Characteristics and Mechanisms Regarding Salinity-Elevating Trend of Groundwater in Funnel Area of Hufu Piedmont Plain in North China

Abstract

The salinity of groundwater in the funnel area of the Hufu piedmont plain has shown an elevating trend over the last 50 years. The chloride and sulfate content have increased dramatically, and local groundwater salinity has significantly exceeded the level III water-quality standards specified by the Quality Standard for Groundwater of the People’s Republic of China. Such phenomena have aroused significant attention among the population. Hence, to reveal the root cause, in this study the evolving characteristics of groundwater level and quality were investigated. The study area was divided into three sub-areas, i.e., the funnel area (Area I), shallow-buried high-value area (Area II), and water-recharging dilution area (Area III). The study time, 1973–2020, was divided into three sub-ranges, i.e., 1973–1980 as the initial stage, 1980–2000 as the middle stage, and 2000–2020 as the final stage. Geomathematical and geostatistical methods, Durov plots, Gibbs diagrams, and the ion ratio coefficient method were used in combination with hydrogeochemical theories to investigate groundwater salinity-elevating characteristics and mechanisms. The results indicated that rock weathering was a key factor contributing to the groundwater salinity-elevating trend in the study area, while human activities also had an influence to some extent. All these factors combined had a multi-dimensional, multivariate, complex influence on the groundwater salinity-elevating trend. In Area I, the groundwater lixiviation increased due to flow-path direction change and burial-depth increase. The groundwater level burial depth in Area II was relatively small, and the hydraulic gradient was relatively large. Hence, Area II was more profoundly influenced by human activities. Area II provided the material source for the salinity-elevating trend of Area I. Area III was the major groundwater-recharging source of the study area. Hence, Area III mitigated the groundwater salinity-elevating trend of Area I through a mixing and diluting effect. However,such an effect was weakened during the middle and final stages due to flow cutoff.