Chemical composition of shallow groundwater of the Poyang Lake catchment (China)

Poyang Lake, the largest Chinese freshwater lake, is a unique ecosystem which is not only a habitat of rare animal species, but also an important part of economy of Jiangxi Province. Freshwater resources are used for domestic, industrial and agricultural needs. These factors determine the significant anthropogenic load to ecosystem including shallow groundwater. However, data about the chemical composition of shallow groundwater are almost absent. Under the circumstances, comprehensive investigations of the chemical composition of shallow groundwater and its forming conditions are the priority line of scientific research in the region, as it will allow to develop effective methods for water quality assessment and to organize monitoring of the dynamics of groundwater chemical composition changes. In the article hydrogeochemical sampling data of shallow groundwater are used. The total number of samples is 54. Concentration of the main ions (HCO₃, SO₄, Cl, Ca+, Mg+, Na+, NO₃) was determined by titration and ionic chromatography (Dionex-900). Electrical conductivity, temperature and pH were measured in situ. Pearson’s correlation coefficients for pH, Eh, Cl, SO₄, HCO₃, Na+, K+, Mg+, Ca+, NO₃ and TDS were calculated in order to identify functional relations between the main components of the shallow groundwater chemical composition behavior. Analysis of the data showed that shallow groundwater of the Poyang Lake catchment is basically ultra fresh (TDS<0.2 g/L) and moderately fresh (TDS 0.2-0.5 g/L). pH-value changes from acid to weak alkaline; however, the majority of sampling points are characterized by weak acid and neutral medium. Predominant ions in shallow groundwater are Ca+ и HCO₃; there are also quite high concentrations of NO₃, Cl, SO₄, K+, Na+. According to the chemical composition of shallow groundwater and association with territories of different economic and agricultural development degrees, two types of shallow groundwater are allocated: relatively pristine shallow groundwater which is localized mainly within weathering crust and less economic and agricultural development areas and conditionally polluted shallow groundwater localized within agrolandscapes. Relatively pristine shallow groundwater is more acid and has a generally low content of the main ions and, accordingly, TDS value compared with conditionally polluted shallow groundwater. Conditionally polluted shallow groundwater of agrolandscapes is formed under the dominant effect of anthropogenic factors; at the same time the chemical composition of relatively pristine shallow groundwater, in authors’ opinion, generally reflect natural background concentrations for the researched territory. However, in relatively pristine shallow groundwater there is a pointwise scattered anthropogenic pollution which is expressed mainly in quite high concentration of NO₃ and, less often, in quite high concentrations of Cl, SO₄, Na+, K+.

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