Wei Zho ,Yn-zhu LinPeng-peng Zhou,*,Gung-i Wng,*,Xue-y Dng,Xio-fn Gu

a China Institute of Geo-Environment Monitoring, China Geological Survey, Beijing 100081, China

b State Key Laboratory of Biogeology and Environmental Geology and MOE Key Laboratory of Groundwater Circulation and Environmental Evolution, School of Water Resources and Environment, China University of Geosciences (Beijing), Beijing 100083, China

c Xi’an Center, China Geological Survey, Xi’an 710054, China

Keywords:Groundwater Global climate change Hydrochemistry Groundwater circulation Groundwater age Groundwater level rising Geological survey engineering Qaidam Basin Delingha Qinghai-Tibet Plateau

ABSTRACT Delingha is located in the northeast margin of Qaidam Basin.Bayin River alluvial proluvial fan is the main aquifer of Delingha,in which groundwater generally flows from north to south.The hydrochemistry results showed that two different hydrochemical evolution paths formed along southeast and southwest directions,respectively.Cl-Na type groundwater was formed in front of Gahai Lake,and SO4·HCO3-Na·Ca type groundwater was formed in front of Keluke Lake.The results of deuterium (D) and 18 O revealed that the groundwater mainly originated from the continuous accumulation of precipitation during geological history under cold and humid climate conditions.In addition,results of 14 C indicated that the groundwater age was more than 1140 years,implying relatively poor renewal capability of regional groundwater.Moreover,our numerical modeling results showed that the regional groundwater level will continue to rise under the warm and humid climate conditions.

1.Introduction

Global climate change affects the spatial and temporal distribution characteristics of regional atmospheric precipitation in varying degrees,which could change the temporal and spatial distribution pattern of water resources and the hydrological cycle process.Since the 1950s,the climate in the arid inland areas of the Qinghai-Tibet Plateau in China has varied from relatively cold and dry to relatively warm and wet gradually.According to statistics from the Climate Change Center of the China Meteorological Administration,from 1961 to 2019,the annual precipitation of the Qinghai-Tibet Plateau area increased gradually.Especially in 2016-2019,the precipitation in this area continued to be abnormally high (Shi YF et al.,2003; Huang L et al.,2006; Wang H et al.,2013; Dang XY et al.,2019;NCC,2020).

The groundwater movement process of inland basins is a typical representative of groundwater circulation and evolution in arid regions,which covers the whole process from recharge to discharge (Li WP et al.,1999).The arid inland basins of the Qinghai-Tibet Plateau have a relatively single source of groundwater recharge,and the ecological environment is relatively fragile.The occurrence and circulation of groundwater are mainly controlled by geological structure,topography,meteorology and climate.In natural conditions,precipitation and melting ice snow in mountainous areas converge into surface runoff at the mountain outlet,and massively recharge groundwater in piedmont alluvial proluvial plain.In alluvial lacustrine plain area,affected by slow topography and low permeability of aquifer,part of the groundwater overflow to the surface to form the discharge lakes,artesian springs,spring rivers,swamps,and wetlands.Under the background of warming and humidification regional climate,the increase of precipitation the melting snow water would aggravate groundwater recharge,which could influence the regional groundwater cycle and the ecological environment depending on groundwater to a certain extent.At the same time,with the expansion and diversification of human activities,the circulation and evolution of groundwater would be affected more directly.All those external conditions change could make the law of groundwater circulation and evolution in the basin more complex.

For arid basins,the impact of climate change on regional groundwater could be generalized into two aspects: (1) The arid climate would reduce the precipitation to a certain degree,which is not conducive to sustainable recharge of groundwater resources.At same time,the arid climate could increase vegetation evapotranspiration and groundwater evaporation,which would lead to the reduction of green area and the continuous decline of groundwater level; (2) The wet climate with increasing precipitation could change the recharge conditions and groundwater regime characteristics of natural groundwater system,which is a favorable condition for water resources supplement in long-term groundwater resource deficit areas (Luong VV,2021; Abdulrahman TM et al.,2020).The impact of human activities on regional groundwater could be generally summarized as two ways: (1)Excessive artificial exploitation of groundwater could induce the formation of local groundwater depression cone,the attenuation of spring flow,the cut-off of downstream,undesirable geological environmental problems such as land subsidence and ground fissures; (2) Unreasonable agricultural irrigation would recharge groundwater excessively and lead to the groundwater level approaching ground surface,which cause ineffective evaporation consumption of groundwater resources,secondary salinization of soil and instability of building foundation.At the same time,the extremely high phreatic water table could be more vulnerable to surface pollution (Qiao XY et al.,2005; Perles RMJ et al.,2009; Isao S,2012; Chen YN et al.,2012; Raposo JR et al.,2013; Wang P et al.,2013; Hou GC et al.,2017; Xiao Y et al.,2017;Scheihing K et al.,2018; Hao AB et al.,2018).

Delingha area is a typical arid inland basin in China,which is located in the northeast margin of Qaidam Basin,northeast of Qinghai Tibet Plateau.It is a small inland depression basin nested in a large basin (Fig.1).The arid climate with less precipitation,large evaporation makes the regional ecological environment is fragile.In recent years,due to the influence of climate warming and humidity and human activities,the regional groundwater level has risen significantly,which resulted in the increase of groundwater salinity and the intensification of surface salinization.Groundwater level rising has a significant negative impact on the normal production and living and ecological environment(Dou Y et al.,2010; Gao Y et al.,2020; Zeng QM et al.,2021).

By using hydrochemical and isotopic techniques,this paper will analyze and identify the hydrochemical and isotopic characteristics as well as the hydrochemical evolution and circulation characteristics of groundwater,and discuss the mechanism of geological environmental problems.The research results could support the scientific development and rational utilization of groundwater resources in Delingha area,and also have reference significance for the study of groundwater circulation and water resources sustainable utilization in other arid inland basins.

Fig.1.Geographic location of Delingha area.

2.Overview of study area

2.1.Natural geographical conditions

Delingha study area starts from Zongwulong mountain in the north,reaches Denan hills in the south,Buhete mountain in the East and Delingha hills in the West.Bayin River alluvial proluvial fan is developed in it and also is the main aquifer of the basin.The terrain is high in the north and low in the south.The highest point is located in Zongwulong mountain with an altitude of 5030 m,the lowest point is located in Keluke Lake with an altitude of 2820 m,and a relative height difference of 2210 m.Spring,groundwater and surface water are concentrated at the lowest point,forming catchment centers such as Gahai Lake,Keluke Lake and Tuosu Lake (Fig.2).

2.2.Meteorological and hydrological conditions

The Delingha area is a typical dry continental climate.According to the observation data of Delingha meteorological station (1956-2013),the annual average precipitation is 169.3 mm,and most precipitation is concentrated from May to September,accounting for 85% of the annual precipitation.The precipitation in July is the most,with an average of 40.2 mm.The annual average temperature is 3.9°C,and July is the hottest month of a year,with average temperature of 16.7°C.The annual average evaporation is 1938.3 mm (Wen GC et al.,2018a).

Bayin River is the main surface water system in Delingha area.It originates from the north foot of Zongwulong mountain and flows southward into the basin through the north of Buhete mountain.In the alluvial proluvial plain area(South of Delingha City),most river water seeps into the ground until at the boundary of alluvial lacustrine plain(Yikeshu),it reappears in the form of spring river and flows westward into Keluke Lake,with a total length of about 188 km.Keluke Lake and Gahai Lake are the main drainage areas of groundwater in the basin.Gahai Lake is the terminal salt water lake in the southeast of the basin,with the average salinity of about 90 g/L; Keluke Lake is located in the west and connects with Tuosu Lake,which is the largest circulating fresh water lake in Delingha area,with the salinity of 0.7-1.8 g/L (Zhang XY et al.,2007).

2.3.Hydrogeological conditions

Bayin River alluvial proluvial fan is the main waterbearing structure in Delingha area.In the Buhete Piedmont area,a large number of gravel and sand gravel of Quaternary Upper Pleistocene and sand gravel and silt of Quaternary Middle Pleistocene are accumulated (Li J et al.,2009).In alluvial proluvial plain,the middle of the basin,due to the decrease of the velocity of Bayin River,the main deposits are thick sand gravel,medium coarse sand and muddy gravel.In alluvial lacustrine plain,the south of the basin,the Bayin River flows very slowly while blocked by the Denan hills,fine-grained medium sand,fine sand,silt and silty clay begin to deposit.The aquifer changed from single layer to multilayer,and the groundwater transited from phreatic groundwater to the coexistence of phreatic groundwater and confined groundwater (Fig.3).

Fig.2.Geomorphological map of Delingha area.

2.4.Development of the water resources

Groundwater in Delingha is mainly used for domestic water supply and livestock.There are many irrigation works in the basin,most of which have relatively adequate supporting facilities.Two major irrigation areas named Delingha farm and Gahai farm are located in the northwest and southeast of the basin,respectively.The water for the canal system is sourced from the Bayin River Valley in the north of Buhete mountain.

3.Materials and Methods

The hydrochemical composition of water could effectively reflect information of hydraulic connection and hydrogeochemical processes (Wang LH et al.,2017; Zhao W et al.,2019).In addition,D and18O stable environmental isotope method is an important way to identify the source,recharge environment and cycle characteristics of groundwater.And the groundwater age could be estimated according to the14C radioisotope characteristics.Combining hydrochemical and isotope analysis,the evolution law of groundwater cycle in the study area could be revealed.

In this study,a total of 120 groups hydrochemical data were obtained,including one group data of atmospheric precipitation,86 groups of groundwater and 33 groups of surface water.Meanwhile,there are 76 sets data of D and18O stable isotopes of groundwater and 37 sets data of14C radioisotope.The sampling sites were shown in Fig.4.

4.Hydrochemistry evolution and circulation characteristics of groundwater

4.1.Characteristics of hydrochemistry evolution

4.1.1.Phreatic groundwater

As shown in Tables 1 and Table 2,Ca2+,HCO3-and SO42-are the dominated hydrochemical components of phreatic groundwater,with a relatively low TDS value.And the hydrochemical characteristics of the phreatic groundwater are basically consistent with that of the Bayin River water.This indicates that there is a close hydraulic relationship between groundwater and river water in the valley area (Tables 1,2).

In alluvial proluvial plain,the main hydrochemical components of phreatic groundwater had changed differently(Table 2).For cations,the concentration of Ca2+changed little while Na+increased significantly along the flowing path.Although the concentration of all anions increased in varying degrees,the concentration of SO42-increased more rapidly than other anions,which constituted the dominant anion together HCO3-.Meanwhile,as shown in Tables 1 and Table 2,it was obvious that the hydrochemical composition characteristics of phreatic groundwater are generally different from that of the river water,implying limited influence of Bayin River on groundwater.In addition,the hydrochemistry types also showed corresponding evolution laws along the axial and transverse directions of alluvial proluvial fan: along the axial direction,hydrochemistry type of phreatic groundwater transited from HCO3-Ca·Mg in piedmont area to HCO3·SO4·Cl-Ca·Mg in upper part of alluvial proluvial plain,then gradually transited to SO4·HCO3-Na·Ca in lower part of alluvial proluvial plain and transition zone; along the lateral direction,as being away from Bayin River,the concentration of SO42-,Na+and TDS of phreatic groundwater gradually increased,and the hydrochemistry type developed from HCO3·SO4·Cl-Ca·Mg to SO4·HCO3·Cl-Ca·Na.

As shown in Table 2,in alluvial lacustrine plain area,Na+gradually became the dominant cation in groundwater.Although the concentration of other cations increased at the same time,their increases were significantly less than that of Na+.In addition,Cl-is the dominant anion,which increases sharply from 53.2-287 mg/L to 51.1-6864 mg/L.And SO42-also increased significantly.Therefore,both Cl-and SO4-constituted the dominant anions of phreatic groundwater in alluvial lacustrine plain (Fig.5).

Table 1.Hydrochemistry data of Bayin River water (mg/L).

Table 2.Hydrochemistry data of the phreatic groundwater (mg/L).

Fig.3.Hydrogeogical profile (from Zongwulong Mountain to Keluke Lake) of Bayin River alluvial proluvial fan.

In the southern margin of the study area,due to the blocking of Denan hills,phreatic groundwater turned to flow towards to the Gahai Lake and to the Keluke Lake,and showed different hydrochemical evolution trends: (1) Along the southeast underground runoff path,the concentration of major anions and cations of phreatic groundwater changed significantly in a relatively short flow distance.After flowing through Gahai farm,the concentrations of Cl-and Na+significantly increased,which sharply became the dominant ions,and the TDS of phreatic groundwater also sharply increased to more than 4250 mg/L.The hydrochemistry type rapidly transited from HCO3·SO4-Ca·Mg·Na to Cl-Na,which means that the phreatic groundwater flowing southeast had entered the rapid mineralization stage before being discharged to Gahai Lake.(2) Along the southwest underground runoff path,the hydrochemical composition has experienced a transition from dominated by HCO3-,SO42-and Ca2+,Mg2+,Na+to dominated by SO42-and Na+,Ca2+.The phreatic groundwater of SO4·HCO3-Na·Ca type with TDS of 1750 mg/L is formed before flowing into the Keluke Lake (Figs.6a,7a).

Overall,the chemical composition and TDS changes of phreatic groundwater in Delingha are affected by thecomprehensive factors of recharge conditions,aquifer structure and regional geomorphic characteristics.

Fig.4.Schematic diagram of sampling sites.

4.1.2.Confined groundwater

Confined groundwater is only distributed in alluvial lacustrine plain.Na+,Ca2+,HCO3-and SO42-are the main ions of the confined groundwater.Table 2 and Table 3 showed that the concentrations of Na+,Cl-and SO42-of the confined groundwater are obviously lower than those of the phreatic groundwater.The main hydrochemistry type of confined groundwater was HCO3·SO4-Ca·Na (Fig.6b).From the Tables 2 and Table 3 as well as Fig.7b,it could also be seen that the values of ions and TDS of the confined groundwater were generally approximated with that of the pheratic groundwater in alluvial proluvial plain,indicating that the single layer phreatic groundwater in alluvial proluvial plain should be the main source of confined groundwater.Besides,according to the groundwater level measurement results,the groundwater level of the confined aquifer is higher than that of the phreatic groundwater in alluvial lacustrine plain,indicating that there was merely less leakage from the upper phreatic groundwater of the alluvial lacustrine plain into the confined aquifer.

4.2.Recharge source and renewal characteristics analysis of groundwater

Fig.5.Ionic concentration curve of typical area.

Fig.6.Durov diagram of phreatic and confined water samples.

By analyzing the characteristics of D and18O stable isotopes and14C radioisotope,the authors tried to identify the source of groundwater recharge,the recharge environment and the hydraulic relationship between surface water and groundwater,to estimate the age of groundwater,and to finally reveal the characteristics of groundwater circulation.The sampling information of D and18O are shown in Table 4.

Zhu JJ et al.(2015) analyzed the D and18O isotopic characteristics and vapor origins in the eastern Qaidam Basin by collecting the precipitation data from June to September,and fitted the local meteoric water line (LMWL,equation (1))of Delingha area:

Fig.8 shows the relationship between LMWL line and18O-D stable isotopes data obtained in this study.All samples are located near the LMWL line,which indicate that the water of Delingha area mainly originated from precipitation.

The least squares method was used to fit the D and18O isotope ratios of the surface water.And the following fitting equation is obtained:

This fitting line could be regarded as the Local Evaporation Line of Delingha (DLEL),with a slope of 5.302 and less than the slope of LMWL line,showing that the surface water is enriched by heavy isotopes due to evaporation (Fig.8).The intersection of DELE and LMWL line approximately reflects the original average isotopic composition of the initial vapor.The evaporated vapor is usually distributed along the LEL extension line to the left of the initial water point,while the residual water is distributed along the LEL extension line to the right of the initial water point (Clark I D et al.,1997).

Fig.9 shows that theδD−δ18O ratios of groundwater located in the left area of the initial vapor on LMWL line.From Bayin River to the two wings of alluvial proluvial fan,δD−δ18O ratios of phreatic groundwater move along the DLEL line in the left direction.As the large water table depth(about 15-50 m) of phreatic groundwater in alluvial proluvial plain,the groundwater merely evaporated in natural conditions,which could maintain the D and18O characteristics of the original recharge source.In addition,depleted D and18O characteristics implies the cold and humid climate environment of recharge period (Liu JD et al.,1997;Nicholas U et al.,2013).Therefore,it could be preliminarily inferred that the groundwater in Delingha area was accumulated by non-modern precipitation under the cold and humid climate condition.

TheδD−δ18O ratios of confined groundwater is similar to that of the phreatic groundwater in alluvial proluvial plain,indicating that the confined groundwater was mainly supplied by phreatic groundwater in alluvial proluvial plain which is consistent with the analysis result of hydrochemical characteristics.For the phreatic groundwater in alluvial lacustrine plain,the shallow depth of water table makes it could be easily recharged from precipitation and surface water,thus theδD−δ18O ratios were relatively close to the initial vapor (Fig.9).

In this study,data of 3714C sample were used to estimate the age of groundwater.The sampling information and dating results are shown in Table 5.

As shown in Table 5,there are obvious differences between the age of phreatic and confined groundwater.Theage of regional phreatic groundwater is in the range from 1.14 to 17.63 ka,while the age of confined groundwater varies from 10.88 to 24.92 ka.In addition,there was spatial variability of the age of phreatic groundwater,with age of 2.19 ka in piedmont area,4.81-15.60 ka in alluvial proluvial plain,and 1.14-15.16 ka in alluvial lacustrine plain.

Table 3.Hydrochemistry data of the confined groundwater (mg/L).

Table 4.D,18O data of precipitation,river water,lake water and groundwater.

Table 5.14C dating results of groundwater in Delingha.

Fig.7.TDS distribution of phreatic and confined groundwater.

Fig.8.δ18 O-δD relationship of surface water.

Fig.9.δ18 O-δD relationship of groundwater.

4.3.Analysis of groundwater renewal intensity

Based on the above results of hydrogeochemical and isotopic analysis,it could be concluded that the local groundwater was sourced from paleoatmospheric precipitation accumulated during the long geological history.And the age of regional phreatic groundwater and confined groundwater were generally more than 1.14 ka and 10.88 ka,respectively.In conclusion,the renewal intensity of groundwater in Delingha area is relatively low.

5.The response of groundwater regime to climate change and human activities

In recent years,the groundwater level of Delingha continues to rise.Groundwater level rising has caused geological and environmental problems such as building foundation damage,farmland inundation and soil salinization(Zhao Z,2014).In this section,we will discuss the controlling effects of climate change and human activities on the groundwater regime.

5.1.Climate warming and humidification

From 1956 to 2013,the precipitation frequency of four seasons in Delingha area increased with the rates of 4.56 mm/10a in Spring,12.35 mm/10a in Summer,3.12 mm/10a in Autumn and 1.24 mm/10a in Winter,respectively.And the temperature in four seasons also increased with the rates of 0.93°C/10a,0.67°C/10a,1.34°C/10a and 2.65°C/10a,respectively (Wen GC et al.,2018a; Liang XY et al.,2019).Both precipitation and temperature showed increasing trends (Figs.10,11).The increasing precipitation and the increasing melt water induced by higher temperature provided more water resources for the Bayin River.According to relevant statistical data,the runoff volume of Bayin River increased at the rate of 0.2 × 108m3/10a.More than 80% of the increased runoff of Bayin River was contributed from precipitation.And then,the increased runoff volume of Bayin River,which is an important recharge source of phreatic groundwater,plays a significant role in controlling the regional groundwater level rise.

5.2.Effect of human activity

The influence of human activities on the groundwater level rise is manifested by the large artificial recharge in irrigation area.The irrigation water of Delingha Farm and Gahai Farm was drawn from Bayin River through two main canals in the East and West (Fig.12).During agricultural irrigation season,canal water leakage and field irrigation water infiltration brought a large amount of artificial recharge to the groundwater,forming the rise of groundwater level.

5.3.Forecasting of groundwater regime

According to the statistical results of meteorology and hydrology in Delingha area,during 1956-2013,the annual precipitation increases at the rate of 21.27 mm/10a,the annual average temperature increases at the rate of 0.38°C/10a,and the runoff volume of Bayin River increases at the rate of 0.2×108m3/10a.By fully analyzing the regional hydrogeological conditions,MODFLOW program was used to predict the trend of groundwater regime change.The predicted result showed that the shallow groundwater level would generally rise by 20-40 cm by 2050,except in the piedmont area (Fig.13).

6.Conclusions

(i) The groundwater in the Delingha area showed different hydrogeochemical evolution along two flow paths towards Gahai and Keluke Lake,respectively.Along the southeast underground runoff path,the concentration of major anions and cations of phreatic groundwater changed significantly in a relatively short flow distance.After flowing through Gahai farm,the concentration of Cl-and Na+significantly increased,which sharply became the dominant ions,and the TDS of phreatic groundwater also rapidly increased to more than 4250 mg/L.The hydrochemistry type rapidly transited from HCO3·SO4-Ca·Mg·Na to Cl-Na,which means that the phreatic groundwater flowing southeast had entered the rapid mineralization stage before being discharged to Gahai Lake.However,along the southwest underground runoff path,the hydrochemical composition has experienced a transition from dominated by HCO3-,SO42-and Ca2+,Mg2+,Na+to dominated by SO42-and Na+,Ca2+.The phreatic groundwater of SO4·HCO3-Na·Ca type with TDS of 1750 mg/L is formed before flowing into the Keluke Lake.In addition,the ion concentrations of the confined groundwater are obviously lower than those of the phreatic groundwater,indicating that there was merely less leakage from the upper phreatic groundwater of the alluvial lacustrine plain into the confined aquifer.

Fig.10.Seasonal variation characteristics of precipitation in Delingha (1956-2013).

Fig.11.Seasonal variation characteristics of average temperature in Delingha (1956-2016).

Fig.12.Distribution of irrigation area and groundwater level rising area.

Fig.13.Prediction result of groundwater level rise in 2050.

(ii) The depletedδD−δ18O ratios show that groundwater was mainly sourced from the precipitation recharge under cold and humid climate.According to the results of14C analysis,the age of regional phreatic groundwater was mainly about 1.14-17.63 ka,while the age of confined groundwater was about 10.88-24.92 ka,implying relatively poor renewal capability of regional groundwater.

(iii) Affected by the warm and humid climate,the rising trend of regional groundwater level is difficult to avoid.In addition,it is much easier to form local groundwater level rising disaster zones in agricultural irrigation areas.The shallow groundwater level would generally rise by 20-40 cm by 2050.

CRediT authorship contribution statement

Wei Zhao conceived of the presented idea and wrote the manuscript.Yan-zhu Lin performed the numerical simulations.Xue-ya Dang and Xiao-fan Gu verified the14C analytical results.Guang-cai Wang and Peng-peng Zhou revised the manuscript.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (41672243) and the China Geological Survey Program (DD20160291,DD20189270).