Groundwater environment variation in the Minqin Oasis

According to the records of 51 observation wells of groundwater table in the Minqin oasis from 1999 to 2008,we investigated temporal and spatial variation of groundwater table.Samples of 30 active water supply boreholes were also collected in 2010 to understand the hydrochemical properties of groundwater.Statistical results indicated that the groundwater table had declined over the past 10 years,with an average decline rate of 0.52 m per year.Under the influences of Yellow River diversion project,water discharge engineering,the decline rate per year of groundwater table was decreased in 2005,2007 and 2008.The decline rates per year of depth to groundwater in Baqu and Quanshan was higher than that of in Huqu,with increments of 0.23m/y and 0.52m/y,respectively.The average TDS(total dissolved solids) in the oasis was 3.34g/L.The hydrochemical types of groundwater were changed along the groundwater flow direction.The dominant anion species of water changed systematically from SO42-HCO3-Na+ or SO42-HCO3-Ca2+ to SO42-C1-Na+-Mg2+ from Baqu and Quanshan to Huqu.