Analysis Of Groundwater Flow System And Modeling Of Hydrogeochemical Evolution In Xinzhou Basin, China

The complexity and concealment of groundwater system make it difficult to fully understandgroundwater system. For a lone time, people have tried to use various methods to understandgroundwater system. But the only part information of the groundwater system can be obtained.Groundwater is the receptor and information carder in the environmental change. Based on thetheory of groundwater system, distilling and assimilating the isolated and scattered informationcan reduce the uncertainty, and can help us to accurately understand groundwater system in theoriginal. This study takes the porous groundwater in the whole Quartemary of Xinzhou basin as anexample to realize and research groundwater system in the integrated and systemic view,combining multitechniques with the traditional hydrogeological analysis to quantitatively distilland assimilate the scattered data.Xinzhou basin is a representative Cenozoic rift basin, located in the northernmost of FenheRiver rift valley in Shanxi Province, and belongs to the arid-semiarid region. Groundwater is theforemost source for water supply. With the double-quick development of the social economy andthe population augment since 1980s, the industrial and agricultural activity and mining made thewater resources requirement increased dramatically. Those have been profoundly changing thenatural environment of groundwater and regional water cycle condition. Groundwater resourcequantity, which can be utilized by human, have reduced by the incessant decline of thegroundwater level and the increasing deterioration of the groundwater quality in the basinQuaternary System. The industrial and agricultural developments in this region have seriouslyinfluenced by those phenomena, and some water resources and environmental problems occur. Theexploitation and utilization of water resources have been increasingly in conflict with theproduction practice due to the unreasonable water resource management and lack of environmentalprotection consciousness. So, it is necessary to research groundwater environmental evolutionunder the interaction between human activities and natural conditions. Most of the formerinvestigation and research in Xinzhou basin have been in the state of qualitative delineation togroundwater system using traditionally analytical hydrogeology methods, and formed differentcomprehensions about the rank demarcation of groundwater system. This research has reknowngroundwater flow system of Xinzhou basin in the view of systematic analysis.The study resultscan help us to correctly understand the three-dimensional hydrochemical field and explain thecomplex phenomena of regional hydrochemical characteristics. These will have certain theoreticalsignificance for revealing the source of groundwater environmental problems, and promoting the development of groundwater flow system theory. It will bring about the practical significance forrealizing the spatial-temporal evolution of groundwater resource and water cycle, and also for thedemonstration and decision-making of water resources development and utilization, and thesustainable use of water resources.Under the guidance of system theory, the porous groundwater in the whole QuarternarySystem of Xinzhou basin can be regarded as the research object to carry through the systemanalysis in this study. Groundwater flow model has been set up by distilling and analyzingmulti-information about the constitutes and structure of the water-bearing medium, boundaryconditions, and groundwater recharge, discharge, hydrodynamics and water balance. Somequalitative or semi-quantitative characteristics have been obtained by systematically analyzing thegroundwater flow field and hydrochemical field, including hydrogeochemical components andisotopes constitutes. The study shows that some hydrogeochemical characteristics can indicategroundwater flow system in the basin and spatial evolutionary rule of the groundwater flow field.Based on multi-information and multi-technique, such as groundwater level trends analysis, thestable isotopes analysis, radioactive isotopic age determination and hydrogeochemistry modeling,we distinguished and validated the groundwater numerical modeling and obtained somequantitative or semi-quantitative results, such as the recharge source of groundwater, rechargepattern, flow velocity, transit way, discharge modes and main hydrogeochemical action.Some major opinions and conclusions obtained in this research are as follows:(1) Groundwater system in Xinzhou basin is the nearly closed one with the relativelycomplex inner structure. According to the tectonic bottom boundary and potential difference, thebasin can be divided into six secondary tectonic units, namely Fanshi depression, Daixiandepression, Yuanping depression, Qicun rupture terrace, Jinyin upheaval and Xinding depression.In the regional scale, the groundwater system of Xinzhou basin was marked off for three ranks.(2) The main water-bearing medium of Quaternary porous-water system in Xinzhou basinare alluvium and proluvium, including coarse sand, middle and coarse sand, middle and fine sand,mealy sand, sabulous sand, mild clay, and clay. The models of stratum structure and lithologystructure of the Quaternary stratum in Xinzhou basin were set up by using TINs code and SOLIDcode in GMS5.0 software to realize three-dimension visualization.(3) According to the analysis of bedrock lithology in the mountain area around the basin, thehydrodynamic permeability of the strata combination of both planes of mountain front fault, wedeemed that porous water-bearing system of the basin gain the lateral recharge from the ambientKarst-water system and fracture water-bearing system. As a whole, groundwater in Xinzhou basingathers from the slant plain in the mountain front zone to the central alluvial plain, and flows alongthe modern valley from the upper to lower reaches of Hutuo River. The recharge source ofgroundwater includes atmospheric precipitation, lateral subsurface runoff from ambient mountainarea around the basin, leakage of rivers and reservoirs, irrigation percolation and channels leakage.The discharge way mainly includes evapotranspiration, surface base flow, springs and groundwaterexploitation.(4) The variation range of the depth of groundwater level in the basin is from more than 20meters in the hilly ground to less than 5 meters in the river valley zone. The dynamic type of the shallow groundwater belongs to the infiltration-runoff type in the upper piedmont plain, theinfiltration-runoff-exploitation type in the lower piedmont plain and most alluvial-lake plain, andthe comprehensive type in the both sides of the modern riverbed of Hutuo river and the localsection of the alluvial transition zone. But for the middle groundwater, the dynamic type is theinfiltration-runoff-exploitation type in the lower piedmont plain, and the subsurface runoff type inthe most area of the basin.(5) The results of the calculation of the water balance during 2004 in Xinzhou basin showthat the total recharge of groundwater is 441.04x10⁶m³/a, mainly including the rainfall infiltrationand lateral influx, about 52.06% and 26.52% respectively of the total recharge. Among the otherrecharges, leakage of rivers and reserviors, channels leakage and seepage irrigation return accountfor 12.36%, 3.92% and 5.15%, respectively. And the total discharge of groundwater is449.18x10⁶m³/a, of which about 56.01% is artificial exploitation. Other discharge items, such assurface base flow and evaporation of phreatic water account for 25.12% and 18.85% respectively.As a whole, balance error is-1.78%, and the mean annual discharge is more than the meanrecharge in Xinzhou basin.(6) Through groundwater flow numerical modeling, we can attain some characteristics on thetypical groundwater flow cross-sections. According to the output of the actual velocity ofgroundwater, the velocity range of＞0.7m/d, 0.5～0.7m/d, 0.2～0.5m/d, 0.05～0.2m/d and＜0.05m/dare defined as supper-strong runoff zone, strong runoff zone, general runoff zone, weak runoffzone and supper-weak runoff zone respectively. The average flow velocity in the middlegroundwater is lower than that in the shallow groundwater. For the shallow groundwater, thevariation range of the average residence time, namely the time that groundwater flow needs alongthe pathline through central node of one cell (1×1km²) , is 1.87～1361.95a. But for the middlegroundwater, the variation range of the average residence time is 3.44～1440.94a. Generally, theaverage residence time in the middle of groundwater is longer than that in the shallow groundwater,and its range is from 1.57 to 78.99a.(7) Integrating groundwater flow field with the characteristics and hydrochemical field todemarcate the groundwater flow system of Xinzhou basin, we found that there exists the obviousranks in the space of the loose porous groundwater flow system. The study result of typicalprofiles of groundwater flow system show that there are local flow system and middle flow systemin the proluvial fan of Yangwu river, and there are local flow system, middle flow system andregional flow system in Daying depression and Qicun rupture terrace.(8) The hydrochemical distribution characteristics, in the different groundwater flow systemsof the basin, can be obtained from the statistical analysis to the hydrochemical index of the watersamples from the different water body in Xinzhou basin. These characteristics include that thespatial distribution of the water type, total dissolved solid (TDS), and the concentration of majorhydrochemical components (such as Ca²⁺, Mg²⁺, Na⁺, SO₄^2-, HCO₃^- Cl^- and NO₃^-), alsoinclude the relationship between groundwater flow field and hydrochemical components. Inaddition, the indicative action of special ion (such as Cl^-) and the different ion concentration ratio,such as rCa/rNa, rMg/rNa, rCa/rCl, rNa/rCl, rSO₄/rCl and rHCO₃/rCl, can be used to judge thespatial hydrogeochemical evolution action of groundwater system, and indicative significance to groundwater flow field. In the lower reaches of the basin, the flow velocity of groundwater isususally slow, and the hydrochemical characteristics can represent the features of the dischargearea in the local flow system or the middle flow system, even the regional flow system.(9) TheδD andδ¹⁸O values for the normal groundwater, thermal groundwater and surfacewater in Xinzhou basin plot in close proximity along Taiyuan meteoric water line. This indicatedthat the recharge of groundwater in this basin is mainly controlled by the atmospheric precipitation.In this study, we obtained the precipitation elevation equation of the study area and calculated therange of recharge elevation of water samples. The concentration of Na⁺ is in direct proportion tothat of Cl^- and in inverse proportion to that of HCO₃^- when the recharge elevation is more than1300m. This also shows that the transit distance of the thermal water is longer than the coldgroundwater. In addition, the content of Ca²⁺, SO₄^2- and TDS increase by the different rate of slopeas the change of the recharge elevation. And it reveals that the cold groundwater and the thermalwater are from the different groundwater flow systems. This point can be identified fromgroundwater system of Daying depression and Qicun rupture terrace, and shows that there are theregional groundwater flow systems indicated by the hydrogeochemical characteristics of thermalwater.(10) The ⁸⁷Sr/⁸⁶Sr ratio of surface water in Xinzhou basin is relatively low, and its range is0.7125～0.7165. There is little change in the ⁸⁷Sr/⁸⁶Sr ratio of the shallow groundwater with therange from 0.7171 to 0.7219. The ⁸⁷Sr/⁸⁶Sr ratio of the middle groundwater is changed from0.7097 to 0.7307. The ⁸⁷Sr/⁸⁶Sr ratio of the thermal water is up to 0.7495 and shows that thethermal waters participate in the regional groundwater circulation with the long residence time inthe thermal storage of deep fault fissures. The correlation analysis to the ⁸⁷Sr/⁸⁶Sr ratio, the contentof Cl^- and SO₄^2- ion shows that SO₄²- and the ⁸⁷Sr/⁸⁶Sr ratiodisplay good information ofgroundwater flow, namely with the positive correlation to Cl^-. By contrasting the range of the⁸⁷Sr/⁸⁶Sr ratio in the different water samples with that in the different lithology, the researchindicates that the solute in the surface water and groundwater mainly from the weathering ordissolving of silicate rocks, and aluminum silicate rock, while there is little impact on thehydrochemical composition of groundwater caused by the dissolved carbonate minerals.(11) Based on the typical groundwater flow system profiles of the Daying and the proluvialfan of Yangwu river, combining the hydrochemical analysis of the representative water sampleswith the result of groundwater flow modeling, the reaction path can be confirmed by flow path,flow velocity and residence time. The "possible mineral phase" can also be confirmed according tothe composition of lithology minerals in the aquifers and its geochemical features. Forwardhydrogeochemistry modeling can be implemented by using PHREEQC2.11 software underone-dimensional invariable velocity. The modeling results show that the flow velocity in theproluvial fan of Yangwu river is faster than that in Daying piedmont plain, and reveal themechanism of hydrochemistry in the different levels in the different groundwater flow system.Shallow groundwater is influenced by the evaporation and concentrated role, and the dissolution ofsoil salts can be used to characterize this impact. The calculated quantitative results fromhydrogeochemistry modeling accord with the measured results. This shows that the majorgeochemical actions, which control hydrogeochemical evolution along groundwater flow path, include CO₂ gaseous escape, the dissolution of gypsum, calcite, dolomite, albite, plagioclase, haliteand fluorite, precipitation of kaolinite and illite, and cation exchange reaction.The major advances achieved in this thesis are as follows: (1) The porous groundwater in thewhole Quarternary System can be regarded as an integrated groundwater flow system and bedemarcated by analyzing groundwater in the whole Xinzhou basin. (2) Based on the frame ofgroundwater system theory, groundwater flow model has been built up by distilling and analyzingscattered and single multi-information about bearing water medium, flow field, hydrogeochemistry(include isotope) and temperature field. (3) A suit of integrated methods for calibratinggroundwater flow numerical modeling have been applied in this thesis, such as contrasting waterlevel contour maps in different depth, fitting water head trends-curves on typical observation well,estimating the average practical velocity of groundwater flow by ¹⁴C age, judging groundwatertransit distance by Cl^- and ⁸⁷Sr/⁸⁶Sr. (4) Coupled evolutionary characteristics have been discussedby groundwater flow modeling and hydrogeochemical reaction-transport modeling on the typicalgroundwater system profiles.