Geostatistical analysis of temporal and spatial variations in groundwater levels and quality in the Minqin oasis, Northwest China

Recognition of the temporal and spatial variations in groundwater levels and quality has become a prerequisite of formulating strategies for the sustainable development and utilization of water resources. In this study, data were obtained from 51 observation wells of depth to groundwater from 1999 to 2008 and 30 sampling wells of hydrochemical characteristics of groundwater in the Minqin oasis. The Kolomogorov–Semirnov test revealed that all data followed normal or log-normal distribution. A set of well-structured semivariograms also confirmed that the data had moderate (only for Mg2+ and K+) or strong spatial dependence. Based on spatial distribution, maps drawn using the ordinary Kriging interpolation method in different periods, the declining trend of the groundwater table was found to have been relieved since 2007, with the mean water table dropping 4.65 m from 1999 to 2008. Spatial comparison results further showed that the variations in groundwater levels in Baqu and Quanshan were more evident than those in Huqu, with mean decline rates of 0.64, 0.93, and 0.41 m/year, respectively. The mean value of total dissolved solids (TDS) was 3.34 g/L, and the groundwater in 76.2 % of the study area was brackish (TDS ≥ 1.0 g/L). From south to north, the groundwater types can be classified into three obvious zones based on the major ion distributions: SO42−–HCO3−–Na+–Ca2+ type in Baqu, SO42−–Na+–Ca2+ type in Quanshan, and SO42−–C1−–Na+ type in Huqu. TDS increased from the 1970 to 2005 and gradually decreased thereafter.