Chemical Signatures and Salinity Evolution of Aquitard Porewater in the Haizhou Bay
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摘要: 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.Abstract: 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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Key words:
- aquitard /
- porewater /
- salinity evolution /
- transport /
- Lianyungang City /
- hydrochemistry /
- environmental geology
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图 3 弱透水层孔隙水主要离子与氢氧同位素垂向剖面
Fig. 3. Profiles of chemical components and stable isotopes in aquitard porewater
图 4 弱透水层孔隙水Cl‒ vs. Br‒ (a)、Cl‒ vs. Cl/Br (b)
Fig. 4. Cl‒ vs. Br‒ (a)、Cl‒ vs. Cl/Br (b) of aquitard porewater
图 5 H4钻孔沉积物TOC与Br‒关系（a）、TOC与Cl/Br垂向变化（b）
Fig. 5. The relationship between TOC in H4 sediment and Br‒ in porewater (a)、profiles of TOC and Cl/Br ratios (b)
图 6 弱透水层孔隙水δ2H vs. δ18O (a)、Cl‒ vs. δ18O (b)
Fig. 6. δ2H vs. δ18O (a)、Cl‒ vs. δ18O (b) of aquitard porewater
图 7 弱透水层孔隙水Na+与Cl‒（a）、Na/Cl与Cl‒（b）、Ca2+与Cl‒（c）、Mg2+与Cl‒（d）、SO42‒与Cl‒（e）、Ca+Mg‒HCO3‒SO4与Na+K‒Cl（f）关系
Fig. 7. The relationship of Na+ vs. Cl‒ (a)、Na/Cl vs. Cl‒ (b)、Ca2+ vs. Cl‒ (c)、Mg2+ vs. Cl‒ (d)、SO42‒ vs. Cl‒ (e)、Ca+Mg‒HCO3‒SO4 vs. Na+K‒Cl(f)in aquitard porewater
图 8 弱透水层孔隙水Sr2+与Cl‒ (a)、87Sr/86Sr与Sr2+ (b)关系
Fig. 8. The relationship between Sr2+ and Cl‒ (a)、87Sr/86Sr and Sr2+ (b) in aquitard porewater
图 9 全新世以来海州湾典型剖面模拟与实测的Cl‒浓度(mg/L)
a~f. 距今10~5 ka时的Cl‒分布（海水淹没本区，孔隙水咸化）；g~i. 距今4~2 ka时的Cl‒分布（海退时期孔隙水淡化）；j. 模拟的现今Cl‒与实测结果（1 ka时近海岸形成蒸发海水）；k. 子模型a运移1 ka时的浓度分布
Fig. 9. The measured and simulated Cl‒ concentration (mg/L) in the typical section of Haizhou bay since Holocene
表 1 模型的参数与边界条件
Table 1. Parameters and boundary conditions in the model
岩性 渗透系数
K（m/d）扩散系数
De（m2/s）运移时间 边界条件 亚粘土
（局部夹粉细砂）0.42 2.0×10‒10 海侵时期10~5 ka 上边界：海水淹没部分，C$ {}_{{}^{\mathrm{C}{\mathrm{l}}^{-}}} $=19 000 mg/L；陆相部分：开放边界； 粘土（局部夹粉细砂） 0.08 2.0×10‒10 海退时期4~1 ka 泄湖区：C$ {}_{{}^{\mathrm{C}{\mathrm{l}}^{-}}} $=25 000 mg/L
下边界：无通量边界粉细砂 0.86 1.0×10‒9 1 ka BP，近岸形成泻湖 左边界：流入边界，C$ {}_{{}^{\mathrm{C}{\mathrm{l}}^{-}}} $=100 mg/L右边界：流出边界 
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