【作者】 李静；

【导师】 靳孟贵； 梁杏；

【作者基本信息】 中国地质大学 ， 地下水科学与工程， 2014， 博士

【摘要】 粘性土弱透水层作为地下水流系统的重要组成部分,是地下水的天然保护屏障。由于粘性土层渗透性低、可限制地下水的更新及循环,因此又作为核废物的储存场址而备受关注。弱透水层的物理化学性质近几年已获得较多的研究成果,但相对含水层地下水,其研究程度和研究范围远远落后,尤其在国内粘性土孔隙水研究甚少。粘性土孔隙水是沉积过程中保留在颗粒空隙间的水,由于粘性土渗透性极差孔隙水交替缓慢,在粘性土中滞留时间长,可以保留沉积时期及其封存后的地球化学特征,成为极有价值的古环境信息载体。渤海湾是研究古环境变化的重要场所,本文选择渤海湾西北岸唐山曹妃甸CFD-100m、天津滨海新区G1-500m和G2-1226m三个钻孔,通过采集钻孔原状土柱,提取粘性土孔隙水,分析了孔隙水化学和同位素信息,绘制钻孔剖面孔隙水化学和氘氧同位素图谱,运用水文地球化学、同位素水文地质学及古水文地质学等研究方法,结合沉积物工程特性、矿物组合和常量元素分析,揭示了粘性土孔隙水的地球化学特征,确立孔隙水与古环境变化有对应关系的指标,探讨了孔隙水所揭示的古环境事件。得到的主要认识与结论如下：1.微量粘性土孔隙水的提取与分析由粘性土工程特性分析表明随着埋深加大,粘性土孔隙水逐渐降低,固结度升高。对于坚硬态的粘性土,孔隙水提取难度加大。针对粘性土特性和测试目的,设计了不同的孔隙水提取方法。用于水化学测试的粘性土孔隙水提取采用机械压榨法,用于氢氧同位素测试的孔隙水采用真空蒸馏法。浅层(一般<200m)高含水量粘性土可用低压压榨仪,最大压力为3Mpa;深层低含水量粘性土用高压压榨仪加压,最大压力为10Mpa。经提取过程发现,压榨法提取的孔隙水化学受大气环境、压榨仪不锈钢材料影响,对氧化还原敏感元素测试有一定的误差。2.曹妃甸100m钻孔孔隙水化学特征及晚更新世以来古环境指示(1)孔隙水化学特征：通过分析孔隙水主离子和微量元素(包括稀土元素),并与相邻含水层地下水进行对比,发现该孔孔隙水均为咸水,TDS变化范围为7.26g/L～26.89g/L,由浅至深TDS逐渐降低,Na+,Cl-,Mg2+等与TDS变化趋势相似,显示出海水垂向扩散影响；与含水层地下水相比,孔隙水具有高TDS,高离子含量,高重金属含量的特征。TDS约是含水层地下水5-30倍,重金属Fe,Mn,Zn含量约比含水层地下水高1-2个数量级,这是由于粘性土富含重金属,孔隙水长期的溶解作用造成。孔隙水中稀土元素含量甚低,页岩标准化模式为“右倾”形态,重稀土富集,主要由粘性土对轻稀土的吸附作用和重稀土的络合作用造成。(2)水岩相互作用及赋存环境：除受海水扩散影响,孔隙水与沉积物间存在强烈的水岩作用。利用各离子与Cl-比值说明离子的相对变化,结果表明孔隙水SO4/Cl比值甚低(小于海水及含水层地下水),结合phreeqc程序计算的孔隙水饱和指数,方解石、白云石和石膏均为过饱和状态,说明存在硫酸盐还原作用；Na/Cl与Ca/Cl比值呈相反的变化趋势,说明存在Na+与Ca2+交换过程；计算的Eu/Eu*值范围为1.34～2.25,Ce/Ce*范围为-0.13～0.26,Ce基本呈正异常和Eu的正异常,表明孔隙水中为相对还原环境,斜长石的溶解造成Eu2+的优先迁移。(3)晚更新世以来古海侵的识别：孔隙水赋存于较强的还原环境,即封存性较好,可能具有古环境指示作用。通过特征离子比Cl/Br, Sr/Ba及孔隙水氢氧同位素分析,探讨孔隙水的古水特征。结果显示孔隙水氢氧同位素比相邻含水层偏负,δ18O随深度逐渐变小且与Cl-呈正相关关系,表明孔隙水可能为古水,并受到海水扩散的影响。Cl/Br,Sr/Ba比值在不同沉积相中变化显著,Cl/Br在37.6m-49.4m,78.5m-100m两个晚更新世海相地层孔隙水中为279～510,与海水值接近,在陆相地层中Br-低于检测限,Cl/Br比无穷大；同时Sr/Ba在海相地层孔隙水显著大于陆相地层孔隙水,表明孔隙水为古水,并未被现代水所驱替,识别了晚更新世两个海相地层,由30-40ka BP和70-100ka BP的两次海侵事件形成。3.天津滨海G1-500m深层钻孔孔隙水化学及上新世以来古环境指示(1)孔隙水化学及同位素特征：通过分析孔隙水的主离子及氢氧同位素信息,得出该孔孔隙水垂向上分为两段：0-100m,孔隙水为咸水,TDS范围为3.69-30.75g/L,主离子Cl-,Na+,K+,Mg2+,S042-,Ca2+垂向变化趋势与TDS相似,在海相孔隙水中含量高,向陆相孔隙水中逐渐降低,与CFD钻孔相似,说明孔隙水保留了古水特征,且运移方式以扩散为主。孔隙水氢氧同位素相对深层孔隙水偏正,δ18O值范围为-5.83～‰-9.09‰,且与Cl-呈正相关关系,说明该段孔隙水受到相邻咸水层的影响；100m-500m,孔隙水为淡水,TDS小于1g/L,各离子含量低,较稳定,水化学类型以HC03-Na为主；孔隙水氢氧同位素贫化,变化范围为-7.57‰～10.66‰,与Cl-不相关。对比岩性来看,粘性土孔隙水δ18O比砂土偏负,主要是砂土渗透性好,受到现代水混合。(2)孔隙水古海侵及古气候指示作用：综合分析孔隙水主要离子,氢氧同位素及特征离子比Cl/Br,Sr/Ba比值,揭示中更新世以来的古海侵事件和上新世以来的古气候。结果显示孔隙水主要离子含量在海相沉积地层中高于陆相沉积地层,氢氧同位素在海相地层孔隙水中偏正,以及Cl/Br,Sr/Ba比值在海陆相地层孔隙水差异明显,表明该孔孔隙水为古沉积水,保留了沉积时期原始溶液的性质,识别了中更新世一个海相地层,晚更新世两个海相地层以及全新世海相地层。深层孔隙水氢氧同位素主要受气候影响。孔隙水δ8O值在上新世地层500m-350m,由-10.6%0增加到-7.87‰,表明逐渐变暖的气候；350m-320m段δ18O快速降低,由-7.87‰降低至-10.22‰,说明气候逐渐转冷；320m-220m,第四纪早更新世地层,δ18O相对新近系地层孔隙水偏负,在相对冷的气候环境下,δ18O值由深到浅从-10.22‰增加到-8.4‰,孔隙水中Fe3+亦出现高值,表明第四纪早期气候相对温暖；220m-150m,第四纪早更新世晚期地层,孔隙水δ18O值变化不大,总体上呈变负的趋势,表明气候向晚更新世逐渐变冷；150m-0,第四纪中更新世～全新世地层,中更新世以来渤海湾气候频繁波动,发生多次海侵,整体气候由晚更新世冷转变为全新世暖。4.天津滨海G2-1226m深层钻孔粘性土及孔隙水的中新世以来古气候指示(1)粘性土矿物特征及古气候指示：粘土矿物种类及含量很大程度上受气候影响。根据粘土矿物含量及组合,分析了晚中新世8.35Ma以来的气候变化。结果得出：沉积物粘土矿物以蒙脱石、绿泥石和伊利石为主,并形成了三种粘土矿物组合。由垂向上粘土矿物含量反映的气候变化为：8.35～7.5Ma BP,晚中新世炎热气候,7.5～5.23Ma BP中新世末期气候明显变干,进入上新世(5.23～3.1Ma　BP)气候向干冷环境转变,约3.1～2.58MaBP为上新世末期的相对温暖期；2.58Ma BP～至今,即第四纪以来蒙脱石含量急剧降低,说明第四纪大冰期低气温环境；早中更新世相对晚更新世更为干冷,全新世气候相对转暖。受气候影响,沉积物颜色由红棕色向灰色过渡；(2)常量元素特征及古气候指示：测试沉积物常量元素,利用风化强度指数CIA值和氧化度“Fe3+/Fe2+”来反映气候特征,并与粘土矿物反映的气候对比,识别三个气候转变期。结果显示沉积物CIA值和氧化度“Fe3+/Fe2+”垂向上变化趋势与蒙脱石相似,说明表现的气候特征一致。识别的三个气候期为：(a)5.23Ma左右,蒙脱石含量和CIA值明显降低,表明向干冷气候转变,反映了亚洲季风进一步增强；(b)2.58-2Ma,蒙脱石含量突降,CIA值低于平均值,表明存在明显的降温事件,即进入第四纪大冰期,全球性大幅度气温降低。(c)0.126Ma左右,从中更新世向晚更新世过渡时期,蒙脱石和CIA含量升高,表明气温升高和降雨量增加,进入末次间冰期。(3)孔隙水氢氧同位素的古气候指示：G2钻孔粘性土埋藏深度大,孔隙水滞留时间长,氢氧同位素受不同时期气候影响,氢氧同位素关系表现出三种模式,位于全球雨水线附近,与雨水线平行和与雨水线相交。由δ180垂向剖面反映的古气候特征为,中更新世气候较暖,上新世存在两次变暖趋势和一次降温事件,第四纪以来气温明显降低,进入晚更新世气温有所回升。对比了孔隙水同位素和沉积物矿物、化学元素所反映的古气候,两者具有一致性,说明深层钻孔粘性土孔隙水为古沉积水,可能受到含水层的扩散影响,但是整体变化趋势可指示古气候变化。本文的创新点为：利用粘性土弱透水层所具有的“封存”性特征,采用沉积物粘土矿物、常量元素、孔隙水化学及同位素剖面图谱,揭示了孔隙水成因及其赋存环境,识别古海侵事件及沉积时期古气候(温度)变化。粘性土孔隙水的研究在水文地质学中既是研究难点又是研究热点。本文设计了孔隙水提取方法,对孔隙水地球化学及其应用开展了初步研究。孔隙水的化学特征及古环境指示,对于探讨弱透水层-含水层相互作用,地下水水质演变等具有重要意义,一定程度上可指导优质水源的选取和丰富全球变化的研究内容。更多还原

【Abstract】 Clay-rich aquitards are natural protective layer for aquifer groundwater, which are important part of groundwater flow system. Based on the low permeability of aqutards, aquifer groundwater can be confined to renew and alternate. Therefore, clay-rich aquitards as potential site for waste reserve have attracted much more attention. These years the study of physicochemical property of clay has gotten few results. However, relative to groundwater, the study of aquitard pore water is far behind, both in extent and scope.Pore waters were trapped in particle gaps during the sediment deposition. Pore water alternate slowly and can be long-time resident in the clayey aquitard as the extremely low permeability, which made pore water to be valuable proxy for paleo-environment. In this paper, we drilled three boreholes in northwest of Bohai bay, i.e. CFD-100m borehole of Caofeidian, G1-500m borehole and G2-1226m borehole of Tianjin coastal plain, where it is an important place for paleo-environment study. By collecting the drill core and extracting the pore water from clay samples, geochemistry and isotopic feature of pore water were analyzed. Through the analysis of the chemical ions,δ18O and δD vertical profiles, combined with engineering properties, clay mineral and elements characteristics of sediment, geochemistry of aquitard pore water was revealed. Indicators of paleo-1environment which correspond to pore water were established, and then we discussed the paleo-environmental events revealed by the pore water, using the hydro-geochemistry, isotope hydrogeology and paleo-hydrogeological research methods. The main conclusions are as follows:1. Extraction and analysis of clayey aquitard pore waterWater content of clay decreased with depth and consolidation showed a reverse trend, which were indicated by engineering characteristics. The pore water extraction is very difficult for hard state soil. Therefore, different methods were designed according to the clay and test purpose. For chemical composition test, pore water was extracted by mechanical squeeze. For stable isotope measure, pore water was extracted by vacuum distillation. Actually, shallow high water content clay samples (<200m) can be squeezed by low pressure device, with the max pressure of3Mpa. Deep low water content clay samples can be squeezed by high pressure device, with the max pressure of lOMpa. Chemistry of pore water are influenced by atmospheric and stainless steel materials, which will cause errors for redox sensitive elements.,2. Chemical signature of pore water in Caofeidian100m borehole and its paleo-environment implications since Pleistocene(1) Chemistry characteristics of pore water:major ions and trace elements (including rare earth elements) were analyzed. Pore water is saline water, with the TDS of7.26-26.89g/L. The profile of TDS shows a decrease trend with depth, and the Na+, Cl-and Mg2+are similar to TDS, indicating that pore water was impacted by seawater diffusion. Compared with groundwater, pore waters are characterized by high TDS, which is about5to30times of groundwater, high ions content and heavy metals, as Fe, Mn and Zn content higher than about1to2magnitude of groundwater. This is caused by the dissolution of clay enrichment with heavy metals. Rare earth elements (REE) content in pore water is very low and the shale-normalized REE pattern shows enrichment in heavy rare earth elements (HREE) relative to light rare earth elements (LREE), caused by the clay adsorption for LREE and complexation for HREE.(2) Water-rock interaction and occurrence environment:In addition to the seawater diffusion, there are other processes to influence pore water as strong water-rock interaction. Use the ratio of ions to Cl-to describe the ions relative changes. The results showed that the ratio of SO4/CI in pore water is very small (less than seawater and groundwater), combined with saturation index calculated by phreeqc code, calcite, dolomite and gypsum supersaturated, indicating the presence of sulfate reduction; The opposite trend of Na/Cl and Ca/Cl profile indicated the process of Na+and Ca2+exchange; Eu/Eu*values are in the range of1.34-2.25and Ce/Ce*values range from-0.13to0.26, showing a positive Ce and Eu anomalies, which indicated the relative reduction environment in pore water and Eu2+priority migration by the dissolution of plagioclase.(3) The recognition of transgression since the late Pleistocene:Pore water occurs in a strong reducing environment, indicating the "better sealed" for clay aquitard and indicative of the paleo environment for pore water. Typical ratio of Cl/Br and Sr/Ba and stable hydrogen and oxygen isotopes were analyzed to discuss the "ancient water" features. The results show that δ18O and8D of pore water are much more depleted than the adjacent aquifer. And δ18O value decreases with depth, together with the positive correlation with Cl-, indicating that pore water may be ancient water and has been affected by seawater diffusion. Cl/Br and Sr/Ba ratios of pore water change significantly in different sedimentary facies along the study profile, i.e., Cl/Br ratio in pore water of depth37.6m-49.4m,78.5m-100m which are two marine sedimentary units of later Pleistocene, is between279and510, close to seawater. And in terrestrial sedimentary unit pore water, Br-is lower than the test limit, thus the Cl/Br ratio trends to infinite. Also the Sr/Ba ratio in marine sedimentary pore water shows much bigger than pore water in terrestrial sedimentary unit. The above features of pore water suggest that pore waters are ancient water and were not replaced by modern water or seawater. Two transgressive events of later Pleistocene30-40ka BP,70-100ka BP, respectively, were recognized.3. Chemical signature of pore water in Gl-500m deep borehole in Tianjin coastal plain and its implications on paleo-environment since Pliocene(1) Pore water chemistry and isotopic characteristics:pore water can be divided into two sections in this borehole according to the ions and isotope information:0-100m depth, pore water is saline water, with the TDS of3.69-30.75g/L. The vertical profile of TDS shows the value higher in marine sedimentary pore water, lower in terrestrial sedimentary pore water. The same trends were found in Cl", Na+, K+, Mg2+, SO42-and Ca2+profile. Just like as CFD borehole, pore water contains the signature of water trapped in sediment during the sedimentation. Isotopes of pore water are much more enrichment compare to the deeper pore water, and the818O values range from-5.83‰to-9.09‰, together with the positive correlation with Cl-, further supporting the found pore water was influenced by adjacent saline groundwater and the transport of pore water by diffusion;100m-500m depth, pore water is freshwater, with the TDS of<lg/L. The ions content of pore water in this section is low and relative stable. The water type is with HCO3-Na. pore water δ18O values range from-7.57‰to-10.66%o, much more depleted, and not relevant with Cl". From the view of lithology,δ18O of pore water in clay formation is more depleted than pore water in sand formation, as the mix role of modern water due to the better permeability of sand.(2) Ancient transgression and Paleo-climate indicated by pore water:through the comprehensive analysis of major ions, stable isotope (δD,δ18O) and typical ratios (Cl/Br, Sr/Ba), ancient transgressive events since the middle Pleistocene and paleo-climate since Pliocene were revealed. The results showed that pore water in marine sedimentary units characterized by higher ions content and enrichment of δ18O and δD compare to the pore water in terrestrial sedimentary units. Also Cl/Br and Sr/Ba ratios of pore water change significantly in different sedimentary facies along the study profile, indicating that pore water are ancient sedimentary water and reflect the properties of original solution. A Pleistocene marine sedimentary unit, two late Pleistocene marine sedimentary units and Holocene marine sedimentary unit are identified.Stable isotope (18O and D) of deeper pore water is mainly affected by climate.δ18O values of pore water in Pliocene strata shows an increasing trend, from-10.6%o at500m depth to-7.87%o at350depth, indicating the gradual warming climate; г18O values decreased rapidly, from-7.87%o at350depth to-10.22%o at320m depth, suggesting the cooling event; in320m to220m depth, Early Pleistocene formation,δ18O values of pore water in this section is much more depleted compare to Pliocene. In the relatively cold climate,δ18O values show an increasing trend, from-10.22%o to-8.4%o with the elevation increases. And Fe3+of pore water also exhibit high content in this section, indicating the relatively warm climate in the early Quaternary; in220m to150m depth, the late of Early Pleistocene formation, the δ18O values of pore water in this section show an decreasing trend, indicating turning cool to the late Pleistocene; in the150m depth to surface, climate changed frequently since the middle Pleistocene, and transgression occurred many times. Overall, the climate trends to warm toward to Holocene from the cold climate of late Pleistocene.4. Plaeo-climate change since Miocene induced by clay and pore water in borehole of G2-1226m in Tianjin coastal plain(1) Clay mineral characteristics and paleoclimate:the species and content of clay minerals are largely affected by climate. The changes of climate since8.35Ma BP were discussed through the analysis of content of clay mineral and mineral assemblage. The results showed that:clay minerals in G2borehole sediments are mainly with smectite, chlorite and illite. And three clay mineral assemblages were formed. By the analysis of clay mineral vertical profile, the climates are reflected as follows:in8.35-7.5Ma BP, hot climate is in the late Miocene, and climate change dry clearly towards to the last phase of Miocene, i.e.7.5-5.23Ma BP. In the early Pliocene period (5.23-3.1Ma BP), climate turned cold and dry, which become relatively warm in the3.1-2.58Ma BP. In the period of2.58Ma BP to this day, i.e., since the Quaternary, smectite content decreased sharply, indicating the low temperature environment of Quaternary glacial maximum. Climate in the early and middle Pleistocene is much colder than the late Pleistocene, and it turned warm in the Holocene. Affected by climate, sediment color turns from red-brown to gray.(2) Constant elements and paleo-climate:Use the weathering intensity index CIA and degree of oxidation "Fe3+/Fe2+" to reflect the climate and compared with the results gotten by clay minerals. Three climate transition events were identified. The results showed that CIA and Fe3+/Fe2+vertical profiles were similar with smectite, indicating the climate is consistent refected by the three of them. The identified three climate transitions are as follows:(a) about5.23Ma BP, the significantly decreased of smectite content and CIA value, suggesting the cold and dry climate, and further enhanced the Aasian monsoon;(b)2.58-2Ma BP, the sharply drop of smectite content and CIA value, reflecting the significant cooling event, e.g., in the Quaternary Ice Age, global temperatures drastically reduced;(c) about0.126Ma BP, during the period of middle Pleistocene transiting to late Pleistocene, the value of CIA and smectite content increased, showing the higher temperature and increased rainfall in the last interglacial.(3) Stable isotope (18O and D) and climate:Due to the deep burial of G2drill core, pore water would be resident in the clay for a long time. Affected by climate of different periods, pore water δ18O and δD relationship showed three patterns, i.e., around the global meteoric water line (GMWL), parallel with the GMWL and intersect with GMWL. The paleo-climate signatures reflected by δ18O are as follows:it is warmer in the Miocene, and there are two warming climate periods in the Pliocene, and one cooling event. Since the Quaternary, the temperature decreased clearly. Becoming to Pleistocene, the temperature has gone up. Compared to the climate reflected by stable isotope of pore water, clay minerals and constant elements of sediments, we found the results are consistent, indicating that pore water as ancient sedimentary water can be used for indicators of paleo-climate, though they maybe influenced by groundwater diffusion.The innovation of this paper:As the "better sealed" signature of clay aquitard, using sediment clay mineral, pore water chemistry and isotopic vertical profiles to reveal the causes of pore water, and identify the transgression events and paleo-climate changes in the sedimentary period.Research of clay aquitard pore water in hydrogeology is both research focus and difficulty. Design the extraction method of clay pore water and carried out preliminary study of pore water geochemistry and its application. Chemical characteristics of pore water and paleo-environment implications have significant to the study of interaction between aquitard and aquifer, and evolution of groundwater quality, which will help to guide the select of high quality water and enrich the content of global change study.更多还原