Study On Hydrochemical Evolution Pattern And Water-filling Conditions In Complicated Carbonate-hosted Deposit

The southwestern China is rich in metallic mineral resources,among which Sichuan-Yunnan-Guizhou area is one of the five supporting production bases for the mining,dressing,and smelting of lead-zinc deposits in China.Maoping Pb-Zn deposit located in Yiliang County,Zhaotong City,Yunnan Province,is a typical representative with high grade,large reserves,long mining history and great mining depth.With the development of deep mining,the groundwater level drops significantly and the mine drainage increases continuously,which has become the main factor restricting the safe development of the mine and also constitutes a typical groundwater system under the influence of human activities.Therefore,it is urgent to systematically and accurately identify water-filling sources and runoff channels in the mining area,which is the basis and key-point of groundwater control in these mining areas and also helps to understand the response and characterization of complex karst water system under the influence of human activities.Maoping Pb-Zn deposit is mainly hosted in the Carboniferous and Devonian carbonate strata,and the Permian limestone is widely distributed in the periphery of the deposit.The three sets of main karst aquifers are separated by the aquicludes composed of the Liangshan Group of lower Permian and the Wanshoushan Group of Lower Carboniferous,respectively.The mining area has a complex structure of karst aquifers and strong regional tectonic activity,and it belongs to a water-filling deposit through karst fissure under complex hydrogeological conditions,whose complex water-filling sources and runoff channels have not been concluded.The mining area is still dominatedly recharged by meteoric precipitation,but the main recharge area and flow direction need to be discussed.The source and cause of the deep low-temperature thermal water are not clear,and it is still doubtful whether the leakage from Luoze River running through the mining area constitutes the main supply source.Hydrogeochemical method is an important and effective tool to recognize the recharge source and runoff channel of karst groundwater system.In this paper,we used a variety of hydrochemical parameters to identify water-filling source according to the hydrochemical sampling and analysis work at different aquifers,different times,and different locations in combination with other hydrological dynamic monitoring data on the basis of the groundwater system theory.Meanwhile some hydrochemical parameters were extended to understand the water-filling conditions better.The characteristics and rules of hydrochemical evolution in the mining area under the influence of intensive mining,and the change of groundwater runoff conditions based on hydrochemical evolution were analyzed and summarized when combined with the historical data.This paper mainly has the following conclusions and understandings:1.The recharge and runoff path of shallow groundwater source in the mining area was summarized on the nitrate distribution.The karst aquifers in the mining area reflected the nitrate pollution of different degrees,which was usually accompanied by the characteristics of low water temperature and heavy isotope values,indicating the shallow recharge from the eastern depression area.The spatial distribution of groundwater nitrate concentration indicated the preferential runoff channels of karst aquifers,and help to reveal the direct hydraulic links via NE-or NNE-trending faults between carboniferous and Devonian karst aquifers.Groundwater nitrate concentration was not observed to increased with depth,but the vertical nitrate pollution in the mining area was still the result of continuous mining.Groundwater nitrate in the mining area showed obvious seasonal characteristics,and the dynamics reflected in each rainfall event which first rose and then fell,were shown as the control of leaching effect.The karst water system in the mining area is dominated by fracture media when combined with other physical and chemical indicators.In general,nitrate component is a good tracer of groundwater recharge,runoff channel and residence time.2.The origin and circulation characteristics of deep groundwater source was understood as a low-temperature geothermal reservoir in the mining area.The hydrogen and oxygen isotope vaules in deep low-temperature thermal water was more negative than that of local springs or cold water,which mainly indicated the atmospheric rainfall recharge in the relatively cooler period under the palaeoclimatic environment,when the general applicable theory of elevation effect was difficult to explain its extremely depleted isotope vaules.The low-temperature thermal groundwater belonged to immature water,which indicated that these cation geothermometers was not applicable,but K-Mg and quartz geothermometers were most suitable for estimating the equilibrium temperature in this kind of geothermal reservoir.The calculated reservoir temperature was 50～65℃,and circulation depth was about 1900m,which meant the formation of low-temperature geothermal water was mainly controlled by the geothermal gradient.The heavier sulfur and carbon isotope values,and their decreased p H and increased HCO3^-and SO4^2-concentration significantly indicated the influence of deep CO2,H2S and other sources related to the metallogenic background,so that carbonate rocks continued to dissolve and sulfide minerals were oxidized to generate groundwater sulfate.The long-term water-rock interaction between deep water and surrounding rock lead to the enrichment of radioactive strontium,and then provided a more suitable mixing model to evaluate the contribution of deep groundwater.In conclusion,the hydrochemical composition of low-temperature thermal water was closely related to the metallogenic background.3.The pattern and degree of surface water leakage to groundwater in the mining area was characterized based on low electrical conductivity.Under the influence of long-term mining activities,the groundwater level continued to decline,and the Luoze River was in a state of losing stream,which constituted an objective condition for groundwater recharge to receive surface water leakage.After the surface water directly infiltrated into the pore aquifer occurred in the riverbed sediments,it mainly ran in the direction parallelling to the Luoze River,according to the immediate and synchronous groundwater level response in the monitoring boreholes in the upper and lower reaches to the riverbed dredging test.Controlled by the lower clay layer of the riverbed sediments,it took several days for pore groundwater to reach a new equilibrium through vertical recharge after riverbed dredging test,and the extreme low eletrical conductivity was generated through cation exchange process.The low EC background indicated the sluggish vertical recharge process between pore groundwater and underlying karst groundwater.The borehole dilution test proved that there was a runoff retardation zone around the karst aquifer under the riverbed,and the groundwater flow rate was estimated by multiple methods at the order of 10^-2 m/d,indicating that it would take several months to disperse and eliminate after the tracer injection.Generally,surface water leakage did not constitute the main water-filling source in the mining area due to the sedimentary structure of the riverbed.4.The changes of water-filling conditions under long-term mining process indicated by hydrochemical evolution of groundwater in the mining area was concluded.The evolution characteristics mainly shown as follows:The mining drainage gradually increased in the mining area,the groundwater level dropped to generater a depression cone,the hydraulic connection between Carboniferous and Devonian karst aquifers was enhanced,and the recharge source and flow direction of Devonian karst aquifer were significantly reshaped.The hydrochemical evolution patterns of Carboniferous and Devonian karst aquifers were different under the influence of mining.The former has a small change in the hydrochemical composition,while the latter had a significant decrease in most hydrochemical components（including sulfate）and salinity,which made the hydrochemical type of SO4 and SO4·HCO3 transform into HCO3 and HCO3·SO4.The oxidation process of sulphide minerals such as pyrite was still one of the main sources of sulphate composition in karst groundwater in the mining area,which was the main reason for the unusuall high SO4^2-composition in the early stage of concentrated mining and the high SO4^2-concentration of southern deep thermal groundwater at present.The water quality improvement of the Devonian karst aquifer was mainly due to the mixing process between the cross-layer recharge from the northern Carboniferous shallow groundwater source and the upward recharge from the southern Devonian deep groundwater source.Hence,the greater the change of groundwater runoff conditions,the greater the change of hydrochemical composition.The change of runoff conditions under long-term mining activities is the main control process of hydrochemical evolution in the mining area.The major advances achieved in this study are given as follows:1.The water-filling sources and its possible contribution in the Maoping Pb-Zn deposit,including the shallow recharge source,the deep low-temperature geothermal groundwater,and possible surface water leakage,was comprehensively discussed based on hydrochemistry and isotope method.The problems related to recharge source was common in these Pb-Zn deposits in the Yunnan-Sichuan area,and this study would provide research reference and practical significance for groundwater prevention and control in these complicated karst groundwater systems.2.The indicator significance of relevant hydrochemical parameters was further expanded based on the analysis of hydrogeological conditions by traditional hydrogeochemical methods,such as the tracing of nitrate components for shallow recharge source,the advantages o strontium isotope mixing model over the stable water mixing model,etc.3.As a typical mining area groundwater system under the influence of intensive human activities,this study combined with historical data analysis of long-term mining activities on the groundwater runoff,and then discussed the hydrochemcial evolution pattern and characteristics controlled by groundwater runoff conditions.The quantitative coupling evolution of hydrodynamic and hydrochemical field under the influnence of human activities should be further discussed.