Xue-ru Wen, Yan-pei Cheng, Jian-kang Zhang, Hua Dong

1 Institute of Hydrogeology and Environmental Geology, CAGS, Shijiazhuang 050061, China.

2 Key Laboratory of Groundwater Sciences and Engineering, Ministry of Natural Resources, Shijiazhuang 050061, China.

Abstract: The natural groundwater recharge in Asia is estimated to be 4 677.74×109 m3/a. However, it features extremely uneven spatial-temporal distribution. Groundwater is distributed in various natural and geological environments, and it is liable to be affected by numerous factors and possesses different properties. Moreover, groundwater faces complex ecological problems. This paper gains a complete understanding of groundwater in Asia in terms of the structure of aquifer systems, the processes of groundwater cycle, and the spatial variation laws of surface ecosystems. Based on this, it proposes the ecological function zoning scheme of groundwater in Asia, aiming to provide guidance for the utilization of regional water resources and the planning for economic and social development, coordinate the relationship between social and economic development and water resource protection, and improve the ecological functions of groundwater. Furthermore, this paper analyzes the problems with regional groundwater management in Asia and puts forward countermeasures and suggestions, thus providing a theoretical basis for the sustainable development and utilization of regional groundwater and environmental protection.

Keywords: Asia; Groundwater resources; Eco-environment index system; Function zoning Received: 07 Sep 2021/ Accepted: 09 Nov 2021

Introduction

The development and utilization of groundwater resources have been constantly increased in most Asian countries/regions. Among the top 10 countries with the largest groundwater abstraction as of 2010, six are Asian countries. The contradictions between the supply and the demand of water resources in Asia are becoming increasingly striking,some groundwater development concentration zones are facing increasingly serious water shortage problems and water environment deterioration,which are mainly reflected in the following aspects: (1) Drying up of the river. According to the 2007 World Wide Fund for Nature (WWF) report World’s Top 10 Rivers at Risk, five of the top ten fast drying rivers listed in the report are in Asia,namely Mekong, Salween, Yangtze, Ganges and Indus rivers (WWF, 2007). This indicated that the river's drying up is particularly severe in Asia; (2)Shrinkage of wetlands and lakes. It is mainly caused by climate warming, increased human activities, and a reduction in rainfall. Most especially,the blind land reclamation around lakes has destroyed the water sources of rivers flowing into lakes; (3) Problems induced by groundwater overexploitation, such as a continuous drop in groundwater level, a reduction in well yield, land subsidence, seawater intrusion into coastal aquifers, and the movement of salt water or polluted water into aquifers. Some Asian countries have even experienced an excessive drop in the groundwater level,including China, Japan, Maldives, South Korea,Sri Lanka, Thailand, and Vietnam (Li and Wu,2010); (4) Poor groundwater quality. Asia features a large area of arid and semi-arid regions and high salt content of shallow groundwater. The silted coastal zones in Asia suffer inferior groundwater quality due to mixing and alternating seawater and salt water. Meanwhile, the groundwater in many Asian regions shows anomalous contents of elements such as arsenic, fluorine, iron, manganese and radon in special geological environments, which exceed drinking water standards and endanger public health. For instance, the arsenic content in groundwater exceeds the drinking water standard in 12 countries, especially in Bangladesh,India and China; (5) Degeneration of permafrost zones. Climate warming is certain to induce permafrost degeneration. Over the past five decades, the permafrost area in the entire northern hemisphere has shrunk by 3-4 million km2. Meanwhile, the permafrost zones have also gradually degenerated in Central Asia, North Asia, and North China; The latest research shows that the permafrost in the headwater area of the Urumqi River in the Tianshan Mountains has rapidly degenerated, with the lower limit depth having dropped by 7.7 m.Permafrost degeneration is also reflected the great changes in active layers. The permafrost active layers in the Siberian Plateau have increased by 20-30 cm and those in the Qinghai-Tibet Plateau have increased by about 1 m over the past five decades(Zhou, 2008); (6) Soil desertification. In Asia, the potentially desertified lands cover an area of 334 000 km2, and the area of desertified lands has increased from 137 000 km2to 176 000 km2since the 1950s; (7) Soil salinization. The soil salinization caused by insufficient drainage or the absence of conjunctive utilization of surface water and groundwater covers about 6 million hm2in India.Meanwhile, the increase in groundwater salinity has become a pressing problem in the Indus Basin in Pakistan (Guo and Zhao, 2005).

The striking problems associated with the groundwater and the ecological environment in Asia becomes a worldwide focus and the challenge faced by the economic integration in Asia. To ensure the safety of the regional ecological environment, meet the demand for water resources, and promote the high-quality development of the Belt and Road Initiative (BRI), it is urgent to ensure the sustainable development of groundwater ecology. Therefore, it is necessary to comprehensively analyze the responses of groundwater and ecological environment in Asia and to investigate how to secure the groundwater and ecological environment in Asia,thus providing a theoretical basis for the healthy and sustainable development of the BRI.

1 Impacts of groundwater on ecological environment

Groundwater serves as an essential resource for human activities and plays an important role in maintaining the natural environment. As a type of water resource, groundwater contains rich information on the changes in the ecological and geological environment system. It is one of the most active parts in the ecological and geological environment and supplied to human society and surface ecosystems directly or indirectly. Besides, groundwater is also a particular ecosystem. It has an inevitable connection and interaction with surface ecosystems, thus serving human society jointly with surface ecosystems.

With the acceleration of economic and social diversification, the rapid development of global industrialization and urbanization has caused serious problems for resources and environments. In particular, the continuous drop in regional groundwater level caused by groundwater over exploitation has induced increasing negative effects, which intensify land desertification, soil salinization, and wetland and grassland degeneration and sharply reduce the water quantity in rivers and lakes. In addition, the water and soil pollution induced by industrial, agricultural, and urban wastes is also imperceptibly affecting the ecological environment in which human beings live. Groundwater is closely related to the survival and prosperity of human beings, and the changes in groundwater have all-around and multiple-level impacts on the environment, including both positive and negative effects. Therefore, it is essential to rationally use groundwater resources and properly protect the ecological environment and curb negative effects,build a harmonious relationship between human beings and the natural resources and environment.

For purposes of benefit-seeking and harm-avoiding, it is necessary to obtain a comprehensive and correct understanding of the ecological functions of groundwater, including its role in supporting the natural environment and maintaining the stability of the geological environment. Based on this, the relationships between groundwater and surface ecosystems, geological environment and social economy should be revealed. Then the value of groundwater in resources and environments should be evaluated. This will provide a theoretical basis for the sustainable development, utilization of regional groundwater and environmental protection.

2 Overview of groundwater resources in Asia

In this study, Asia was firstly divided into regions with different groundwater infiltration coefficients according to different infiltration recharge conditions of groundwater, such as landforms, terrain,average multiyear precipitation, hydrologic conditions, lithology of aeration zones and aquifer media. Then the groundwater recharge intensity (mm/a) was converted into the unified natural ground-water recharge (runoff) modulus. Given the conversion relationship between surface water and groundwater, groundwater resource evaluation was carried out in the areas in plains and basins where groundwater is recharged through surface water infiltration, especially in the downlands in the Siberian Plain with significant groundwater recharge through alternate freezing and thawing in seasonally freezing layers (Dong et al. 2011). According to the continuity of groundwater flow, aquifers in Asia can be grouped into three types:Unconsolidated-sediment continuous aquifers in plains and intermountain basins, bedrock intermitten taquifers in hills and mountains ,and other sporadic aquifers. The natural groundwater recharge modulus of these three types was divided into five grades, namely ＜10 × 104m3/a·km2, 10 × 104- 20× 104m3/a·km2, 20 × 104- 30 × 104m3/a·km2, 30 ×104- 50 × 104m3/a·km2and ＞50 × 104m3/a·km2.Fig.1 shows the aquifer distribution in Asia.According to this figure, aquifers in the eastern regions and southeastern coastal regions are dominated by bedrock intermittent aquifers in hills and mountains, with a natural groundwater recharge modulus of 20 × 104- 30 × 104m3/a·km2. Meanwhile, aquifers in the central and southwestern coastal regions mainly include other sporadic aquifers, with a natural groundwater recharge modulus ＜20 × 104m3/a·km2. In addition, aquifers in the northern regions mainly consist of unconsolidated-sediment continuous aquifers in plains and intermountain basins, with a natural groundwater recharge modulus ＜30 × 104m3/a·km2. This differential aquifer distribution is mainly induced by complex landforms and terrain and uneven atmospheric precipitation in Asia. The natural groundwater resources in these three types of aquifers were determined using runoff modulus (Table1).

Table1 Summary of natural resources in groundwater aquifers in Asia

Fig.1 Map showing the natural recharge modulus zoning of groundwater in Asia

According to the table above, natural groundwater resources in Asia are distributed in an extre-mely uneven manner. The natural groundwater recharge resources of other sporadic aquifers are only 386.14 × 109m3/a, accounting for merely 8% of the total natural groundwater resources in Asia,while those of unconsolidated-sediment continuous aquifers in plains and intermountain basins and bedrock intermittent aquifers in hills and mountains account for 52% and 40%, respectively. Moreover, the unconsolidated-sediment continuous aquifers in plains and intermountain basins have the maximum natural recharge resources.

In this paper, the groundwater in Asia was divided into 11 primary groundwater systems and the natural water recharge of these aquifer systems was determined using the groundwater runoff modulus. Firstly, the regional precipitation data of each primary aquifer system were analyzed. Secondly,the average groundwater recharge intensity of each primary groundwater system was converted into the average recharge modulus of the regions, according to the landforms, terrain, average multiyear precipitation, hydrologic conditions, lithology of aeration zones, and aquifer media in these regions.Finally, the natural recharge of each primary groundwater system was obtained using the groundwater runoff modulus (Table2).

The recharge modulus in Table2 represents the natural groundwater recharge occurring in the groundwater system per unit area. The average recharge modulus across Asia is 10.55 × 104m3/a·km2.The minimum recharge modulus is merely 5.35 ×104m3/a·km2, which occurs in the tropical arid groundwater system in the Arabian Peninsula-Mesopotamian Plain (i.e. the Mesopotamian Plain and Arabian Peninsula). This is closely related to the arid climate, extremely high evaporation, and extremely low precipitation in the region. The maximum recharge modulus is 23.08 × 104m3/a·km2,which occurs in the alpine groundwater system in the Qinghai-Tibet Plateau that covers the Kashmir-Himalayan-Qinghai-Tibet Plateau and the South-Tibet river basin. The reason is that the abundant glacier resources formed from precipitation in the region can provide abundant groundwater recharge resources. As a result, the groundwater system has a very high recharge modulus despite its small area.

As shown in Table2, the natural groundwater recharge across Asia is 4 661.28 × 109m3/a in total. The maximum natural groundwater recharge occurs in the cold-temperate semi-humid groundwater system in platforms and plateaus in North Asia, accounting for 22% of the total natural groundwater recharge across Asia. This groundwater system is located in a rather cold region and covers the largest area. Due to low evaporation and relatively high recharge through precipitation, this groundwater system boasts the highest natural groundwater recharge in Asia. The minimum natural groundwater recharge occurs in the subtropical arid and semi-humid groundwater system in the Iranian Plateau-Asia Minor Peninsula, accounting for only 3% of the total natural groundwater recharge across Asia. This is caused by the high evaporation and relatively small recharge through precipitation in this groundwater system since it covers a small area and is located in an arid region.

Table2 Average recharge modulus and natural recharge resources of primary groundwater systems in Asia

3 Index system and ecological func tion zoning scheme of groundwater in Asia

The ecological function zoning of groundwater in Asia was conducted based on the alternation, waterrock interactions, and ecological types of ground water under the natural environment (i.e. climate zoning, landforms and terrain), as well as the impacts of human activities. The zoning scheme should focus on the relationship between groundwater and surface ecosystems, and the distribution characteristics of them, meanwhile fully considers the spatial distribution of the human economic and social development, and the support of the groundwater for the development, thus fully reflecting the sustainable development of the groundwater ecological service functions. Besides the zonal differentiation of natural elements or natural systems, the zoning bases also include the hierarchical structure and scales of ecosystems. In this zoning scheme,the impacts of human activities on the ecosystems are assessed and measured in terms of ecosystems integrity. Moreover, the scientific base of the zoning scheme is achieved on the platform of "ecosystem-based management". See Table3 for details (Ge et al. 2012).

Table3 Index system for groundwater ecological function zoning in Asia

Based on groundwater systems in Asia, the ecological functions of groundwater were defined according to the spatial distribution of groundwater resources and the distribution characteristics and rules of ecological environments. Then the ecological service function zones of groundwater in Asia were determined through the comprehensive analysis of the above-mentioned index system. The major ecological service function zone types are as follows: (1) Groundwater regulation and storage zones. They are mainly distributed in coastal ports,plain basins and other areas with highly recoverable groundwater and highly developed society and economy, Groundwater plays an important role in supporting the economy and ecological environment in these zones; (2) Groundwater conserva-tion zones (in which groundwater sustains the ecological environment). Zones of this type are widely distributed, such as the Qinghai-Tibet Plateau, the Indian Peninsula in south Asia, and the Eastern Siberian Plain. In these zones, groundwater mainly acts as ecological barrier and maintains the ecological environment. (3) Fragile ecological zone with deficient groundwater. Zones of this type are mainly distributed in the vast desert basins in Central and West Asia with dry climate and less precipitation. In most of these zones, the groundwater resources are deficient and difficult to develop and utilize. Therefore, groundwater plays a minor role in supporting society (Table4 and Fig.2).

Fig.2 Map showing the groundwater ecological function zoning in Asia

Table4 Ecological function zones of groundwater in Asia

4 Analysis of groundwater and ecological protection in Asia

With the increasing shortage of water resources and the rising economic and social value of water,it is undoubtedly the right path that Asia should urgently take to place equal focus on the development & utilization of groundwater and the governance & protection of groundwater to realize integrated groundwater management. The management of groundwater resources and ecology is a complex and highly comprehensive task. Therefore, it is necessary to employ technical, economic, legal,and administrative means to reduce the harm of human production and living to groundwater, coordinate the relationship between human activities and groundwater, and prevent undesirable environmental and hydrogeological events during the development and utilization of groundwater. The purpose of better protection of groundwater resources and ecological environment will be achieved (Liu et al. 2011).

Currently, the major problems with groundwater management in Asia are as follows. (1) Although many Asian countries have formulated laws and regulations on groundwater (Table5), they are poorly systematic, applicable, and practical. There are no targeted regulations on groundwater pollution in some countries. (2) Some countries lack the unified groundwater management of authority, and the functions of departments are overlapped and separated. So it is difficult for them to work out water resource planning; (3) Over a long time in the past,surface water and groundwater were always separately considered, and unified planning was unavailable for the planning of the development, utilization and management of water resources, thus leading to unreasonable exploitation and utilization and chaotic groundwater management.

Table5 Groundwater-related laws and regulations in some Asian countries

Countermeasures for the above problems are as follows. (1) Improve relevant systems for groundwater protection. It is necessary for Asian countries to establish authorities for water management based on their specific conditions and to make efforts to take management measures emphasizing both the development & utilization and the governance & protection of groundwater. The purpose is to achieve unified planning and management of groundwater and surface water, to ensure rational development, utilization, and effective prevention of groundwater pollution. (2) All Asian countries should jointly establish an effective network system to identify and share information on groundwater resources and river basins while managing their groundwater resources independently. The aim is to mutually supervise and learn advanced experience from each other and avoid one-sided understanding of the resources and environment in watersheds, which may affect long-term overall planning of cross-watershed control of environmental pollution. (3) Make efforts to follow new and effective ways to develop, utilize, and manage water resources. These new and effective ways are(i) Establish a water-saving social and economic structure system (including water-saving agricultural and industrial production systems and watersaving urban and rural resident living systems),adjust and optimize the industrial structure, and plan production and city development based on water. The purpose is to achieve the rational distribution of the limited water resources in the economic system and maximize the benefits of water resources. (ii) Conduct artificial regulation and storage of groundwater by converting more water resources such as surface water into groundwater using underground storage spaces. (iii) Turn wastewater into resources. Wastewater can be turned into usable water resources after water treatment, which can reduce pollution, protect the environment, increase water resources and alleviate the water shortage crisis. The treated wastewater can be used as non-potable water, such as industrial circulating water, agricultural water, and water for urban greening and sanitation. (iv) Implement quality-based water supply and water recycling. The so-called shortage of water resources is essentially the lack of high-quality water. Therefore, the quality-based water supply shall be implemented based on the water quality requirements imposed by different uses (such as water for industry, agriculture, or living) to ensure highquality water can be properly used. Meanwhile,unified planning and reuse should be implemented for inter-industrial water utilization to eliminate inter-industrial barriers and promote multiple water uses. (v) Transfer water among watersheds. In the case that the water resources in one region cannot meet the need for life and production after optimal allocation, transferring surface water from watersheds with surplus water resources is an option.However, the cross-watershed water transfer projects are complex, influential and costly. They require the construction of numerous water conservancy facilities and will change the original conditions for water balance, thus affecting the climate,ecology, and environment. Therefore, it is necessary to conduct complete surveys, detailed research,and scientific prediction and demonstration (Zeng et al. 2021); (vi) Enhance scientific researches on water resource management. Based on the comprehensive consideration of water resource systems and social and economic development, it is necessary to build practical water resource management models as well as the information-linkage systems,decision support systems, early warning systems and so on by using systematic engineering methods. The purpose is to gradually establish and improve a scientific water resource management and control system (Wang, 2011). (vii) Popularize the general knowledge on groundwater protection to enhance the awareness of governments, enterprises, and the public on the protection of the groundwater environment (Ma et al. 2021).

5 Conclusions

This paper focuses on the research and exploration into the rules of groundwater resources and environments on a continental scale and establishes the macro-scale research concepts conforming to the actual situations in Asia.

(1) The groundwater in Asia was divided into 11 primary groundwater systems primarily according to continental-scale geological structures, climate,landforms, and hydrogeological structure using the system theory. Specifically, the principle that groundwater systems should be determined in line with the collectivity, relevance, and macro purposiveness of system elements was followed, the regional natural geographical zoning was taken into account, and the natural attributes of climate and landforms and the spatial distribution of main aquifer systems in hydrogeological tectonic domains were considered. Furthermore, the principle that groundwater systems should be relatively independent and has complete water-cycle, hydrodynamic,and hydrochemical systems was followed. Besides social attributes were also covered to facilitate the evaluation and management of groundwater resources on the continental scale.

(2) Based on the theory on groundwater seepage, this paper discovered the water storage characteristics of aquifer systems and analyzed the conversion relationships among atmospheric precipitation, surface water, and groundwater. Meanwhile,the differences in underground seepage fields between continuous aquifers in unconsolidated sediments and intermittent or sporadic aquifers in bedrock fissures and Karst fissures (caves) were determined using water storage characteristics and seepage conditions of underground aquifer systems. By referring to the groundwater resource evaluation results of various regions (countries), natural groundwater recharge was rationally assessed using the natural groundwater recharge (runoff)modulus in a unified manner, thus providing reliable data for the study on groundwater resources in the world.

(3) Based on the complete understanding of groundwater aquifer systems structure, the groundwater cycle processes, and the spatial distribution regulation of the surface ecosystems in Asia, this paper conducted the ecological function zoning of groundwater, aiming to provide guidance for regional geological environment protection, industrial layout, water resource utilization, and economic and social development planning and to improve ecological functions of groundwater.

(4) The impacts of groundwater changes on the ecological environment in Asia were analyzed.Relevant measures were put forward to achieve the rational utilization of groundwater resources, the protection of the ecological environment, and the establishment of a harmonious relationship between human beings and natural resources and environment. Meanwhile, the relationships of groundwater with surface ecosystems, geological environments, and social and economic development were revealed to promote the sustainable development of the regional economy and society.

Acknowledgements

This study was supported by Map Series Compilation of 1 : 5 000 000 Asian Geological Environment and Investigation of Major Geological Environmental Issues (12120111200137); Maps Series Compilation of Groundwater Resources and Geological Environment in China and Its Surrounding Regions (12120113014200), and Cooperative Mapping of Hydrological and Environmental Geology in Five Central Asian Countries (DD20160106).