| Dalongdong underground river catchment is located in Western of Hunan priovince,China and belongs to the subtropical zone.It has the unique physical geography,climate,and hydrogeology characteristics.Karst is widely spread in the catchment and developed intensely.Individual and combination karst forms are rich.There was multiple variety groundwater types such as descending springs,ascending springs,underground rivers etc.karst groundwater system boundary clearly but has complicate structures.As a typical underground river catchment in the Southwest of China,it has the double structure.Rainwater supplies to the groundwater by mainly ways of injection,infiltrate and leakage.The flow field is natural and groundwater flow rate dynamic changing with rainfall largely.The study of exploit groundwater resources and its influence mechanism has been attracted much attention.In recent years,impacted by global climate change,human activity,land use and land change,and hydrological condition et al.,multiple factors,the quality of groundwater in Dalongdong underground river catchment has been degenerated seriously.A special case was that land use and vegetal cover pattern impact on karst groundwater hydrogeochemistry in a short time scale,which was particularly apparent for all.Based on collecting the materials and data in the past investigation and research,combined with recharge-runoff-discharge condition,land use form,vegetation status,groundwater type and natural eco-environment conditions et al.This paper took a whole basin as object,selected typical sub-catchments(basins)in Dalongdong underground river catchment,that were two ascending spring basins(basin a and basin c),two descending spring basins(basin b and basin d),and three underground river basins(basin e,basin f and basin g)respectively.Monitoring and taking sample to laboratory to measure multiple indexes from every study basin monthly from July 2011 to June 2012,statistic and analyzed various indexes dynamic characteristic and it relate to land use and vegetal cover pattern in different basins.Focus on human activities impact on groundwater hydrogeochemistry characteristic and its mechanism research.The ultimate goal of this paper was to study dynamics and influence mechanism of groundwater hydrogeochemistry characteristics,and analyze its relationship with land use and vegetal cover pattern in Dalongdong underground river catchment.Provide evidence for protecting Dalongdong underground river catchment,scientific and rational use the land,vegetation restoration etc.Mainly research contents and conclusions as follow:(1)We selected 7 basins in Dalongdong underground river catchment as study areas.Based on foundation materials combine with remote sensing maps and field investigation.Analyze variety basins geology back ground,recharge-runoff-discharge condition,eco-environment characters,land using type,vegetation coverage etc.the results showed that the development and distribution of karst basins in this region were mainly subject to stratum lithology,regional geological structure,karst development et al.7 study basins spread in karst developed intense zone and karst developed medium zone that groundwater quantity abundant and moderate respectively.Percentage of land use and land coverage in different basins were 2%grass land,14%paddy land,84%forest land in basin a.7.7%grass land,24.3%paddy land,68%forest land in basin b.71%grass land,29%forest land in basin c.68%grass land,32%paddy land in basin d.76.5%grass land,3%paddy land,3.7%dry land,14.4%forest land,2.4%reservoir area in basin e.60.7%grass land,0.8%paddy land,3.8%dry land,0.6 reservoir area in basin f.81%grass land,8.3%paddy land,3.2%dry land,7.5%forest land in basin g,respectively.(2)Adopting advanced monitor methods and measure techniques,we titrated,detected in situ and collected water sample to test elements in laboratory monthly from July 2011 to June 2012.In field monitor,titrated Ca2+ and HCO3·,measured water flow rate,water temperature,pH value and conductivity.In laboratory,we tested mainly ions,fixed CO2,TOC,DIC,TDS,?13C,and ?18O etc.Through statistic and comprehensive analysis the data from monitor and test,we found that hydrogeochemistry indexes in different basins groundwater were distinguish and dynamic changing with time obviously due to geological background,land use and land change etc.multiple factors integrated impact.The groundwater type was HCO3·-Ca,pH values changing from 6.53 to 8.30 and most groundwater showed slightly alkaline have drop-offs values in November.The epikarst springs water temperature as a whole was higher about 0.5?1.5? than that in underground rivers.Groundwater conductivity value was also show the same size comparison as temperature.Basins those have more forest and grass land coverage on land surface present stable dynamic change curve of runoff modulus for a whole year.Which indicated that vegetation on land surface have a good adjust and store ability for groundwater.Especially for Epikarst zone groundwater adjusting and storing.While the basins that were mainly covered by farmland and residential area have higher K+,Na+,Cl-,SO42-,NO3,TOC content than the other basins.It is showed out that the groundwater in these basins were suffered from excess farmland fertilization and wastewater discharge by human activity.According to monthly detect ?13C and ?18O content in study basins groundwater for a year,we found that the it was difficult to determine the exactly factor that impact the groundwater isotope,especially for ?13C and ?18O,because there were different original resources such as atmosphere,plant root respiration,soil microbial degradation and so on.But from the analyzed of relationship between ?13C and DIC,?13C and ?18O,it can be seen that land use and vegetal cover pattern impact the karst groundwater to a certain extent.(3)Karst process is actually a course of "carbon sink",which can be a new way for people to find "the missing sink" of carbon dioxide in global.Through monitoring,testing and analyzing,it was indicate that variety basins have different carbon sink potential due to distinct geology background,land use and land coverage,land quality etc.Basins that covered by more forest and grass land have more karst carbon sink intensity.In summary,the groundwater quality in Dalongdong underground river catchment was serious effected by anthropogenic in recent years.Human activity impact on karst groundwater were mainly of land use and vegetal cover pattern.Which has both positive and negative effect,but negative effect becomes more dominant,mainly manifested as vegetation degradation and karst rock desertification and lead to epikarst zone decreased its function for water adjust and store,result in groundwater violence up and down and cause drought and flood disaster in this region.Meanwhile,frequent cultivation made the land can't rehabilitate by itself and degenerate the soil eco-system,and then reduced its ability to degrade pesticide and fertilizer.Harmful substance easily accompanying up-layer water flow down to underground karst space,and made the quality of groundwater becomes poor.In addition,one bad result of land use and vegetal cover pattern was the rate of plant root respiration and microbial degradation decelerate,which could weaken the karst reaction at the water-rock-soil interface,and then reduce karst carbon sink intensity.In summary,from our study results we can see that in Dalongdong underground river catchment,land use and vegetal cover pattern influence karst groundwater hydrogeochemistry obviously.Which mainly present in hydrology,chemistry,isotopic and karst carbon sink,not only showed diversity between different basins,but also showed as danymic changing in our study indexes.In addition,karst groundwater in the given catchment presents sensibility and vulnerability to land use and vegetal cover pattern on land surface. |
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