Evolution characteristics of total dissolved solids in the groundwater level funnel area in the Hufu piedmont plain

In order to reveal the response characteristics of total dissolved solids (TDS) increase to groundwater level decline and the main chemical components that drive TDS to increase in the core area of the groundwater level funnel in the Hufu piedmont plain, geological statistical analysis, time series variation analysis and GIS spatial feature analysis technology are applied to analyze the variation characteristics of groundwater level depth, TDS and chemical compositions in the study area in the past 50 years (1972-2017). The results show that (1) the increase of TDS is characterized by stages along with the decrease of groundwater levels. In the initial stage of the funnel formation (1972-1980), the annual decrease of groundwater levels was the largest, and the annual increase of groundwater TDS was also the largest. In the middle period (1981-2000), the average annual decrease of groundwater level was the lowest, and the average annual increase of groundwater TDS was also the lowest. At the end of the period (2001-2017), the average annual decline of groundwater level was in the middle, and the average annual increase of groundwater TDS was also in the middle. (2) The influence of groundwater level on the increasing TDS presents a decreasing effect with the increase of the depth of groundwater level. For each increase of groundwater level of 1.0 m, the average growth rate of groundwater TDS increased by 21.96 mg/L at the initial stage of the funnel formation (the groundwater level buried depth ranged from 9.10 m to 23.20 m), the average growth rate of groundwater TDS increased by 13.54 mg/L at the middle stage (the groundwater level buried depth ranged from 13.40 m to 42.28 m), and the average growth rate of groundwater TDS increased by 12.32 mg/L at the end stage (the groundwater level buried depth ranged from 21.41 m to 50.29 m). (3) The increase of concentrations of Na⁺ and \rmSO_4^2- in groundwater is the main reason for the increase of TDS in the funnel core area. From the initial stage to the final stage of the funnel formation, with the increasing depth of groundwater level and TDS, the ratio of the increment of Na⁺ and \rmSO_4^2- in groundwater to the increment of TDS in each stage shows an increasing characteristic. However, the total ratio of the increment of Ca²⁺, Mg²⁺, \rmHCO_3^- and Cl⁻ contents in the final stage to the increment of TDS is still 65.88%, which still plays a crucial role in the increasing of groundwater TDS in the funnel core area.