Abstract
The baseline quality of pre-drilling shallow groundwater is essential for the evaluation of potential environmental impacts of shale gas development. The Xishui region in the northern Guizhou Province of Southwest China has the potential for shale gas development but there is a lack of commercial production. As for the future environmental concerns in this undeveloped area, this study presented the hydrochemical and isotopic characteristics of shallow groundwater and its dissolved gas before shale gas development and determined the sensitive monitoring indicators. Results showed that shallow groundwater with an average pH of 7.73 had low total dissolved solids (TDS) ranging between 102 and 397 mg/L, with the main water chemistry types of HCO3-Ca and HCO3-Ca·Mg. The quality of most groundwater samples satisfied the drinking water standards of China. The mass concentration of dissolved methane in groundwater was below the detection limit (<0.01 mg/L), suggesting the low baseline value of hydrocarbon. The shallow groundwater was mainly recharged by local precipitation based on water isotopes. Water chemistry was modified by the dominant dissolution of carbonate rocks and partial dissolution of clastic rocks, as indicated by δ13C-DIC, 87Sr/86Sr, and δ11B. Evidence from carbon isotopes of dissolved methane and CO2 (δ13C-CH4 and δ13C-CO2) and noble gas isotopes (3He/4He and 4He/20Ne) demonstrated that the biogenic methane mainly originated from acetate fermentation and the dissolved noble gas was a result of the dissolution of air. Based on the geochemical and isotopic differences between shallow groundwater and flowback and produced water (including shale gas) from the Weiyuan and Fuling shale gas fields as well as shale gas from Xishui, this study has provided the sensitive monitoring indicators and methods for identifying potential pollution of regional shallow groundwater related to shale gas development in the future.
Highlights
Due to advances in horizontal drilling and hydraulic fracturing, the rapid increase of shale gas production in North America has affected the global energy pattern [1,2]
In the Guizhou Province of Southwest China, the Longmaxi Formation is mainly located in Northern Guizhou and its shale gas reserves can be evaluated to 1.83 × 1012 m3, which has the potential for large commercial shale gas development [7,8,9]
This study aims to present a case for the comprehensive evaluation of shallow groundwater quality before shale gas production
Summary
Due to advances in horizontal drilling and hydraulic fracturing, the rapid increase of shale gas production in North America has affected the global energy pattern [1,2]. Some earlier studies indicated that dissolved CH4 of shallow groundwater in Pennsylvania increased as it moved closer to shale gas wells [13,19]. This is most likely due to the small sample size, which was from a planned sampling campaign in a known contamination area [20]. Establishing geochemical baseline datasets of shallow groundwater systems before shale gas production is necessary, which can provide direct evidence when contamination occurs, but can reduce controversies of shallow groundwater pollution after hydraulic fracturing [28,29,30]. A few sites, such as Pennsylvania, USA [31], Colorado, USA [32], Quebec, Canada [33], North Yorkshire, UK [34], and Chongqing, China [35,36], have developed baseline investigation of groundwater before drilling shale gas wells
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