The origin of Na+, Cl- and thermal source of karst groundwater in the stagnant area of Liulin spring basin

Karst groundwater constitutes the major water supply source for industry, agriculture and urban domestic water use for six counties in the Liulin spring basin. The value of the water supply is influenced by high temperature and high concentrations of sodium (as Na+) and chlorine (as Cl-) of karst groundwater in the stagnant area. Thus, the research on the origin of Na+ and Cl- and thermal source is of significance for the understanding of the karst groundwater system, reasonable exploitation and conservation of the water resource. Based on analyses of regional geographic and geomorphic, geologic and hydrogeologic conditions, the spatial distribution and origins of Na+, Cl- and temperature of the karst groundwater are explored using methods of Kriging interpolation, chemical thermodynamics and stoichiometry. The results show that:（1）The temperature and concentrations of Na+ and Cl- of the karst groundwater in the stagnant area increase in the flow direction with a significant gradient of 0.95 ℃/km、140 mg/km and 190 mg/km，respectively, which is higher than those in the recharge, flow-through and discharge areas. （2）In groundwater recharge and flow-through and the discharge area represented by Shangqinglong, Longmenhui, Yangjiagang and Zhaidong spring groups, Na+ and Cl- is mainly from the dissolution of halite. Furthermore, the concentration of Na+ is higher than Cl-, which is perhaps attributed to the processes of ion exchange, dissolution of Na-bearing minerals in loess and contribution from the discharge of urban domestic sewage. （3）In the Liujiageda spring area and the stagnant area, the high concentrations of Na+ and Cl- mainly derive from the dissolution of halite. And the cause of concentration of Cl- higher than Na+ could be the ion exchange between Na+ and Ca2+or Mg2+ because of higher concentration of Na+.（4）The heat sources of the karst groundwater in stagnant area consist of local geothermal gradient, local normal ground temperature, radiogenic heat of rocks and exothermic process via gypsum dissolution; and the contribution of these sources are 45%, 28%, 20% and 7%, respectively.