The lowest reaches of a large-scale basin could be the discharge areas of local, intermediate and regional groundwater flow systems with significantly distinct travel distances and travel times. This study aims to delineate the groundwater chemical characteristics and the mechanism controlling the chemical evolution in the lowest reaches of the Wushenzhao Cretaceous basin, NW China. A total of 38 groundwater samples were collected and were chemically classified into five distinct water types by means of a Piper Plot. According to the hydrogeological setting and groundwater age, the spatial distribution of these water types is found to be associated with hierarchically nested groundwater flow systems (local and regional system): Types 1, 2, 3 and 4 belong to the local groundwater flow system, while type 5 belongs to the regional flow system. Graphical plots, stable isotopes and geochemical modeling techniques were used to interpret the observed compositions. The results show the dominance of carbonate and gypsum dissolution in type 1 waters; ion exchange in types 2, 3 and 4; and evaporite dissolution in type 5. In addition, human activities in the form of extensive irrigation also affect the chemical compositions of type 1 water. These findings are important for the sustainable management of groundwater resources in the study area.

Studies have been carried out to investigate the baseline radioactivity level (gross alpha, gross beta and ²²⁶Ra) of soil, rocks and groundwater in the Fuling block, Chongqing, the largest shale gas exploitation area of China. The results show that there is a general activity concentration trend of gross alpha, gross beta and ²²⁶Ra: shale > soil > limestone due to the high content of uranium, thorium and potassium in shale and low content in limestone. The average activities of shallow groundwater from a limestone aquifer are 0.14, 0.13 and <0.008 Bq/L for gross alpha, gross beta and ²²⁶Ra, respectively. The radioactivity concentrations of gross alpha, gross beta and ²²⁶Ra (4.37, 1.40 and 0.395 Bq/L, respectively) of the formation water were far lower than those of formation water in the Marcellus shale in the USA (with ranges of 86–678, 23–77 and 16–500 Bq/L, respectively). One polluted shallow groundwater source and its associated stream sediments had been polluted due to leakage of drilling fluid with relatively high radioactivity levels and high concentration of main ions. Overall, this study provides an important baseline radioactivity level to assess the impact of shale gas exploitation on a shallow environment.

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.

Acid rain with a relatively high concentration of ammonium and nitrate can accelerate rock weathering. However, its impact on groundwater nitrate is uncertain. This study evaluated the dual isotopic composition of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) from precipitation to groundwater in a rural mountainous area affected by acid rain. The average concentration for NH₄⁺ is 1.25 mg/L and NO₃^- is 2.59 mg/L of acid rain. Groundwater NO₃^- concentrations ranged from <0.05 to 11.8 mg/L (baseline), and NH₄⁺ concentrations ranged from 0.06 to 0.28 mg/L. The results show that groundwater δ¹⁸O-NO₃^- values (-4.7‰ to +4.2‰) were lower than the values of rainfall δ¹⁸O-NO₃^- (+24.9‰ to +67.3‰), suggesting that rainfall NO₃^- contributes little to groundwater NO₃^-. Groundwater δ¹⁵N-NO₃^- values (+0.1‰ to +7.5‰) were higher than the values of δ¹⁵N-NO₃^- derived from the nitrification of rainfall NH₄⁺ (less than -4.7‰ in the study area), suggesting that nitrification of rainfall NH₄⁺ also contributes little to groundwater NO₃^-. This implies that rainfall NO₃^- and NH₄⁺ have been utilized. The dual isotopic composition of nitrate shows that baseline groundwater NO₃^- is derived mainly from nitrification of soil nitrogen. The denitrification process is limited in the groundwater system. This study shows that the rainfall NO₃^- and NH₄⁺ contribute little to groundwater NO₃^-, improving the understanding of the nitrogen cycle in areas with a high concentration of NH₄⁺ and NO₃^- in rainfall.