地下水停采后地面沉降区地下水氟的演化规律: 以沧州市为例

Evolution of groundwater fluoride in land subsidence areas after groundwater cessation:A case study at Cangzhou

南水北调工程极大改善了我国北方的用水问题,进一步减少了深层地下水的开采量,缓解了华北平原地下水长期亏损的情况,但其对区域地下水水化学演化的影响尚未可知。以地面沉降典型分布区沧州市为研究对象,研究南水北调工程通水及地下水停采后对地下水水化学的影响。采集2017年及2021年区域第Ⅲ、Ⅳ层承压地下水样品,探究水化学特征,并通过SBAS-InSAR技术进一步评估区域年均地面形变量,分析地下水停采后区域水质及氟时空变化的影响。研究发现:压缩开采后深层地下水氟含量略微降低,高值区面积减小,高pH、TDS和ρ(HCO_(3)^(-)),低ρ(Ca^(2+))的地下水环境有利于氟的富集;水化学类型没有改变,地下水盐分含量升高,岩盐、萤石溶解更充分;同时,全区地面沉降量及沉降速率较南水北调工程实施前明显放缓,东南部存在小面积地面抬升区。地面沉降的减缓抑制了隔水层黏土压密释水,减弱黏土孔隙高氟水的直接释放,侧向径流补给占比上升,含水层得到的有效补给变多,使得区域地下水中氟浓度降低。但较长的水力停留时间及水岩相互作用,可促使沉积物蒸发岩溶解迁移进入地下水中,使得近海区域深层地下水中盐分含量升高。研究成果对沧州市饮用水安全和水资源管理提供了科学依据。

[Objective]The South-to-North Water Transfer Project(SNWTP)has improved water use in northern China and further reduced deep groundwater extraction in the North China Plain.However,its impact on the evolution of regional groundwater hydrochemistry is still unknown.[Methods]In this paper,Cangzhou,which has experienced severe land subsidence due to groundwater overexploitation and has received water resources from the SNWTP since 2015,was selected as the study area to investigate the effects of the SNWTP on groundwater chemistry.In 2017 and 2021,groundwater samples fromⅢandⅣconfined aquifers were collected to determine the hydrochemical characteristics.Moreover,the average annual land subsidence of the regional layer was further evaluated by using SBAS-InSAR to identify the correlation between variations in land deformation and changes in groundwater chemistry.[Results]The results showed that in comparison with those before the SNWTP,the groundwater fluoride concentration was slightly decreased,and the area of the high value zone was reduced after the SNWTP.The groundwater environment characterized by high pH,TDS and HCO_(3)^(-) concentrations and low Ca^(2+)concentrations favors fluoride enrichment in groundwater.The water chemistry type did not change,and the salinity concentrations in groundwater increased after the SNWTP.Groundwater receives more dissolution of halite and fluorite.Meanwhile,the amount and rate of land subsidence slowed after the SNWTP.A small degree of land uplift was even observed in the southeastern part of the region.The inhibition of land subsidence constrains clay compaction and the release of high fluoride porewater trapped in the clay layer.As a result,aquifersⅢandⅣreceived more effective lateral recharge,thereby causing a slight decrease in groundwater fluoride.However,it needs to be noted that the longer hydraulic residence time and stronger water-rock interaction after the SNWTP interaction promote the dissolution of sediment evaporites into groundwater,leading to an increase in groundwater salinity.[Conclusion]The results of the study provide scientific support for drinking water safety and water resource management in Cangzhou.