Spectral characteristics and indication of dissolved organic matter in the karst water system of Jinan
Dissolved organic matter(DOM)plays an important role in the biochemical cycle of karst water systems.In this paper,the karst water system of Jinan spring,a typical karst area in North China as well as an important source of drinking water in Jinan,was selected as the research area.In the southern part of the area,the metamorphic rocks of the Taishan Group of Archean Eonothem are exposed.The carbonate rock strata of the Cambrian and Ordovician,which incline toward the north with monoclinic structures and are hidden under the Quaternary in the north part,are located above the metamorphic rocks.The regional groundwater flows from south to north under the control of topography.The main aquifers in the sampling area of this study are the middle Cambrian Zhangxia Formation,the upper Cambrian Fengshan Formation and the Ordovician aquifers,which contain carbonate fissure water with lithology mainly composed of limestone,dolomite limestone and argillaceous limestone.According to the hydrogeological conditions in the source area and the formation process of the source,the study area was successively divided into the indirect recharge area(IRA),direct recharge area(DRA)and discharge area(DA)from south to north.The direct recharge area can directly be recharged by surface water leakage.In the indirect recharge area,recharged by atmospheric precipitation,surface water and groundwater flows into the direct recharge area in the form of surface and subsurface runoffs.Groundwater was blocked by magmatic rocks in the north,and fissures were developed in the contact zone between aquifers and the magmatic rocks,where karst groundwater rose and was exposed,and springs came into being.This study aims to investigate the spectral characteristics,spatial distribution and indicator of DOM in karst groundwater in recharge,runoff and discharge areas at a spring-area scale.The composition,source and spatial distribution in the karst water system of Jinan spring were analyzed by absorption spectrum,fluorescence spectrum and PARAFAC,combined with water chemical index and correlation analysis.In this study,three main fluorescence components have been identified in the Jinan spring area,namely,Cl,C2 and C3,which are humus,protein-like tryptophan and protein-like tyrosine,respectively.Groundwater in the indirect recharge area is dominated by humic substances in low molecular weight,while groundwater in the direct recharge area and the confluence and discharge area are dominated by protein-like substances.In these two areas,the change in protein-like substances caused by microbial activities is the main driving force for the change in geochemical characteristics of DOM.The order of endogenous contributions is DA＞DRA＞IRA,which is consistent with the order of water circulation quantity in the karst water system.In this karst water system,aromatic substances are mainly present in hydrophobic components.The humus component C1 is an exogenous input,and is significantly positively correlated with the protein-like tryptophan component C2.These two components from different sources are subject to the same factors.The protein-like tyrosine component C3 has no obvious correlation with all indices.The humus component C1 and the total fluorescence intensity can be used as natural indicators to trace the karst water cycle.Besides,the protein-like tryptophan C2 and the protein-like tyrosine C3 may provide important biogeochemical information to evaluate the vulnerability of karst aquifers.
dissolved organic matterspectral characteristicsspatial distributionfluorescent componentthe Jinan spring area
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