Impacts Of The Groundwater Flow Path On The Patterns Of Dissolved Organic Carbon Export In The Cold Alpine Area

Permafrost area is the important global carbon reservoir.Large portion of the soil organic carbon in the permafrost layer will transport to aquatic ecosystems as dissolved organic carbon(DOC)due to permafrost degradation.The transport of organic carbon from the permafrost area is a crucial part of global carbon cycling.The seasonally frozen layer and permafrost layer widely spread in the cold alpine area.Groundwater is the main contributor of alpine stream,and the groundwater flow path is one of the main factors controlling DOC export in cold alpine catchments.The groundwater flow path is complex,and it is still not clear how the groundwater flow path influences the DOC transport in the cold alpine catchment.Understanding the impacts of the groundwater flow path on the patterns of DOC not only provides information about the DOC export in the cold alpine area,but also encourages the studies about the hydrogeology in the permafrost area under the global warming condition.The study area is the Hulugou catchment,which is a typical cold alpine headwater catchment of Heihe River,located at the northeast of Qinghai-Tibet Plateau.In this research,the field investigation,drilling,in-site monitoring and sampling work were carried out,combined with the analysis of hydrogeochemical and isotopic data and hydrogeochemical inverse modelling.Finally,we figured out the aquifer structure in the Hulugou catchment and built the groundwater flow path conceptual model.In addition,the spatial and temporal variation of DOC concentration and dissolved organic matter(DOM)composition were explored,and the mechanism of the controlling impact of groundwater flow path on DOC export were revealed.A few conclusions were drawn from the results.1.Groundwater flow path in the cold alpine catchmentHydrogeochemical and isotopic data of rainwater,glacier-snow meltwater,snow meltwater,supra-and sub-permafrost groundwater,well groundwater in the seasonally frozen area,spring water and stream water were analysed,and hydrogeological data were combined to explore the the aquifer structure and the groundwater flow path in the seasonally frozen area and permafrost area.The results indicate that in the permafrost area,the groundwater and stream water were mainly recharged by glacier-snow meltwater from the south mountains.The perennially frozen layer extends from 2 m to 20 m below ground surface and separates the aquifer into the supra-and sub-permafrost aquifers.Suprapermafrost groundwater flowed through the shallow organic layer and the chemical components were mainly affected by calcite dissolution and evaporation,while the geochemistry of subpermafrost groundwater was controlled by dolomite and gypsum dissolution,calcite precipitation,and albite and halite dissolution.Distinct hydrogeochemical characteristics and controlling processes suggest a poor hydraulic connectivity between the supra-and sub-permafrost groundwater.Groundwater in the seasonally frozen area was mainly recharged by the infiltration of surface water,including stream water,seeps and springs,mostly from the permafrost area.In the seasonally frozen area,a silty clay layer(5~20 m below ground surface)with a depth about 3~6 m with low permeability separates the aquifer into the deep(depth > 20 m)and shallow(depth < 20 m)flow paths.The deep groundwater was characterized by the enhanced dedolomitization and enhanced cation exchange processes compared to the shallow groundwater.Groundwater in the seasonally frozen area finally discharged as base flow into the stream.In the cold season,groundwater from the seasonally frozen area was the main source of stream water in the main stream.The widely spread Quaternary porous aquifers play an important role in the alpine catchment hydrological cycle,and lead to the frequent groundwater and surface water(GW-SW)interaction.(1)Porous aquifers provide a good reservoir for groundwater storage;(2)the delayed release of groundwater from the loose sediments maintain the stream of alpine streams;(3)the uneven distribution of loose sediments leads to the frequent GW-SW interaction and influence the export of DOC and dampen the influence of spring snowmelt water on DOC export.2.The pattern of DOM and DIC transport in the cold alpine catchmentThe DOC and dissolved inorganic carbon(DIC)concentrations,DOM compositions features and the seasonal variations of stream water and groundwater along the flow path were analysed to reveal the pattern of DOM and DIC transport in the Hulugou catchment.The largest DOC and DIC export occurred in summer in the permafrost area and seasonally frozen area,and were greatly influenced by the summer precipitation and coordinated with the increase of stream discharge.The daily DOC and DIC flux at the catchment outlet were 32 kg/d and 1768 kg/d,respectively.The DOC and DIC flux were much lower than that at the catchment outlet,and the seasonally frozen area showed comparable DOC and DIC flux with that of the catchment outlet.GW-SW interaction is the main factor influencing the DOC export.In the permafrost area,groundwater and stream water both had high DOC concentrations,and were generally terrestrially derived.The percentage of terrestrial DOM of well groundwater was higher than that of spring water;and during the warm season,the DIC and DOC concentrations in the well groundwater were more variable than stable spring water,indicating the influence of the recharge of stream water and groundwater from permafrost area.Stream water in the seasonally frozen area showed smaller increase of DOC concentrations,and larger increase of DIC concentrations in the spring than that of stream water in the perennially frozen area,suggesting the influence of GW-SW interaction.The groundwater from the seasonally frozen area sustained the catchment discharge during the cold season,leading to the content of microbially derived DOM increase in the catchment outlet stream water.The stream water and groundwater from permafrost area showed significant influence on the groundwater in the seasonally frozen area,instead of direct recharge to the catchment outlet.3.The comparison of DOM transport between cold alpine area and arctic and panarctic areaThe arctic and pan-arctic area show significant differences in DOM export with the cold alpine area,including(1)the greatest DOC flux occurs in spring every year;(2)the DOC concentrations show positive relationship with DOC flux;(3)with the increase of the thickness of active layer,the DOC concentration and composition will change in the stream water.These differences should be attributed to(1)the great differences lying in the pattern of precipitation input;(2)different distributions of the seasonally frozen area and the permafrost area affecting the structure of aquifers and the groundwater and surface water interactions;(3)the distinct influence of freeze-thaw process on the active layer in the perennially frozen layer.4.The prediction of the influence of permafrost degradation on DOC exportThe permafrost degradation have been widely observed in the Qinghai-Tibet Plateau area,and the will definitely change the aquifer structure and groundwater flow paths,then influence the DOC export.The DOC and DIC concentration will greatly increase in the stream water and groundwater from permafrost area,and lead to the increase of DOC and DIC flux.The thickness of active layer will increase,which will intense the interaction between supra-and sub-permafrost groundwater and increase the groundwater storage capacity.The groundwater storage and DOC concentration in the seasonally frozen area will rise due to the GW-SW interaction.The catchment DOC flux will grow significantly,and the discharge will increase during the cold season.On one hand,this research clearly described the groundwater flow path in the typical cold alpine catchment by using multiple independent geological and hydrogeochemcial and isotopic data,and revealed the important role of the Quaternary porous aquifers in hydrological cycle.On the other hand,our work explain how the complex groundwater flow path influence the DOC export in the cold alpine area,and pointed out that the GW-SW interaction play an important role in controlling the DOC export.The deficiency of this research lies in following two parts:(1)lack of monitoring of pattern of DOC transport in the Heihe River,and cannot efficiently show how the Heihe River DOC export coordinate with that of the small typical catchment;(2)lack of the observation data of discharge and DOC export from other typical small catchment,and decrease the applicability of the conclusions.