Present state and change of phreatic water quality in Dali County, Shaanxi

A total of 236 samples of phreatic water were collected from Dali County and analyzed, and on the basis of previous data, the authors used the statistical processing and single index evaluation methods to analyze the present situation of phreatic water quality in the study area and its main influence factors and preliminarily discussed the change trend of the phreatic water quality. The results show that: the phreatic water quality in Dali County is generally poor, and the main factors that affect water quality are as follows: the TDS content is too high, there occurs nitrate pollution and sulfate, chloride and fluorine ions exceed the standard limits greatly. Phreatic water covering an area of about 1230.24 km2 (which accounts for 4/5 of the total area) cannot be directly drunk and used for irrigation. The drinkable phreatic water is mainly distributed along both banks of the Luohe River and in the wind-borne sandy area south of the river. it covers an area of 110.06 km2, taking up only 1/15 of the total area. The phreatic water quality south of the Luo River nearly all cannot meet the standards of potable water. Although the phreatic water in the wind-borne sandy area is of good quality and suitable for irrigation, because of the long-term irrational fertilization, large amount of NO3－ is concentrated affecting the phreatic water quality. Comparative analysis of water quality has revealed that in the last thirty-odd years the phreatic water quality in Dali County has showed a trend of turning better; however, because of the hydrogeological conditions, the transformation has been very slow.