Analysis of hydrochemical characteristics and formation mechanism of karst water in Feicheng City

Feicheng City, located in the central part of Shandong Province, is rich in mineral resources and is a famous hometown of peach at home and abroad, and is one of the important industrial areas in Tai’an prefecture-level city. The topography of the study area shows the basic trend of high in east and low in west, and high in north while low in south. It inclines from northeast to southwest with a uplift topography in the middle. In the south, there is the Wenyang Plain, which is characterized by the Dawenkou basin; and in the north the Kanghui plain is characterized by the Feicheng basin. The city is divided into two major regions, i.e. the north and the south parts, which naturally comprises four terrain types as low mountains, hills, plains and waterlogged depressions. As an important industrial area in Tai’an City, the requirement for industrial and agricultural water in recent years has been increasing extremely, which has caused some environmental hydrogeological problems such as excessive groundwater exploitation and deterioration of water quality.This paper takes the chemical composition of groundwater in Feicheng as the research object. It combines the hydrogeological conditions and the historical data of groundwater in the study area, and uses method of statistics for the study of hydrogeology and hydrogeochemistry on the basis of long-term measured data. The water hydrochemistry characteristics of the study area and its temporal and spatial distribution, the formation mechanism of water chemistry and hydrogeochemical processes were discussed. The results show that,(1) The chemical types of groundwater in the study area are mainly HCO3—Ca and HCO3—Ca·Mg.The hydrochemical composition of water changed continuously, and the proportion of SO42- and Cl- in groundwater increased continuously. The main hydrochemistry type has changed to HCO3·SO4－Ca·Mg, accounting for 64.29%,which also contains water types of HCO3·SO4·Cl—Ca·Mg and HCO3·Cl—Ca·Mg. The main anions shifted from HCO3- to SO42- and Cl-, TDS and total hardness showed a significant increase trend.(2) The spatial distribution and variation of the hydrochemical components in the study area are in line with the traditional characteristics from the recharge area to the discharge area through runoff area, and the distribution in discharge area is higher than that of recharge area. TDS is gradually increased from recharge area in northeast part to groundwater runoff zone and discharge zone in the southwest. The areas with poor groundwater quality are mainly distributed in Wangzhuang town and Shiheng town, which are the main industrial parks in the northwest. The water quality in the southern Wenyang plain is relatively good.(3) Through the application of hydrogeological, hydrogeochemical and statistical methods, Gibbs diagram and ion scale coefficient are used to map and analyze the main hydrogeochemical processes that form the groundwater hydrochemical characteristics of the Feicheng City. It shows that the leaching of calcite, dolomite and gypsum is the main factor causing the change of groundwater quality in the study area. At the same time, some hydrogeochemical processes occurred, such as salt rock dissolution and cation alternating adsorption, which makes the concentration of Na+ and K+ decreased and the concentration of Ca2+ and Mg2+ increased. Changes in aquifer structure caused by human activities and industrial wastewater and urban domestic sewage are also important factors in the transformations of groundwater composition.