| 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. |
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