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Hydrogeochemical and Isotopic Indicators of Hydraulic Fracturing Flowback Fluids in Shallow Groundwater and Stream Water, derived from Dameigou Shale Gas Extraction in the Northern Qaidam Basin Zhaoxian Zheng, Hongda Zhang, Zongyu Chen, Xufeng Li, Pucheng Zhu, and Xiaoshun Cui Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017
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Hydrogeochemical and Isotopic Indicators of Hydraulic Fracturing Flowback Fluids in Shallow Groundwater and Stream Water, derived from Dameigou Shale Gas Extraction in the Northern Qaidam Basin Zhaoxian Zheng,1,2 Hongda Zhang,3 Zongyu Chen,1* Xufeng Li,4 Pucheng Zhu,1 Xiaoshun Cui1 1. Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang, 050061, Hebei, P.R. China 2. China University of Geosciences (Beijing), Beijing, 100083, P.R. China 3. Oil & Gas Survey, China Geology Survey, Beijing, 100029, P.R. China 4. Center for Hydrogeology and Environmental Geology, China Geological Suvery, Baoding Hebei, 071051, P.R. China Correspondence to: *Zongyu Chen: Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, No. 268 Zhonghuabei Street, Shijiazhuang, 050061,
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Hebei, P.R. China; Phone: +86-311-67598558; Fax: +86-311-67598661; E-mail:
[email protected]. TOC Art
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Abstract: Most of the shale gas production in northwest China is from continental shale.
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Identifying hydrogeochemical and isotopic indicators of toxic hydraulic fracturing
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flowback fluids (HFFF) has great significance in assessing the safety of drinking water
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from shallow groundwater and stream water. Hydrogeochemical and isotopic data for
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HFFF from the Dameigou shale formations (Cl/Br ratio (1.81×10−4–6.52×10−4), Ba/Sr
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(>0.2), δ11B (-10–1‰) and εSWSr (56–65, where εSWSr is the deviation of the
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ratio from that of seawater in parts per 104)) were distinct from data for the background
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saline shallow groundwater and stream water before fracturing. Mixing models indicated
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that inorganic elemental signatures (Br/Cl, Ba/Sr) and isotopic fingerprints (δ11B, εSWSr)
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can be used to distinguish between HFFF and conventional oil-field brine in shallow
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groundwater and stream water. These diagnostic indicators were applied to identify
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potential releases of HFFF into shallow groundwater and stream water prior to fracturing
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and flowback. The monitored time series data for shallow groundwater and stream water
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exhibit no clear trends along mixing curves towards the HFFF end member, indicating
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that there is no detectable release occurring at present.
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1. Introduction
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87
Sr/86Sr
Energy demand is continually rising in China as a result of rapid economic
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development. Natural gas consumption in China has increased from 110.5 billion m3 in
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2010 to 185.5 billion m3 in 2014, and is predicted to increase to ~400 billion m3 in 2030.
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However, natural gas production in China has only increased from 99.0 billion m3 in
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2010 to 134.5 billion m3 in 2014, and is predicted to be ~250 billion m3 in 2020.1, 2
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Because of the large shortfall between supply and demand, more attention is being given
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to the large reserves of organic-rich shale gas in China. At present, large-scale
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commercial shale gas development is still in its infancy in China. Nonetheless, the
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extraction of gas from low-permeability organic-rich shale formations through hydraulic
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fracturing has dramatically increased with the rapid development of shale gas
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industrialization and commercialization.3 Hydraulic fracturing flowback fluids (HFFF),
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which are produced by ubiquitous high-volume hydraulic fracturing, comprise injected
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and formation water released adjacent to the shale formations.4 HFFF are typically highly
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saline and contain human-made additives and naturally occurring chemicals at toxic
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concentrations.5, 6
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The toxic substances potentially associated with HFFF have led to strong public
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concerns about their impact on groundwater and surface water during fracturing, storage,
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transport, and disposal.5 The occurrence of HFFF release can be identified if specific
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fracturing fluids that are rare in the area and/or artificial chemical tracers added to the
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fracturing fluid are found in the water under scrutiny. However, patent protection and/or
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fracturing efficiency is constraining the application of these identification methods.7
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Therefore, a robust methodology for identifying of HFFF spills is currently a key
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research topic given the global interest in shale gas extraction. Many studies have focused
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on the characterization of HFFF from the world’s largest unconventional natural gas
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play—the Marcellus Formation in the Appalachian Basin, USA. Data from the USGS
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produced water database reveal that the formation water is typically hypersaline and
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characterized by a Natrium-chloride composition with high Br/Cl ratios that reflect
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different degrees of seawater evaporation and water–rock interactions8. Chapman stated
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that strontium isotopic ratios from the Marcellus Formation HFFF fall within a narrow set
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of values (87Sr/86Sr = 0.710148 to 0.712119), and observed that this isotopic range is
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distinct from most oil and gas brines in the Upper Devonian Venango Group associated
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with western Pennsylvania acid mine drainage9. Warner suggested that trace element
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ratios (B/Cl >0.001, Li/Cl >0.002) and isotopic fingerprints ( δ11B = 25 to 31‰, δ7Li = 6
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to 10‰) for HFFF from the Marcellus and Fayetteville black shale formations were
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distinct in most cases from produced water sampled from conventional oil and gas
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wells.10 We observed that most shale gas formations in the United States were deposited
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in an oxygen-deficient marine environment 11 and that in previous studies the major
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signatures of HFFF are closely linked to the depositional environment and the origin of
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shale formation water.
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However, a large number of shale gas plays in northwest China (e.g., the Qaidam,
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Ordos, Tuha, and Tarim Basins) were deposited in a continental environment,12 and the
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origin of formation water is different from most shale gas plays in the United States (e.g.,
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the Marcellus formation water is of ancient seawater origin10). The continental origins of
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the shale gas plays in northwest China has given the HFFF distinctive hydrogeochemical
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and isotopic characteristics compared with HFFF produced from the Marcellus marine
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shales in which the values of 87Sr/86Sr, δ11B and Cl/Br are strongly affected by formation
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water that originated from evaporated seawater10, 13. Furthermore, the shallow
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groundwater in arid areas of northwest China is highly saline because of intense
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evaporation.14 This salinity has masked the presence of HFFF when it has migrated to
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shallow groundwater and stream water. Finding unique indicators for HFFF is, therefore,
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critical for distinguishing HFFF salts from autochthonous salts in shallow groundwater
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and surface water in shale plays in northwest China. These indicators could be used for
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establishing a diagnostic index system for identifying accidental releases and potential
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migration pathways during shale gas development in the Qaidam Basin.
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This study focused on the hydrogeochemical and isotopic characteristics of shale
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formation water that might strongly influence the composition of HFFF. The variations of
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inorganic elements such as Br, Ba, Sr, and B in the HFFF that can be affected by the
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hydraulic fracturing process were also investigated. The injection of fresh water into the
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shale formations causes the release of elements from exchange sites on the mineral
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surfaces.10 The injection of fresh water could also alter the isotopic compositions of
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elements (e.g. Sr and B) in flowback fluids during mixing and water-rock interactions.15
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The overall aim of the current study is to distinguish the altered HFFF from the shallow
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groundwater and stream water, and to provides key information for forensic evaluations
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of HFFF accidental releases. Thus, this study is addressed by three objectives: (1) study
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the origin of Dameigou shale formation water to characterize its chemical and isotopic
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compositions; (2) identify the unique hydrogeochemical signatures of HFFF that could be
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used as environmental tracers differentiating it from conventional oil-field brine (COB)
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in shallow groundwater and streams; and (3) apply the hydrogeochemical and isotopic
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tracers as environmental indicators at field sites to distinguish the contamination of
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shallow groundwater and streams by HFFF, and evaluate the sensitivity of HFFF
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indicators. Our study investigated the characteristic indexes of HFFF associated with
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hydraulic fracturing of continental shale for the first time. Our hydrogeochemical and
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isotopic indicators, including Br/Cl, Ba/Sr, δ11B, and 87Sr/86Sr, are consistent in HFFF
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found across the Qaidam Basin shale gas play in which the Dameigou shale constitutes
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the most favorable reservoir for exploitation. When combined with mixing models, these
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indicators may be applied elsewhere in northwest China to identify shallow groundwater
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and stream contamination by HFFF in extensive continental shale gas plays.
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2. Study area
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The study area lies within the northeastern margins of the oil-bearing and gas-rich
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Qaidam Basin (Figure 1a). The study area has an arid continental climate with cool
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summers and frigid winters, with daily highs in the coldest/warmest months (January/July)
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of −28.0 °C and 28.1 °C, respectively16. Precipitation ranges from 8.4 to 87.7 mm/year
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across the region, while evapotranspiration varies from 2065 to 3040 mm/year16. The
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terrain of the study area slopes downwards from the northwest to the southeast. Tectonic
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erosional mountains and hills with medium to low relief, and alluvial and proluvial plains
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and valleys are well developed in the study area. The surficial geology is dominated by
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unconsolidated gravel, coarse sand, and fine sand of the Upper Pleistocene, which ranges
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from a few meters at the piedmont to ~40 m in the valleys.
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A few Neogene and Eogene sedimentary strata are exposed downstream of the Naoer
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River. Chai Ye 1 (CY1) well, the first shale gas exploration well in terrestrial Jurassic
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sediments in northwest China, was hydraulically fractured at the study area in August
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2014. The Dameigou Formation (comprising interbedded fractured shale, coal, and
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siltstone) is part of the Middle Jurassic stratigraphic sequence that was deposited in an
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oxygen-deficient semi-deep lake environment.17 The Dameigou Formation dips to the
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south and lies 1920 m to 2150 m below the ground surface in the study area. A typical
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synthesized columnar section of Qaidam Basin can be seen in Figure S1. The area
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contains multiple faults and lineaments as mapped in Figure 1b. Confined shallow
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groundwater in the semi-consolidated Neogene strata is dominated by pore flow through
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primary intergranular porosity from the topographic highs to the lowland discharge areas.
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Phreatic pore water in the Upper Pleistocene strata drains as multiple springs along the
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Naoer River. Generally, the shallow groundwater has a high TDS (>10 g/L) and a Na-Cl
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composition in both aquifers, although isolated areas of low salinity upstream of the
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phreatic groundwater are found.18
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3. Materials and Methods
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3.1 Sampling
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Five categories of water samples were used in this analysis: (1) a single sample of
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formation water was obtained before hydraulic fracturing; (2) five samples (CY1-1d to
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CY1-7d) of HFFF from CY1 were obtained as a time series from the beginning of
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flowback; (3) ten samples of shallow groundwater (Neogene aquifer) from two
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monitoring wells (CJ1, CJ2), located downstream and upstream of CY1, were collected
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as time series commencing the day after fracturing; (4) four samples from a single spring
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(CJ3) discharged from the Upper Pleistocene were collected as time series f commencing
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the day after fracturing; (5) five samples from the same sampling location in the Naoer
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River (R1) were collected as time series commencing the day after fracturing. The
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specific day of sample collection from fracturing event can be seen in Table S1.
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Background samples were also collected before fracturing to monitor potential mixing
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relationships between the HFFF and the shallow groundwater and stream water.
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Formation water was collected, before fracturing, from perforated holes made through the
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casing and cement, and extending into the shale formation, using deep penetrating
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charges. There are two main Neogene shallow aquifers in the study area: the upper
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aquifer is composed of semi-consolidated siltstone and is located at a depth of 96.2 to
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105.0m, and is accessed by monitoring well CJ2; and the lower aquifer is composed of
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semi-consolidated fine sandstone at a depth of 145.0 to 150.5m, and is accessed by CJ1.
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HFFF samples labeled 1d through 7d indicate the number of days from the hydraulic
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fracturing event.
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Samples were field filtered through a 0.45 µm nylon filter into pre-cleaned HDPE
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bottles with no head space. Samples for cation, Sr, and B isotopes were acidified with
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ultrapure concentrated HNO3 to a pH of less than 2. All samples were kept on ice while
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in the field and refrigerated in the lab at 4 °C until the analyses were completed.
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3.2 Analytical Methods
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Major anions, except HCO3−, were analyzed by a Thermo Scientific Dionex ICS-4000
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(precision, ±1%). The HCO3− concentration was determined by phenolphthalein titration,
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and major cations and minor elements were analyzed by a PerkinElmer Inductively
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Coupled Plasma Optical Emission Spectrometer (ICP-OES) Model Optima 8300
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(precision, ±1%) at the National Research Center for Geoanalysis, Chinese Academy of
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Geological Sciences. The δ18O and δD measurements were completed on a Picarro
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L2130-i Analyzer (precision, ±0.025‰ for O and ±0.1‰ for H) at the Institute of
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Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences.
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Strontium and Boron isotopes were measured using an Isotopx Phoenix Thermal
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Ionization Mass Spectrometry (TI-MS) and PerkinElmer ELAN DCR-e Inductively
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Coupled Plasma Mass Spectrometry (ICP-MS), respectively, at the Center of Analysis,
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Beijing Research Institute of Uranium Geology. The average 87Sr/86Sr ratio of NIST
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SRM 987 over the period of these analyses was 0.710244 ±0.000015 (n= 30). The
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average 11B/10B ratio of NIST SRM 951 during this study was 4.0436 ±0.0016 (n= 25).
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The long-term standard deviation of δ11B in the standard through replicate measurements
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was
