滇中红层膏盐溶蚀特征及其对水质的影响——以楚雄谢家河河谷为例

Dissolution characteristics of gypsum salt in the red beds of central Yunnan Province and their effects on groundwater quality:A case study of Xiejiahe valley in Chuxiong City

滇中红层多含有石膏、岩盐等可溶盐矿物,在可溶盐富集地段,地下水往往表现为咸水或微咸水而不适宜饮用。文章以楚雄市谢家河河谷为例,基于钻探、物探、测井、水质分析等综合勘察资料,研究红层膏盐在地下水循环中的溶蚀特征并分析膏盐溶蚀对水质的影响。结果表明:膏盐溶蚀形式主要为散点状膏盐的溶蚀孔洞和裂隙状石膏的溶蚀裂隙,溶蚀强度主要受控于含水介质孔隙发育程度和地下水循环速度;浅层地下水循环速度快,虽可溶盐溶蚀强烈,但对地下水水质影响较弱,水质超标面较小;深层地下水循环速度慢,可溶盐溶蚀后富集于深层承压含水层中,导致地下水水质超标面大;断裂及其影响带导水造成深层承压水向浅层地下水越流,导致浅层地下水水质变差。

Red beds in central Yunnan Province are mainly deposited of continental red clastic rocks.Soluble salts in the red beds are enriched in some sections,generating undrinkable salt water or brackish water.Besides,during water resources exploitation,the improper design and construction of water supply drilling will often lead to serial pollution of groundwater or the formation of inferior wells,affecting the project benefit and safety of water supply.To explore dissolution characteristics of gypsum salt in the red beds of central Yunnan and their effect on groundwater quality,a representative area of Xiejiahe Valley in Lucheng Town of Chuxiong City was selected in this study.Comprehensive survey methods such as geological survey,mapping and drilling were used to analyze lithology,geological structures,aquifers,groundwater types,classification and distribution and dissolution characteristics of gypsum salt in the study area.Based on indicators of sulfur isotope(δ^(34)S),concentration and TDS of groundwater,the groundwater quality was studied and the effect of gypsum salt dissolution on groundwater was analyzed.The Quaternary loose sedimentary layer(Qp^(al)),early Cretaceous Puchanghe formation(K_(1)p)mudstone intercalated with sandstone,and Gaofengsi formation(K_(1)g)sandstone intercalated with mudstone compose the main strata in the study area.Groundwater media types include pore water of loose layer,bedrock fissure water and karst water.Bedrock fissure water is composed of weathering fissure water,interlayer fissure water and structural fissure water.Pore water of loose layer and weathered fissure water constitutes a shallow circulation of groundwater with fast circulation speed.The deep circulation system is mainly composed of interlayer fissure water with pressure and slow circulation speed.The Yi-gan fault(F8),mainly containing vein fissure water,runs through the whole study area.Associated structures developed near monitoring wells lead to water diversion in this section of the fault.There are two types of gypsum salt in red beds of the study area:scattered gypsum salt and fissure gypsum.K5,K7 and K9 boreholes reveal the pseudocrystal of gypsum salt in Puchanghe formation.K1,K2,K6,K8 and K19 boreholes reveal fissure gypsum in Gaofensi formation.Sulfur isotope(δ^(34)S)is positively correlated with SO_(4)^(2-) concentration of borehole water samples,indicating that the high concentration of SO_(4)^(2-) in groundwater is derived from dissolution of gypsum filling in the surrounding rock of aquifer.The plane distribution of fissure gypsum shows that gypsum mainly occurs in the Yi-gan fault and its associated tectonic fractures.Dissolution forms of gypsum and salt are mainly dissolution pores and cracks.Dissolution pores are mainly formed by dissolution of scattered gypsum salt,and dissolution cracks are mainly developed above the underground section of 100m depth.The dissolution intensity of gypsum salt is mainly controlled by the development degree of pores in water-bearing media and the groundwater circulation rate of groundwater.The effect of gypsum salt dissolution on groundwater quality in the study area is significant.A fast speed of shallow fissure groundwater circulation results in basic dissolution of gypsum salt,and soluble salt minerals are strongly dissolved,hence weakly influencing groundwater quality,and contributing to a generally high quality of groundwater.Due to the slow circulation of deep confined groundwater,sulphate is enriched in confined aquifer after gypsum salt dissolution.The concentration of is high and most of groundwater exceeds permitted levels.Deep confined water in the fault and in the water diversion section of the influence belt of fault is transported to shallow groundwater,which results in the pollution of shallow groundwater and poor water quality in the area.Dissolution forms of gypsum salt mainly include dissolution holes of scattered gypsumsalt and dissolution cracks of fissured gypsum,and the dissolution intensity is mainly controlled by development degrees of pores in water-bearing media and groundwater circulation rates of groundwater.Because the speed of shallow fissure groundwater circulation is fast,although soluble salt minerals are strongly dissolved,the influence on groundwater quality is still weak and the area of groundwater exceeding permitted levels is small.Conversely,if the circulation speed of deep fissure groundwater is slow,and soluble salt is enriched in deep confined aquifers after dissolution,resulting in low groundwater quality.Water diversion in the fault and in its influence belt causes deep confined water to flow over to shallow groundwater,which leads to the deterioration of shallow groundwater quality.