the Permian Emeishan formation basalts in China have an area of 13.4×104 km2.The Leiqiong area?the Leizhou peninsula and the northern part of Hainan Island?studied in this paper has the largest area among the ten Quaternary volcanic groups in China.Basalt experienced unique degassing effect in the process of diagenesis,and formed abundant pore and fissure network,so the groundwater migration path in the basalts is short,spring discharge is large flow,and water-rock interactions is insufficient.The springs waters are colorless and odorless,with large quantity and low salinity.They are often used as supply water and agricultural irrigation.Through field sampling and indoor testing and analysis,this paper makes an in-depth analysis of the basalt lithology,geological structure characteristics and spring hydrochemical characteristics in the study area,summarizes the regional geological structure background and spring hydrochemical types in Leiqiong area,analyzes the spring genesis model,and puts forward reasonable development and utilization suggestions.The basalts in Leiqiong area are composed of quartz tholeiite and olivine tholeiite.The main minerals are plagioclase,pyroxene and amphibole.Water temperature of the springs in the study area ranges between 26 and 32?,which are close to the outdoor temperature.TDS range from 144 to 313 mg/L,and the springs are of the low salinity.The mainly hydrochemical type is of HCO3–Mg·Ca type.The analyses,show that HCO3-comes from the atmospheric precipitation and magmatic rocks of aluminum silicate minerals.Mg2+comes from the water-rock interaction between groundwater and the basalts,and the reason why Mg2+content is greater than Ca2+is that Mg2+is more easily dissolved and migrated than Ca2+.The strong leaching effect of groundwater in the basalt area leads to the higher content of magnesium than calcium in groundwater after long-term groundwater circulation.The hydrogen and oxygen isotope of the springs show that groundwater is of meteoric origin.The slight deviation of the values of?2H and?18O can be attributed to the influence of three processes.The first one is because of the influence of near source water vapor.The second one is strong surface evaporation in Leiqiong area.And the last one is that in rock strata?18O is more enriched than?2H.stable isotopes shift to the right or upper right after the action of water rocks.Then the values of?2H and?18O shift to the right or upper right after water-rock interaction.By comparing the rare earth elements in groundwater and rocks,the variation in contents of the element is consistent.It is inferred that the groundwater source of each spring is the same and the water-rock interaction process is similar.The odd-even effect of rare earth elements in groundwater can be attributed to the difference of element solubility.The genesis mode of the springs in the Leiqiong basalt is similar,which can be summarized as follows:the meteoric water infiltrates the groundwater from the surface along the basalt holes and fissures,groundwater undergoes shallow circulation depth,fast runoff speed and short migration path,and does not mix with the groundwater of other aquifers,and finally appear on the surface in low-lying terrain or along the structural fissures in the form of springs.The content of metasilicic acid in the spring water in the study area ranges from24.4 to 81.8 mg/L,Conforming to the naming standards of natural drinking meta-silicic acid mineral water in China.The Nasong well and Shennvquan meet the standards of strontium mineral water.Therefore,reasonable development and utilization can be carried out in combination with local conditions. |
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