Formation mechanism of metasilicate mineral water in Chengde, Hebei Province: Evidence from rock weathering and water-rock interaction

This paper is the result of the hydrogeological survey engineering.

Objective The orientation of Chengde City's primary function of ecological environment in Beijing-Tianjin-Hebei Urban Agglomeration is a water conservation area due to its abundant water resource endowment.It is of great significance to clarify the occurrence, spatial variation, and formation mechanism of metasilicate mineral water for the sustainable utilization of water resources and the optimization of water conservation.

Methods Multivariate statistical analysis, rock-wreathing lithogeochemical, hydrogeochemical, and multiple isotopic (δD, δ¹⁸O and δ¹³C) approaches were systematically conducted to identify the influencing factors of the enrichment and spatial variation of metasilicic acid in groundwater in the study area under a water-rock interaction perspective of the Earth's Critical Zone.

Results The results showed that the normal temperature water samples with soluble SiO₂ concentrations higher than 30 mg/L accounted for 5.16%, while the average concentration of soluble SiO₂ of geothermal water reaches 61.76 mg/L.The enrichment of soluble SiO₂ in groundwater was controlled by the rock-weathering desiliconization process and water yield property and transmissivity of geological structures, simultaneously restricted by the recharge of weathering medium acidic substances and hydrochemical formation process in aqueous porous media.The higher the weathering sensitivity of water-bearing media, or the higher the content of easily weathered minerals in exposed strata, the higher the concentration of soluble SiO₂ in aquifers tend to obtain.The hypergene silicate rock of the study area was generally in the primary chemical weathering stage that kaolinite, montmorillonite, and illite were formed during the dissolution of feldspar minerals, pyroxene, and other mafic minerals.The mantle-derived CO₂, exogenous sulfuric acid, nitric acid generated by mining, artificial and agricultural activities were demonstrated to be jointly involved in the rock weathering process.The metasilicate mineral water and geothermal springs were mostly occurring or exposed in the composite parts of the structure or the intersection of main and secondary faults.

Conclusions The genetic model of metasilicate mineral water in Chengde City can be generalized into three types: Deep circulation leaching of tectonic faults, shallow circulation leaching of weathered fissures, and interlayer pore and fracture-recharge enrichment burial type.The intersection zones of the piedmont wide-gentle valley and the water-transmitting faults in the basalt, pyroclastic rock, terrigenous clastic rock basin with high vegetation coverage, water-thermal conduction fracture zone of intrusive rock, contact zones between the intrusive rocks and surrounding rocks, carbonate rocks and granite, gneiss or terrigenous clastic rocks turn out to be the potential exploitation areas of metasilicate mineral water in the bulk horizons.