Geochemical Characteristics And Evolution Mechanism Of Thermal And Mineral Springs In Arxan, North Eastern China

The study area is located in Arxan,Hinggan League,Inner Mongolia.Effected by the new huaxia intersecting fracture.Thirty-six springs are located within 4 km²with totally different output temperatures,the previous researchesof the genesis mechanism are not deep enough.Also,economic development and groundwater over-exploitationhave resulted in the waste of geothermal resources and declines of temperature as well.Thus,it is necessary to get insight into the formation mechanism and circulation characteristics of groundwater sytem to make up for the gaps in researchand improve the sustainable development of Arxan springs.This study is based on a group of cold and hot springs exposed in the urban area of Arxan.The hydrogeological conditions and geothermal-geological background of the study area were analyzed using geological investigation and geophysical prospecting soil radon method.The genesis mechanism of Arxan springs and the formation mechanism of geothermal system were obtained by comprehensive application of hydrogeochemistry,geochemistry analysis,isotopic method and groundwater numerical simulation,etc.A numerical model was eastablished to quantify the variation of groundwater thermal field under natural and anthropogenic conditions,along wih the relationship between outlet temperature and groundwater exploitation.The results can provice theoretical support to draw up a reasonable plan for water resources utilization and the sustainable development of geothermal resources in the study area.The results show that the surrounding rocks are a set of potash-high-K calc-alkaline volcanic rocks,which originated from the crust.The area with high Rn values constitutes a feathered facture area?50×30m?and provides conditions for the infiltration of surface water and shallow groundwater.Under the influence of lithology and geological structure,the cold springs in the southern part are low salinity neutral soft waterwith output temperature ranging from 6.5to 13?,while springs located in the northern part are low salinity weak alkaline soft water with output temperatue of 13.3⁴8?.The water type changes from HCO₃-Na·Ca in the southern part to HCO₃-Na in the northern area.The spring water in the study area is of meteoric origin and the hot springsalong with the cold springs have similar supply environment.The thermal reservoirs in the spring water are of the same type as Jurassic tuff.The age of cold springs and shallow groundwater is around 10yearsand the renewal circulation rate is about 2.5-5.3%/a.The residence time of hot springsis70-90 years and the renewal circulation rate is about 0.23-1.95%/a.The reservoir temperatures of hot springs is calculated using different geothermal temperature methods.Combined with the mineral-fluid chemical equilibrium judgment and SiO₂ mineral saturation index calculation,the range of reservoir temperature is estimated as60-90?,the mixture ratio is between55%and89%.Based on the simulation results of the mineral transfer in the groundwater system,it is concluded that the cold springs and the hot springs are in different circulation systems,and the main control factors of the water-rock reactions from the recharge zone to the discharge zone are different.Using Feflow software,the groundwater profile model of groundwater coupled with the typical section of the study area is established.The results show that the error between simulated temperature and measured temperature is less than 5%and the fitting effect is good.According to the variation of water table,distribution and topography of the study area,groundwater flow system is divided into three system.The temperature of hot springs under natural conditions has experienced a long time of geological history and the hydrothermal conditions have reached a state of dynamic equilibrium.The exposed temperature of hot springs fluctuates around 48°C.While the human activities will affect equilibrium state of hydrothermal field.Under the condition of mining for 50 years,the temperature of the hottest spring has dropped by 4.1°C.Considering the prediction results of the water-heat coupling model and the status of economic development,it is recommended that the reasonable range of hot water extraction is 480⁶40 m³/d.At the same time,it is necessary to carry out water-recycling system and establish a long-term monitoring network system for geothermal water,in order to provide a basis for the sustainable development and utilization of spring resources in the study area.