Source characterization of nitrate in groundwater using hydrogeochemical and multivariate statistical analysis in the Muling-Xingkai Plain, Northeast China.

The source characterization of nitrate (NO3-) in groundwater of Muling-Xingkai Plain (MXP) and the influence of NO3- on the water environment were studied by hydrogeochemical and multivariate statistical analysis. A total of 164 groundwater samples were collected, and the samples were classified into three clusters by using hierarchical cluster analysis. Cluster 1 (C1), accounting for 13% of total samples, was mainly located in local residential zones where the top soils were the medium-textured sediments. Cluster 2 (C2) and cluster 3 (C3) were mainly located in farmlands and residential zones where the clay sediments were overlaying the aquifers. The soil media covering the aquifers was an important factor controlling the concentration of NO3- in groundwater, which determined the infiltration rate of wastewater and the redox environment of aquifers. Only the samples in C1 exceeded the WTO standards for NO3- (50 mg/L), and the samples in C2 and C3 had low NO3- concentration (less than 10 mg/L). The excessive NO3- in groundwater was observed in the shallow groundwater under local residential zones, and it was closely related to the anthropogenic activities since the 1950s. The domestic sewage was responsible for the elevated NO3- contents in the MXP. Then, it was still necessary to construct the sewage disposal system in rural areas to further protect the groundwater resource to avoid the formation of extensive nitrogen pollution. At present, NO3- in the groundwater mainly shows a fertilizer and natural rainwater origin and is not demonstrating the significant deterioration of groundwater qualities and water environment.

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.

Most of the shale gas production in northwest China is from continental shale. Identifying hydrogeochemical and isotopic indicators of toxic hydraulic fracturing flowback fluids (HFFF) has great significance in assessing the safety of drinking water from shallow groundwater and streamwater. Hydrogeochemical and isotopic data for HFFF from the Dameigou shale formations (Cl/Br ratio (1.81 × 10-4-6.52 × 10-4), Ba/Sr (>0.2), δ11B (-10-1‰), and εSWSr (56-65, where εSWSr is the deviation of the 87Sr/86Sr ratio from that of seawater in parts per 104)) were distinct from data for the background saline shallow groundwater and streamwater before fracturing. Mixing models indicated that inorganic elemental signatures (Br/Cl, Ba/Sr) and isotopic fingerprints (δ11B, εSWSr) can be used to distinguish between HFFF and conventional oil-field brine in shallow groundwater and streamwater. These diagnostic indicators were applied to identify potential releases of HFFF into shallow groundwater and streamwater during fracturing, flowback and storage. The monitored time series data for shallow groundwater and streamwater exhibit no clear trends along mixing curves toward the HFFF end member, indicating that there is no detectable release occurring at present.