Groundwater Cycle Patterns And Its Response To Human Activities In Nalenggele Alluvial-proluvial Plain

Nalenggele River Alluvial Plain is located in the southwestern of the Qaidam Basinwhich is part of the arid region in western China. Groundwater is not only animportant source of water supply but also the safeguard water of the vulnerableeco-environment in arid region due to scarce precipitation and strong evaporation.Therefore, this study relies on the thematic project of China Geological SurveyBureau “The study of transforming relationship between groundwater and surfacewater and the renewable ability of groundwater in Nalenggele River basin” andNational Natural Science Foundation project “Biogeochemical behavior of Fe and Mnin the hyporheic zone affected by groundwater exploiting (Grant Number:41372238)”. Based on Sufficient analysis of the hydrogeological conditions ofNalenggele River Alluvial plain, this study reveals the transforming relationship ofgroundwater and surface water, identifies the characteristics of water infiltrationinfluence on groundwater using multiple environmental tracing techniques, predictsand evaluates the impact of human activities on the groundwater circulation usingnumerical simulation of groundwater flow. Conclusions that be got in this study areshown as follows.1. The groundwater of study area is mainly recharged by Nalenggele river seepagewhich is formed by Kunlun Mountains snowmelt and mountain precipitationconfluence with a percentage of87.6%and accepts a small amount of lateral runoffrecharge with a percentage of12.3%.Scarce precipitation and strong evaporationoccur in the piedmont plain, the calculated potential of precipitation to rechargegroundwater is only0.027~0.043mm/a which is difficult to form a substantialrecharge.2. There is a complex transforming relationship between groundwater and surface water in the study area which can be divided into three sections according to thedifferent landforms to study the transformating characteristics of river andgroundwater. The transforming relationship between river and groundwater of eachsection is identified according to the indicating of hydrochemistry, oxygen stableisotope, radioisotope222Rn and other environmental tracers. From Piedmont upliftregion to upstream mountain basin(sectionⅠ), river water is closely linked togroundwater which can be infered from multiple conversions between river water andgroundwater within this section; Vertical seepage from river to groundwater mainlyoccurs from piedmont uplift areas to Alluvial tilt plains of the front edge ofdownstream Alluvial Fan (section Ⅱ); There is a strong groundwater overflow whichrecharges the river water from groundwater overflow belt to the alluvial plain of thetaijinaier river(section III). Combined with the calculation result using massconservation principle of222Rn and temperature tracing, the amount of river rechargeis determined accounting for about52.3%of river runoff and the groundwateroverflow is calculated accounting for about20%of the river recharge in the conditionof recent years.3. Close hydraulic connection between river water and groundwater shows somecorrelation between river runoff dynamics and groundwater level dynamics. The riverflow and the groundwater dynamics have shown a cyclical fluctuation, while the leveldynamics of groundwater away from the river shows a decline on cyclical fluctuation,and the delay of groundwater with respect to river flow also increases with distancefrom the river. The lag time of groundwater compared with river flow dynamics isabout50to150days in the nearshore zone. Characteristics of hydrochemistry andisotope have also shown the influence of river infiltration on groundwater, reflected ina higher value of TDS and14C and an enriched stable isotope distribution ingroundwater closer to the river. Affect sections of river infiltration to groundwater aredivided based on the above affect characteristics. The strongly influenced section is located in the center of the river and the adjacent of riparian zone, the lag time ofgroundwater dynamics compared with river dynamics is about50to100days; Theweak influenced section is far away from the center of the river with a lag time about100to150days; The hydrochemistry and isotopic characteristics exhibited a mixingprocess of river water and groundwater in the mixing section of the leading edge ofthe alluvial fan with a lag time of spring flow dynamics compared to river dynamicsabout200days.4. A local groundwater flow system, an intermediate groundwater flow system and aregional groundwater flow system occur in the study area under present conditions.The study area can be divided into three kinds of groundwater circulation patternsaccording to the distribution of water flow system and groundwater circulationfeatures of each water flow system. Local groundwater circulation patterns: thelongitudinal extension length of groundwater flow system along the groundwater flowdirection is30-35km with a strong water cycle alternating ability; Intermediategroundwater circulation patterns: the longitudinal extension length is about50km witha poor circulation alternating ability; Regional groundwater circulation patterns: thelongitudinal extension length is more than80km and after accepting the river rechargefrom the upstream, groundwater flows directly to the outside of the study area throughthe deep with no hydraulic connection with surface water and a poorest alternatingcycle ability.5. The characteristics of groundwater cycle is affected by river water andgroundwater extraction from human activities in the study area.The effects of miningwater resources on groundwater circulation are determined through numericalmodeling of groundwater flow. When a large exploitation of water resources happens,circulation characteristics of local water flow system is most strongly affected.Reflected in a gradually increasing decline of groundwater level of the center ofalluvial fan with the river flow reduces and the mining amount increases, a reducing local water flow system distribution range and a decreasing amout of groundwaterresource which is positively involved. Reduce in the amount of active circulation is adirect impact on the distribution area of vegetation suitable water level. When theriver runoff by extreme weather conditions of continuous flow of dry years and riverwater and groundwater extraction is set according to the largest water demand,alluvial fan central groundwater level decline of more than10m, and activelycirculating groundwater resources with a42%reduction relative to current conditions,vegetation suitable water level distribution range of22%relative reduction in currentconditions.