Effects of Land Use on Hydrochemistry and Contamination of Karst Groundwater from Nandong Underground River System, China

摘要

The Nandong Underground River System (NURS) is located in Southeast Yunnan Province, China. Groundwater in NURS plays a critical role in socio-economical development of the region. However, with the rapid increase of population in recent years, groundwater quality has degraded greatly. In this study, the analysis of 36 groundwater samples collected from springs in both rain and dry seasons shows significant spatial disparities and slight seasonal variations of major element concentrations in the groundwater. In addition, results from factor analysis indicate that NO₃−, Cl−, SO₄2−, Na+, K+, and EC in the groundwater are mainly from the sources related to human activities while Ca2+, Mg2+, HCO₃−, and pH are primarily controlled by water–rock interactions in karst system with Ca2+ and HCO₃− somewhat from anthropogenic inputs. With the increased anthropogenic contaminations, the groundwater chemistry changes widely from Ca-HCO₃ or Ca (Mg)-HCO₃ type to Ca-Cl (+NO₃) or Ca (Mg)-Cl (+NO₃), and Ca-Cl (+NO₃+SO₄) or Ca (Mg)-Cl (+NO₃+SO₄) type. Concentrations of NO₃−, Cl−, SO₄2−, Na+, and K+ generally show an indistinct grouping with respect to land use types, with very high concentrations observed in the groundwater from residential and agricultural areas. This suggests that those ions are mainly derived from sewage effluents and fertilizers. No specific land use control on the Mg2+ ion distribution is observed, suggesting Mg2+ is originated from natural dissolution of carbonate rocks. The distribution of Ca2+ and HCO₃− does not show any distinct land use control either, except for the samples from residential zones, suggesting the Ca2+ and HCO₃- mainly come from both natural dissolution of carbonate rocks and sewage effluents.