Water Geochemistry Of The Huguangyan Maar Lake: Insights From Dissolved Organic Carbon,Ionic Composition,and Heavy Metal Content

The Huguangyan Maar Lake is one of the well-preserved Maar lakes in the world that kept on its closed hydrological and independent aquatic ecosystems with minimum external interference.Therefore,this unique lake has been regarded as an ideal site for studying past regional climate variation and carbon cycling&preservation based on its relatively simple influencing factors.The dissolved organic carbon concentration(DOC)of lake water is an important feature of lake geochemistry and it plays an important role in the primary production and sedimentary organic matter formation of the lake.This thesis had carried out a continuous sampling of Huguangyan Maar lake water from June 2018 to May 2019,and the surface water of the lake was taken once a week from four locations of the lake and subjected to the measurements of DOC,ionic composition,and five heavy metal contents.During the sampling,water temperature and p H values were measured and the daily rainfall data obtained from a nearby meteorological station.The DOC,ionic composition,and heavy metal content and their variation are discussed and the following conclusions could be drawn:1.The DOC concentration of lake surface water varied from 8.15 to 32.65 mg/L with an annual average concentration of 17.96 mg/L.There were no significant spatial variations in the average monthly concentration among the four sampling locations except occasional variation during the wet season.The reason for the wet season DOC differences among four sampling locations is likely due to the uneven runoff and underground water inputs and the relatively slow circulation of lake water.2.There was significant seasonal variation in the average DOC concentrations between the raining and dry seasons as the monthly average DOC concentration was 21.72 mg/L for the wet season compared to the dry season concentration of 14.21 mg/L.The rainfall shows a significant effect on the DOC concentration of lake water,as DOC concentration was much high during the wet season.The seasonal changes of DOC concentration in this closed lake water suggest that dissolved soil organic matter inputs through the rainfall-related surface runoff and subsurface infiltration from the surrounding watershed land are important to the primary production and organic matter deposition in the lake sediments3.A total of four anions(Cl^-,SO₄^2-F^-,NO₃^-),and cations(Ca²⁺,Mg²⁺,Na⁺,K⁺)were measured.Among the ions,Cl^-,SO₄^2-,NO₃^-,Mg²⁺,Ca²⁺and Na⁺,are dominant ions.There were no significant spatial variations in the average monthly ion concentrations among the four sampling locations except occasional variation during the wet season.4.Except for K⁺and NO₃^-,most ion concentrations have no significant seasonal changes.The concentration of K⁺ions presents rainfall-related changes,indicating that rainwater may bring about changes in the concentration of K⁺.The concentration of NO₃^-presents a unique pattern of changes,which suddenly increases significantly after October.This change may be related to lake biogeochemical activities or agricultural activities relevant,but further research is needed5.Five heavy metals(Cu,Cr,Cd,Zn,and Pb)were measured.Based on their annual mean concentration they were in the order,Cr >Zn >Cu>Pb>Cd.These five heavy metals were homogeneously distributed in the lake water as there was no significant variation in their average monthly concentration among the four locations.6.Regional climate-related changes in rainfall did not affect the five heavy metal concentrations of this lake water as there was no significant difference in the heavy metal concentration between the raining and dry seasons.Correlation analysis further suggests that factors such as changes in water temperature,and pH values,as well as DOC concentration,do not also influence the heavy concentration of these metals in the lake.Principal component analysis(PCA)suggests that the five heavy metals may originate from a common source(s),while there was no correlation among the metals suggesting that their concentration is not controlled by a single factor.As a closed lake with no external water sources coupled with the obvious lack of seasonal variation in the metal concentration,it is suggested that these five heavy metals may be from atmospheric deposition from anthropogenic activities relevant while the weathering-related contribution is limited,but further research is needed.In conclusion,both the heavy metal and ionic concentration of this lake's water appear to be within permissible levels of the water quality(China Environment Water Quality Standards)and meet the US-EPA and WHO standards for freshwater quality.