Changes and driving factors of groundwater environment in Lanzhou since 1998

Based on groundwater pollution survey data, groundwater monitoring data and remote sensing data, the authors analyzed the groundwater environment change trend in different areas of Lanzhou City since the 1990s (1998-2013). The results show that, since 1998, the main groundwater environment change in Lanzhou has been characterized by deterioration of groundwater quality and rising of groundwater level. Typical hydrochemical indexes such as TDS, TH, nitrate and fluorine have different trends in different areas of Lanzhou, and most of them show a steady upward trend. Nitric acid groundwater has appeared in some areas. The increase of urban pollution load is an important driving factor for the rise of typical hydrochemical indicators, and there is a direct relationship between the characteristic indicators and the main functions of different urban areas. Chengguan District is the main residential and commercial area, with the typical index of water quality deterioration being nitrate. Xigu District is a concentrated area of heavy industries such as chemical industry and smelting, with the highest degree of groundwater pollution, and the increase of fluoride suggests that industrial pollution is significant. Qilihe District and Anning District are comprehensive distribution areas of commerce, residence and light industry, and TDS, TH, nitrate and other indicators have all risen in varying degrees, but the increase is not significant. The rise of groundwater level in Xigu and Santan area is more obvious, with that in other areas being relatively stable. Urban impervious interface hinders the vertical infiltration of rainfall, irrigation and other surface water, making the annual fluctuation of groundwater level gentle. Due to the reduction of groundwater exploitation, increasing construction along the Yellow river and the leakage of underground pipe network, the condition of groundwater recharge and discharge is changing. It not only causes the rise of groundwater level in the urban area, but also enhances the leaching of pollutants and the evaporation and concentration of groundwater, which has a certain impact on the hydrochemical environment.