基于水化学及稳定同位素的西藏察雅地下热水成因研究

Formation Model of Geothermal Water in Chaya of Tibet:Perspective from Hydrochemistry and Stable Isotopes

西藏察雅分布有两处地下热水,其中娘曲热水流量达23356 m3/d,温度达36℃,掌握其成因以及地下水循环模式对铁路隧道的规划建设具有重要意义。为查明地下热水水化学特征及其成因模式,采用同位素水文地球化学方法进行研究。结果表明:两处地下热水主要阳离子为Ca^(2+)和Mg^(2+),主要阴离子为SO_(4)^(2-)和HCO3^(-),溶解性总固体含量为1255~2051mg/L,水化学类型分别为SO_(4)·HCO3^(-)Ca·Mg型和SO_(4)-Ca·Mg型。氢氧同位素分析结果表明,地下热水补给来源主要为大气降水,并具有18O漂移现象,反映了热水与围岩的氧同位素交换效应。地下热水的补给高程为4146~4185 m,热储温度为53.1~61.0℃,循环深度为1409~2020 m。其成因模式为:地下水在东北部高山区接收大气降水入渗补给,沿岩溶裂隙管道径流,经深循环获得大地热流加热,受构造及岩层阻水影响沿断层上升,在上升过程中与份额达0.79~0.91的浅层地下水混合,于沟谷等地势切割处出露成泉。综合水文地质条件与隧道位置分析,隧道穿越的两处岩溶富水条带,东部岩溶富水区对隧道突涌水威胁较小;西部岩溶富水区对隧道存在构造岩溶水高压突涌水风险,后期应注意防范。

Two occurrences of geothermal waters have been identified in Chaya of Tibet,and the one at Niangqu has a flow rate of 23,356 m^(3)/d and a temperature of 36℃.Understanding its hydrochemical and isotopic characteristics and groundwater circulation model is of great significance to tunnel design and construction.To clarify its hydrochemical characteristics and formation model,we analyzed the isotope hydrochemistry of water samples from these two geothermal water occurrences.The two geothermal waters contain main cations of Ca^(2+)and Mg^(2+),main anions of SO_(4)^(2-) and HCO3^(-).They are classified into SO_(4)-Ca·Mg and SO4·HCO3-Ca·Mg water-type,and have TDS content of 1,255 to 2,051 mg/L.Stable hydrogen and oxygen isotopes indicate that the main source of geothermal waters is meteoric precipitation,as featured by a 18O drift,reflecting O-isotope exchange between hot water and the wall rocks.Elevation of the recharge area is of 4,146 to 4,185 m,the temperature of the geothermal reservoir is about 53.1 to 61.0℃,and the water circulation depth is approximately 1,409 to 2,020 m.Formation model of the geothermal water is proposed as follows:after receiving recharge from meteoric water infiltration in high mountainous area from the northeast,groundwater may have flowed along the fractures between the karst layers and was heated by the geothermal heat flow through deep circulation.The water may have risen along faults due to the aquitard structure,mixed with 0.79 to 0.91 shallow groundwater,and formed springs in valley topographic incision.Integrating hydrogeology and tunnel location,the eastern karst-rich water area poses less threat on tunnel water incursion.The western karst-rich water area has tectonic karst water for the tunnel,and the risk of high-pressure water incursion should benoticed.