Impact of land use change on groundwater quality in a typical karst watershed of southwest China: a case study of the Xiaojiang watershed, Yunnan Province

The impact of land-use change on the quality of groundwater in the Xiaotjiang watershed, China was assessed for the period 1982-2004. Groundwater samples were collected from 30 monitoring points across the watershed, and were representative of the various changes, determined by remote sensing and geographical information systems. The results indicate that 610 km(2) ( 60% of the total watershed area) were subject to land-use change during the period. The most important changes were the conversion of 135 km(2) of forested land to cultivated land, and 211 km(2) of unused land to cultivated land. The main impact was ascribed to diffuse pollution from fertilizers applied to newly cultivated land, and from building development. Overall the groundwater pH value was significantly increased, as were the concentrations of ions NH4+, SO42- , NO3-, NO2-, and Cl- in groundwater whilst the concentrations of Ca2+ and HCO3- declined. More precisely, in the regions where forested land and unused land were converted into cultivated land, the pH value and the concentrations of Mg2+, NH4+, SO42-, NO3-, NO2-, Cl- increased whilst the concentrations of Ca2+ and HCO3- declined. However in the region where cultivated land was converted into construction land, the pH value and the concentrations of Ca2+, Mg2+, NH4+, HCO3-, SO42-, NO3-, NO2-, Cl- increased.