Hou Guangcai ^a,b,Liang Yongping ^c, Yin Lihe ^d, Su xiaosi ^a, Tao Zhengping ^b,Zhao Zhenghong ^b, Yang Yuncheng ^b, Wang Xiaoyong ^b

a. College of Environment and Resources, Jilin University,Changchun, 130026, P.R. China

b. Xi’an Institute of Geology and Mineral Resources, Xi’an, 710054, P.R. China

c. Institute of Karst Geology, CAGS, Guiling, 541004, P.R. China

d. China University of Geosciences, Beijing, 100083, P.R. China

Abstract: The Ordos Basin is a large-scale sedimentary basin in Northwestern China. The hydrostratigraphic units from bottom to upper are pre-Cambrian metamorphic rocks, lower-Paleozoic carbonate rocks, upper-Paleozoic to Mesozoic clastic rocks and Cenozoic deposits. The total thickness is up to 6000m. Three groundwater systems are present in the Ordos Basin, based on the geological settings, i.e. the karst groundwater system, the Cretaceous clastic groundwater system and the Quaternary groundwater system. This paper describes systematically the groundwater flow patterns of each system and overall assessment of groundwater resources.

Key Words: groundwater system, circulation and evolution, groundwater resources, the Ordos Basin

1 Introduction

The Ordos Basin is located in the east of northwestern China with the area of about 270 000 km² and extends to five provinces, i.e. Shaanxi, Gansu, Ningxia, Inner Mongolia and Shanxi(Fig.1). As one of the large scale groundwater basins in the world (Habermehl, 1980; Brinkmann et al., 1987; Herczeg et al., 1991 ;Habermehl, 1996 and Li et al., 1995), Ordos basin is composed of different aquifer systems. Karst water is stored in the Carmbrian-Ordovinian carbonates occurred along the outer margins of the basin; fissured-pore water is buried in the Cretaceous strata in the central-west basin and pore water is stored in Quaternary deposits discontinously overlying the Carboniferous and Jurassic clastic rocks. All of these aquifer systems are superimposed together vertically and contacted laterally. As the result of erosion locally by the drainage system, there is a close hydraulic connection among these aquifer systems, which makes

the Ordos basin a semi-open groundwater basin.

 

 

 

The Ordos Basin contains a wealth of natural resources including coal, petroleum, and natural gas and it is one of the most important bases for China’s energy and heavy chemical industries in the 21st century (Hou et al., 2004). Unfortunately, because of arid and semi-arid climate and rare precipitation and strong evapotranspiration, there is a severe shortage of water resources in the Ordos Basin that has hampered industrial development and eco-environmental improvement and adversely affected people’s livelihood. This paper describes systematically the major features of surrounding Carmbrian-Ordovinian Karst Aquifer System (KAS), the Cretaceous Aquifer System (CAS) and the Carbonate-Jurassic and Quaternary Aquifer System (C-JAS and QAS) and the groundwater circulation patterns in these aquifer systems are also discussed. Additionally the groundwater resources and its exploitation in the Ordos basin have been evaluated.

Because of regional distribution of these aquifer systems, shallow buried-depth, rich productivity, good water quality and suitability for exploitation, these aquifer systems can be served as the important target aquifers to supply plentiful groundwater for mineral exploration and energy base construction(Liu, et al., 1996). Rational development of groundwater resources will significantly mitigates the conflict between supply and demand of groundwater.

2 The Characteristics of Aquifer Systems in the Ordos Basin

The Ordos basin is dominantly a large scale synclinal sedimentary basin composting of different kinds of rock from Mesozoic to Paleozoic era with the N-S length of 620 Km and E-W width of 400 Km.

The Ordos Basin is also an asymmetric syncline. The east limb is a flat monoclinal structure dipping westward with the angle of 1~2⁰ and the west limb is made up of many faults striking north-south, which forms the mountain ridges striking north-south. Besides the margin of the southern margin is Weibei uplift, north to Weibei uplift is the monoclinal declining northward and south to weibei uplift is the stepped depressions. Yimeng uplift in the north basin has been rising since Paleozoic, which results in lack of lower Paleozoic strata and the metamorphic basement is outcropped in the northwest of the uplift and contacted with Hetao basin by faults. The basement of the Ordos basin is mainly composed of Precambrian crystalline and metamorphic rocks and the lower Paleozoic carbonate rocks ,upper Paleozoic –Mesozoic clastic rocks and Mesozoic and Cenozoic rocks are sedimented in basin with the total thickness of more than 6000 m. The general geological structure of the Ordos basin makes it a large groundwater basin consisting of multiple aquifer systems. According to theory of groundwater systems (Toth, 1963; Ma et al., 2002; Wang et al., 2005 and Liu et al., 2005), the water-bearing unity with the same media and close hydraulic connections is often thought as an aquifer system, the Ordos Basin can be divided into3 aquifer systems as shown in Fig.2.

2.1 The karst aquifer system of cambrian-ordovician carbonate rocks (KAS)

The karst aquifer system of Cambrian-Ordovician carbonate rocks is mainly distributed in U shape along the surrounding margins of the Ordos basin with covering area of about 735 000 km². The KAS is confined by the underlying Precambrian crystalline rocks at the bottom and the overlying Carboniferous aluminous shale at the upper. The karst water is stored in the fissures and karst occurred in the carbonate rocks (Hai, 2001). According to the geological and hydrogeological structures, KAS can be categorized into three patterns, i.e. the monoclinal pattern, the stepped fault pattern and the fault pattern in the east, in the south and in the west of Ordos basin respectively (Liang et al., 2004, Liang et al., 2005).

Monoclinal pattern in the eastern basin: It is distributed along the west edges of Lulian Mt.,such as Tianqiao, Liuling and Yumenkou spring fields. Carbonate rocks generally occurs in monoclinal structure dipping slightly toward west and the depth of carbonate rocks can be up to over 1000m below the land surface in the west of Yellow river in Shannxi province. The permeability of KAS may be homogeneous and each spring field has the uniform hydrodynamic regime and hydrogeochemical regime as the result of occurrence of stratified or quasi-stratified aquifers. In natural state, more than 70% recharge is from precipitation and groundwater flows westward parallel to the occurrence of the strata. The discharge area is in the form of large spring in the sections where the aquifer is cut by the Yellow river and its tributaries. The highly yielding locations are mainly concentrated in the lower reaches in the groundwater-blocking sections by the confining bed in the west and most of wells in this area can be artisan with yielding capacity of 1 000 and 10 000 m³/d for single well and the maximum up to 50 000 m³/d.The water quality is good with TDS concentration normally less than 1g/L. Therefore, this pattern is an important water supply site.

Stepped fault pattern in the southern basin: Controlled by the faults, carbonate rocks are occurred in step downward or the form of alternation of graben and horst. Besides outcropping areas in north mountains, most rocks are buried more than 800-1200m below the land surface or deeper. The networked and veined aquifers are inhomogeneous and there is a uniform hydraulic (the potentiametric head of 380 m above sea level) and hydrogeochemical regime only in the east part north to Weihe uplift. Groundwater is mainly recharged by the seepage of surface water, accounting for 60% of the total recharge. Groundwater moves in the direction of Fenwei basin trend,which is inverse to the tilt of the aquifers and discharges as form of springs because of the fault blocking and also discharge as the large spring along river valleys of the Yellow river and its tributaries, such as Jing river and Luo river. Groundwater is mainly rich in river valleys or along faults at the foot of mountains with yielding capacity of 1 000 and 5 000m³/d for single well and the maximum up to 10 000 m³/d. The water quality is good with low TDS concentration. This area is also an important water supply site.

Belt fault pattern in the western basin: It is located between Zhuozi Mt. and Qianyang-Pengyang basin and the distribution of carbonate rocks in this area are separated by faults, which results in the occurrence of small scale spring domains. The veined aquifers are significantly inhomogeneous with non-uniform hydraulic and hydrogeochemicl regime. In some places with the favorable recharge conditions, such as the Zhuozi Mt. in inner Mogonia,Pengyang city in Ningxia province and Pingyang city in Gansu province., the yielding capacity of single well can be up to 1 000 and 5 000m³/d with good water quality. But in other places, the KAS has low productivity owing to weak yielding capacity.

2.2 The cretaceous aquifer system (CAS)

The Cretaceous aquifer system (CAS), located in the middle-west part of the Ordos Basin,is a sub-sedimentary basin overlying the Jurassic and pre-Mesozoic basin. The CAS is distributed in a rectangle shape with the length of 640 Km in north-south direction and the width of 200-265 Km in east-west. The total area of this basin is about 132,100 Km², bordered by Pingliang-Dengkou fault in west, the southern margin of Hetao fault in north, the contacted zone between the Paoan group and Jurassic strata in east and south. The Baiyu Mt. as the result of intermitted uplift of crust since late Cretaceous era has an obvious effect on the sedimentary environment, lithology-structure, litho--paleo-geography and hydrogeological conditions of the Cretaceous basin, which results in an obvious difference between the south part and north part of the Cretaceous basin. Bordered by the Baiyu Mt., the CAS can be divided into two separate parts (Xie et al., 2003 and Sun et al., 2004), desert plateau aquifer system (DPAS) in the north and loess plateau aquifer system (LPAS) in the south.

DPAS: DPAS is distributed in desert plateau north to the Baiyu Mt. and is mainly made up of alluvial sandstones and gravels with cementation and semi-cementation, loose structure, simple lithology , less content of mud and unobvious rhythmicity (Lu et al., 2004). No regional continuous and stable aquiclude makes a close hydraulic connections between shallow and middle-buried aquifers (less than 300m below the land surface ) and deep aquifers, which makes the CAS a uniform water-bearing system with the maximum thickness of more than 1000 m. Precipitation is the major recharge source and groundwater flow is vigorously controlled by topography and drainage systems. Groundwater flows from the surface divide (Dongsheng Ridge and Sishili Ridge) to rivers in the outside of basin (Dusitu river, Wuding river and Wulanmulun river). Groundwater is discharged by evaporation and seepage to surface water systems. The groundwater quality is good in most places. The shallow burial, large thickness, well recharge conditions and high yielding capacity make the CAS in north basin a good candidate for groundwater supply.

LPAS: The LPAS is distributed in loess plateau south to the Baiyu Mt with heavy erosion of surface water systems. The LPAS has high mud content and clear rhythmicity and it can be divided into three aquifer formations: the Luohandong aquifer formation, the Huanhe aquifer formation and the Luohe aquifer formation from top to bottom. The Luohe aqufier formation is made up of middle and fine sandstone with the thickness of 20~160 m , the buried depth of 400~800 m below the land surface, loose structure and high porosity, which makes the Luohe aquifer formation a good aquifer. Except in the east and south margin, the groundwater quality is good with TDS content of less than 1g/L, the groundwater quality is not good with TDS content of 1~3g/L in most places. Huanhe aquifer formation is mainly made up of mudstones of lucustrine facies and the dense cementation of mudstone makes it a regional aquitard/aquifuge. The concentration of TDS of groundwater in Huanhe aquifer formation is 2-4g/L due to high salt content in the media and slow circulation velocity. Luohandong aquifer formation consists of sandstone of desert and fluvial facies and distributed locally. Groundwater yield capacity and quality in Luohandong aquifer formation varies greatly spatially. Groundwater is recharged by surface water and lateral recharge and flows towards the rivers, such as river valleys of the Jinghe river and Luohe river. The LPGS is a typical artesian basin.

2.3 The carbonate-jurassic and quaternary aquifer system (C-JAS and QAS)

The Carbonate-Jurassic aquifer system (C-JQAS) is located between the karst groundwater system and the Cretaceous groundwater system with covering area of 64 700 km².Lithology is mainly alternative layers of sandstones and mudstones. Due to cementation, water yielding capacity is low. In the shallow or weathered area (50-100m depth), groundwater can receive recharge from precipitation, practically in river valleys where groundwater in Jurassic-Carboniferous is interconnected with groundwater in Quaternary (Liu et al., 2002). Therefore, groundwater has good quality and high yielding capacity in river valleys.

Quaternary aquifer is overlying the Jurassic-Carboniferous rocks with discontinues distribution. In the south of the Baiyu Mt., groundwater is mainly stored in loess and in the north mainly in unconsolidated deposits of Quaternary. Groundwater resources are fairly rich in unconsolidated deposit of Quaternary. Unconsolidated deposits of Quaternary mainly include siltstones of Pleistocene with 60-80 m thick. Groundwater in unconsolidated deposits of Quaternary is generally interconnected with groundwater in overlying Jurassic rocks. Yielding capacity of wells can be up to 1000-3000m³/d with TDS content less than 1g/L. It is the major groundwater supply site. Groundwater in loess is mainly stored in large loess plains. In other loess-covering areas, as the result of deep cut by rivers, groundwater system occurs locally and has less yielding capacity.

In addition, groundwater in Quaternary is present in the marginal basins, i.e. the Guanzhong Basin, the Yinchun Basin and the Hetao Basin as well as groundwater in fissured rocks in the Qing Mt., the Helai Mt. and the Yin Mt., which are beyond the scope of the paper.

 

 

3 Regional Groundwater Circulation and Evolution

The features of regional groundwater circulation and evolution have great effect on the attributes of groundwater resources and are important for local groundwater resources evaluation and eco-environmental protection. Regional groundwater circulation and evolution in the Ordos basin are mainly controlled by modern geomorphology, drainage systems and paleo-environment. Especially the tectonic activities and regional uniform uplift since Cenozoic era has resulted in the general geomorphology of typical modern plateau surrounded by mountains in east, south and west parts of the basin with the altitude of 1000~2800 m above sea level (a.s.l) in mountains and 1000~1700 m a.s.l in the main parts of the basin. And the Yellow river is passing through the western ,northern and eastern parts of the Ordos basin in “几” shape and the drop of Yellow river water level is about 800 with the water level of about 1120 m a.s.l when entering into the basin and 320 m a.s.l. when leaving the basin. Based on the above analysis, regional groundwater circulation patterns can be divided into the shallow active circulation system and the deep stagnant circulation system as shown in figure 3. The shallow active circulation system is involved with the Quaternary groundwater system, the Cretaceous System, karst groundwater system and the shallow Jurassic-Carboniferous groundwater system with the circulation depth of 100~1800 m. And the shallow active circulation system can further divided into shallow circulation sub-system, middle circulation sub-system and regional circulation sub-system. The deep stagnant system is involved with deep karst groundwater system and Jurassic-Carboniferous groundwater system with the circulation depth of 300~1800m below the land surface. Generally, groundwater is of meteoric origin and recharged by precipitation directly or undirectly. The groundwater is flowing to the local discharge surfaces in different aquifer systems and finally discharged to the Yellow river and its tributaries.

3.1 Groundwater circulation and evolution in KAS

Because the maximum buried depth of carbonate rocks is more than 4000 m and there is no regional fault as the groundwater conduit,it is impossible for the karst groundwater to flow from the western basin to the eastern basin through deep circulation based on the regional flow regime, hydrogeochemical and groundwater temperature regime. Modern karst groundwater circulation only occurs within the karst entity in the vicinity of the basin. According to the buried conditions and circulation features, from the margin to the central basin, three zones can be defined as shown in figure 4.

1). Actively renewing zone (I): This zone mainly distributes in the peripheral area where carbonate rock is outcropped or shallow-buried and karst groundwater has a close relationship with rainwater and surface water. Circulating rate of karst groundwater is high and the circulation depth can be up to 800m below local erosion basis. Karst groundwater temperature is 10-45℃, groundwater age is less then 10 000 years and groundwater type is dominated by bicarbonate with TDS content less than 1g/l. This zone is the main site for current groundwater exploitation

2). Slowly renewing zone (II): This zone is buried 800-1800 m deeper below local erosion surface with low groundwater renewing rate. Groundwater temperature in this zone is 40-75℃ and TDS ranging from 1.2 to 5.0g/l and dominated by the type of Cl·SO₄ and is of over 10 000 years old. Karst groundwater within this zone has not been exploited currently in most areas except where geothermal water being exploited.

3). Stagnant zone (III): the burial depth of this zone is 1800-2500 m below local erosion surface where groundwater is the connate water in petroleum and gas fields. Temperature, TDS content and chemical type are 90-105℃, 10-50g/l and chloride type respectively.

3.2 Groundwater circulation and evolution in CAS

Groundwater in CGS is mainly recharged by precipitation. Controlled by the higher topography than surrounding areas, there is no lateral recharge from the surrounding groundwater systems except in the eastern foothills of Liupan Mountain in the southwestern corner of the CGS. Recharge conditions mainly depend on the ground relief (surface water divides), vegetation and soil property of vodose zone. North basin has good recharge conditions and can receive direct rainwater recharge where the Cretaceous aquifer is outcropped or buried shallow.

However, south basin has worse recharge conditions and can not receive direct recharge from rainwater where the Cretaceous aquifer is underlain by 100m thick loess and 20m Neocene

mudstones,.

Groundwater circulation and evolution in CGS is mainly controlled by surface divides, recharge conditions and drainage systems. Piezometric surfaces of the groundwater in CGS are similar to ground surface in that the ground divide is also the divide of shallow groundwater and the ground depression is the discharge zone of shallow groundwater. The CGS can be divided into 5 independent groundwater systems by 4 surface divides, among which the Baiyu Mt. divide

separates the basin into the south and north parts with distinct features of circulation. In the north, groundwater receives recharge in the divides, flows to rivers (lakes). In the south, groundwater receives recharge at the aquifer outcropping area and flows to rivers. In order to well understand groundwater circulation and evolution on the basis of newly acquired hydrogeological and hydrogeochemical information, particularly data of water level, isotope and hydrogemchemistry from different depth taken by the Packer systems(Yang et al., 2005), three circulation patterns are summarized, that is, local, immediate and regional circulation systems as shown in Fig.5.

Shallow circulation system (III): This system occurs between ridges and nearby rives or lakes with circulation depth of 100-150m. This system is characterized by fast circulation, high renewal capacity and good water quality. Generally, groundwater age is less than 1000 years. For example, , CFC model ages of groundwater samples at 100 m and 170 m below the land surface in Borehole 14 are 14 years and 40 years respectively (Yang et al., 2004).

Intermediate circulation system (II): Compared with the shallow circulation system, intermediate circulation system is involved with more extent and deeper with the maximum circulation depth of up to 300m. The middle-scaled river and larger lakes are the main discharge zones. Groundwater age varies between several hundreds to several thousands years with majority younger than 1000 years.

Regional circulation system (I): In this system, groundwater flowing rate are extremely low and the circulation depth is between 500-1000m below land surface up to the lower bottom of Cretaceous aquifer. In the north basin, groundwater quality is good with TDS content less than 1g/L and groundwater age is within 10 000-20 000 years which indicates that groundwater was recharged before Holocene. In contrast, in south basin groundwater quality is bad with TDS content varying from 1 to 3g/L and groundwater age is between 10 000 to 30 000 years.

3.3 Groundwater circulation and evolution in C-JAS and QAS

Fissured groundwater circulation in C-JAS is mainly controlled by the strata-structure, lithology and the relief of the aquifer outcropping areas. Active groundwater circulation zone is only distributed in the aquifer outcropping area or shallowly buried areas with circulating depth of 50-1000m. The weak circulation zone is between 100-300m and the stagnant zone is below 300m. Groundwater in Quaternary Salawusu formation can receive direct recharge from precipitation and flow to nearby lakes. Due to short circulation path, groundwater age is within 20 years using CFC dating methods.

4 Groundwater Resources and its Potential

Groundwater resources evaluation in the Ordos basin by water balance and numerical simulation methods shows that the amount of total recharge resources, available resources and the current exploitation are 10.148 ×10⁹ m³/a ,5.123×10⁹ m³/a and

 

1.036×10⁹ m³/a respectively and the amount of potential resources in the Ordos basin is 4.088×10⁹ m³/a(Table 1 and Figure 2).

In KAS, the amount of total natural resources are 1.345 ×10⁹ m³/a and the amount of available resources is 1.076×10⁹ m³/a. the current mining rate is 0.198×10⁹ m³/a，and the potential is 0.879×10⁹ m³/a. The greatest potential is in the Tianqiao spring domain and the Fuping-Wanrong system.

In CAS, the amount of total natural resources are 7.133 ×10⁹m³/a and the amount of available resources is 3.628×10⁹ m³/a. The current mining rate is 0.729×10⁹ m³/a and the potential is 2.899×10⁹ m³/a. The greatest potential area is located in the north of CGS and Jingbian of Shaanxi province and bordering area of shaanxi and Inner Mongolian.

In C-JAS and QAS, the amount of total natural resources are 1.67×10⁹ m³/a and the amount of available resources is 0.419×10⁹ m³/a. The current mining rate is 0.109×10⁹ m³/a and the potential is 0.31×10⁹ m³/a. The greatest potential area is located in desert covering area and large river valleys.

 

Table 1 Groundwater resources in the Ordos Basin (10⁹ m³/a)

Groundwater system	Area (104km2)	Natural resources	Available Resources	Current mining rate	potential
KGS	7.35	1.345	1.076	0.198	0.879
CGS	13.21	7.133	3.628	0.729	2.899
CQGS	6.47	1.670	0.419	0.109	0.310
Total	27.03	10.148	5.123	1.036	4.088

 

 

 

 

 

 

5 Conclusions

1. The Ordos basin is dominantly a large scale synclinal sedimentary basin composting of different kinds of rock from Mesozoic to Paleozoic era. Based on the water-bearing media and hydraulic features, the groundwater system of the Ordos basin can be divided into Karst Aquifer System of Carmbrian-Ordovinian carbonates (KAS), the Cretaceous Aquifer System (CAS) and the Carbonate-Jurassic and Quaternary Aquifer System (C-JAS and QAS). All of these aquifer systems are superimposed together vertically and contacted laterally. As the result of erosion locally by the drainage system, there is a close hydraulic connection among these aquifer systems, which makes the Ordos basin a semi-open groundwater basin.

2. Groundwater is of meteoric origin. Groundwater circulation and evolution are mainly controlled by geomorphology, drainage system and

paleo-environment. Generally, groundwater flows from the recharge area to the Yellow river and its tributaries. Circulation depth is different in different systems. The maximum circulation depth is 1200-1800 m,500-1000 m and less than 300m below the local erosion basis surface for KAS,CAS ,C-JAS and QAS respectively. Knowledge about the groundwater circulation and evolution will benefit to understand the attributes of groundwater resources, to evaluate the groundwater resources and protect the eco-environment.

3. Groundwater resources are rich in the Ordos basin and the current mining rate is low. With the development of the national energy base, large amount of groundwater will be exploited progressively. In order to make sustainable development and prevent geo-environmental problems, rational development strategies are made to meet the need of sustainable development of resources, environment and society.