Evolution Of Niangziguan Karst Water System Under The Impact Of Coal Mining Activity

Karst groundwater has always been the primary water-supply resource in the karst area.Affected by the industrialization and global climate change in recent years,the need for groundwater is increasing significantly more than ever before,especially in the northern China where carbonate rocks spread widely and surface water resources are extremely not well-distributed.The severe contamination in groundwater makes it critically important to protect an explore karst water in a reasonable way.Niangziguan spring is one of the biggest karst springs in the northern China,as well as water-supply resource for industry and agriculture development and civic need.This resource is critical for the sustainable development of local society.However,since 1980 s,with the booming of economic,the groundwater table declined sustainably owing to the periodical change of rainfall amount,the overexploitation of groundwater,and mining activities.More seriously,under the impacts of climate change and anthropogenic factors,the water quality of karst groundwater had a significant contamination phenomenon with excessive hardness and sulfate,as well as other toxic constituents which threaten the ecological environment.Acid mine drainage in the study area has significant impacts on both surface and groundwater quality,which is characterized by?1?low pH value,?2?high concentration of sulfate,?3?high hardness,?4?high total dissolved solids,?5?high concentration of iron,and?6?high contents of toxic matters.Most of mine waters possess low pH value,showing acid feature.This is because there are high contents of sulfates in coal strata.Under natural condition,karst groundwater aquifers are separated from coal strata.When mining activities broke the confining bed,sulfate-containing coal makes contact with groundwater,and the oxidation process improves the rate to form acid water.This thesis attempts to study the hydrogeochemical evolution characteristics of Niangziguan karst groundwater under the influence of mining activities by methodology of hydrogechemical analysis,isotope charateristics,and core-flooding experiment.The research contents of this thesis are summarized as follows:1.Hydrochemistry and coal mining activity induced karst water quality degradation in the Niangziguan karst water systemHydrogeochemical,statistical analysis and geochemical modeling were employed to evaluate the impacts of coal mining activities on karst water chemistry in Niangziguan springs catchment,one of the largest karst springs in northern China.Significant water quality deterioration was observed along the flow path,evidenced from the increasing sulfate,nitrate and TDS content in karst water.Karst water samples are Ca-Mg-HCO3 type in the recharge areas,Ca-Mg-HCO3-SO4 type in the coal mining areas,and Ca-Mg-SO4-HCO3/HCO3-SO4 type in the rural areas and discharge areas.A four-factor principal component analysis?PCA?model is conducted which explains over 82.9% of the total variation.Factor one,explained the largest portion?45.33 %?of the total variance,reveals that coal mining activities and natural water-rock interaction as the primary factors controlling karst water quality.Anthropogenic effects were recognized as the secondary factor with high positive loadings for NO3-and Cl-in the model.The other two factors are co-precipitation remove of trace elements and silicate mineral dissolution,which explained 20.96% of the total variance.A two end mixing modeling was proposed to estimate the percentage of coal waste water giving on karst water chemistry,based on the groundwater sulfate chemistry constrains rather than sulfur isotopes which could lead to an overestimation of coal mining water contribution.According to the results of the modeling,the contribution of coal mining waste on karst water chemistry was quantified to be from 27.05 to 1.11% which is ca.three times lower than the values suggested by sulfur isotope method.2.Constrains of D,18 O,⁸⁷Sr/⁸⁶ Sr and Sr/Ca ratio on the sources of solutes in Niangziguan karst groundwater systemWater samples were taken in the Niangziguan spring catchment for major ion hydrochemcial and stable isotopic(D,18 O,and ⁸⁷Sr/⁸⁶Sr)analysis.The combined use of major ion hydrochemistry and stable isotopes is to identify?1?the sources of chemical solutes in both groundwater and surface water,?2?the hydrogeochemical processes that karst water might go through,and?3?mining activities influence on karst groundwater.In terms of karst water,fissured water and spring water,their hydrochemical types tend to evolve from Ca-HCO3 to Ca-SO4 closing to that of surface and mine water,indicating a mixing process in the flow path from recharge area to coal mining area.The linear relationship between Ca and Sr suggests a possible resource for strontium in most of the water samples through carbonate dissolution.Two end members could be defined according to ratio of in water samples.End member MW3 is characterized as high ratio of both Sr/Ca and Sr/Mg,while end member RW1 as remarkably low Sr/Ca ratio.Samples such as KW8 and Sf8 which are plotted close to MW3 could be explained by mixing with mine water,and those close to RW1,such as Sf6,KW17 and FW2,could attribute to the input of meteoric water.The stable oxygen and hydrogen isotopic compositions of karst groundwater samples show a wide range of ?18O?from-9.93 to-6.85?and ?D?from-69 to-50.2?.As discharge of groundwater,with ?18O ranging from-9.67 to-8.96 and ?D from-70.5 to-66,the isotopic compositions of spring waters are much depleted as compared to karst waters,which is close to that of rain water.According to the allocation of 18 O and D,most of the water samples are scattered along the global meteoric water line?GMWL,?D=8?18O+10?,indicating a meteoric source with no exception.The large variability and light deviation of isotopic composition in karst groundwater samples reflect the occurrence of isotopic fractionation impacted evaporation or mixing with other evaporated sources.The high ⁸⁷Sr/⁸⁶ Sr ratio and low Sr/Na ratio confirm that a few karst water and mine water samples such as KW21 and MW3 are derived from dissolution of silicate,while low ⁸⁷Sr/⁸⁶ Sr ratio and high Sr/Na ratio reveal a carbonate source for karst water like KW8.Since the co-impact of both rain water and mine water to karst system,a ternary mixing model could be established.Karst groundwater samples are located in the area where the mixing ratio of mine water is lower than 10%,indicating a minor impact by mining activities.The most significant mixing with mine water is KW8 whose mixing ratio is nearly up to 20%.3.Hydrogeochemical evolution and changes in pore size distribution during a core-flooding experiment with lab-made acid mine drainage?AMD?and carbonatesCarbonate dissolution controls the major constitutions of groundwater in the Niangziguan karst system.Mineral dissolution or precipitation processes caused by the flow of a reactive fluid through a porous medium often produce volume and surface variations at the pore level that change the flow and transport properties at the macroscale,such as saturated and relative permeability,porosity,reactive surface area,capillary pressure curve and pore size distribution.The increase of Ca,Mg and alkalinity in all the 3 core-flooding experiments is relative to the dissolution of dolomite and calcite.A slight elevation of concentration for Na+ was obtained only in experiment for RD2,together with the presence of Si in the outlet solution,which suggests a possible source of albite.It is worth noting that Fe3+ in S1 and Fe2+ in S2 declined down close to detection limit after percolation,which indicates the precipitation of iron hydroxides.The peak-shape curve of iron concentration suggests that wormhole might occur over 13-day's percolation,leading to the reduction of resident time in core RD2.However,the preferential flow channel did not maintain,owing to the precipitation of iron hydroxides and blocking of the channel.This is coincident with the drop of iron concentrations during the last 5 days.In the coarse dolomite core RD1,attacked by Fe3+-rich solution S1 at pH 3.27,the total porosity declined from 3.1% to 1.7%,permeability declined slightly from 0.7751×10-3 ?m2 to 0.7469×10-3 ?m2 over 15-day's percolation.In the dolomite core RD2,attacked by Fe2+-rich solution S2 at pH 2.3,the total porosity declined from 4% to 2.7%,permeability reduced from 0.⁸⁶51×10-3 ?m2 to 0.8471×10-3 ?m2 over the 20-day's percolation.This indicates that some minerals might precipitate at the surface of pore wall and pile up to reduce the effective reactive surface area while lab-made AMD made contact with dolomite core samples.Additionally,we conducted N2 Gas Adsorption for dolomite cores before and after percolation experiment.The isotherms are a combination of type I and IV with two distinct regions.At low relative pressure,the isotherms exhibit high adsorption,an indication that the rocks contain micropores?type I?and at high relative pressures?P/P0 > 0.45?the curve exhibits a hysteresis loop indicating the multilayer range associated with capillary condensation in mesopores?type IV?,however,with unlimited adsorption at large relative pressures.The inferred processes involving with the distribution of pore size in carbonates are?1?acid fluid attacking carbonate rock could stimulate the micro-fracturing,further to increase the amount mesopore and?2?nanoparticles of ferric hydroxides formed owing to elevated pH could precipitate at the pore surface and shrink the average pore size.