Chemical Signatures and Salinity Evolution of Aquitard Porewater in the Haizhou Bay
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为查明海岸带弱透水层孔隙水的盐分来源与演变机制,在连云港海州湾钻探采集了 4个钻孔的粘性土柱,采用压榨法采集孔隙水,测定了孔隙水化学‒同位素组分,建立了二维剖面孔隙水全新世以来的溶质运移数值模型.由陆向海孔隙水的总溶解固体由 0.9增加到 41.4 g/L,垂向上浅层高、深层低.孔隙水Cl/Br比为 170～533(均值 267),87Sr/86Sr比值为 0.709 3～0.711 6,Cl‒与δ18O呈正相关关系,表明孔隙咸水为海相成因,同时还受到硅酸盐矿物风化和阳离子交换作用影响.全新世弱透水层孔隙水为 10～5 ka BP海侵时期的古海水,向下入渗造成更新世孔隙水咸化.4 ka BP海退后,孔隙水接受淡水补给,但古海水并未被完全驱替.近海岸处孔隙水受持续蒸发影响而形成盐水.数值模拟表明海侵‒海退事件是控制弱透水层孔隙水盐分演变的主要因素,海侵时海水以"指状"模式向下入侵,造成了咸淡水的不均匀分布.
To ascertain the salinity origin and evolution of aquitard porewater in the coastal zone,this paper collected four borehole core samples in the Laizhou Bay,Lianyungang City,and porewater was then extracted by mechanical squeezing.The chemical and isotopic components of porewater were measured and a solute transport model of porewater since Holocene in a 2D typical section was established.The results show that the total dissolved solids of porewater range from 0.9 to 41.4 g/L from land to sea,and it is higher in shallower sediments and lower in deeper ones.Porewater has Cl/Br ratio of 170-533(267 in average)and 87Sr/86Sr of 0.709 3-0.711 6,together with the positive correlation between Cl‒ and δ18O,indicating that saline porewater is of marine origin and has been affected by silicate weathering and cation exchange.In the Holocene aquitard,porewater is trapped ancient seawater in 10-5 ka BP and the downward infiltration led to salinization of the underlying Pleistocene porewater.About 4 ka BP,porewater was diluted by freshwater in the regression,yet paleo-seawater has not been totally flushed.While in the area close to the coastline,the continuous evaporation led to the occurrence of much saltier porewater.Porewater simulation suggests that aquitard porewater salinity evolution is mainly controlled by transgression-regression events,and during transgression,seawater downward invaded in a"finger-like"pattern,resulting in the heterogeneous distribution of brackish and fresh groundwater.
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