Analysis Of Hydrochemical Characteristics And Formation Mechanism Of Shallow Groundwater In North Ordos Basin

The Ordos Basin is located in the arid and semi-arid region of northwestern China,which is a water deficient area.Groundwater is the main source of residential,agricultural and industrial requirements.Large-scale exploitation of coal resources,frequent agricultural activities,over exploitation of groundwater and other human activities have had a certain impact on groundwater in the northern part of the basin.To understand the chemical characteristics of groundwater and its formation mechanism is especially important for recognizing the interaction between the groundwater and environment and exploiting the water resource rationally.This paper is a branch of “Investigation and Assessment of Groundwater Pollution in Ordos Basin” project of the China Geological Survey.Groundwater samples were collected from 84 active phreatic wells of clastic rock aquifer of Carboniferous-Jurassic and 134 samples were collected from pore-fissure aquifer of the clastic rock of Cretaceous in 2014.In this paper,cluster analysis,discriminant analysis,correlation matrices,descriptive statistics,together with hydrochemical types,ionic ratios,Gibbs plots and some other methods are employed to gain an understanding of the hydrochemical characteristics and processes of two different aquifer system in north of Ordos Basin.Results showed that the groundwater of Carboniferous –Jurassic aquifer system is neutral and weak alkali with medium TDS,the whole groundwater quality in this aquifer is acceptable,but few indicators of some samples exceeds standard.Hydrochemical types mainly belongs to Ca-Mg-HCO₃,which is mainly caused by silicate weathering and less subjected by halite,gypsum,mirabilite or other sulfate rock dissolving.There are evaporation-concentration process and cation exchange and adsorption in water,explaining the reasons of the sources of groundwater chemical compositions.Nitrate pollution is identified in this aquifer system.The groundwater of Cretaceous aquifer system is neutral and weak alkali with higher TDS,the whole groundwater quality in this aquifer is relatively poor,with excessive sulfate,fluoride,chlorine and nitrates.The hydrochemical types of groundwater in this aquifer are complex: Ca·Mg-SO₄·Cl,Ca·Mg-HCO₃,Na-SO₄·Cl,Na-SO₄.Rock-water interaction and evaporation—precipitations are the dominant mechanisms for hydrochemical processes;and there are weathering of silicate,dissolution of sulfate rock,mirabilite and other minerals in water.Nitrate pollution is the major pollution factor in this aquifer system,while some samples’ groundwater which contained with extremely high sulfate may be affected by the oxidation of pyrite,oil and gas drilling or agricultural fertilizer.