Nguyet, Vu T. M._hydrogeological Characterisation and Groundwater Protection of Tropical Mountainous Karst Areas in NW Vietnam

8/11/2019 Nguyet, Vu T. M._hydrogeological Characterisation and Groundwater Protection of Tropical Mountainous Karst Area

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Hydrogeological Characterisationand Groundwater Protection

of Tropical Mountainous Karst areasin NW Vietnam

by

Vu Thi Minh Nguyet

Department of Hydrology and Hydraulic Engineering

V U B HYDROLOGIE (48)

2006

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This dissertation is dedicated to Mr. Thai Duy Ke,

our respectful, good-hearted, beloved colleague and great friend,

who is always alive in our memory.

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Acknowledgments

This thesis owes much to the help and support of many people, all of whom have contributed

in different ways.

First of all, I would like to express my sincere gratitude and appreciation to my promotors,

Prof F. De Smedt and Dr. N. Goldscheider, for their valuable guidance, fruitful discussions

and consistent support that made it possible for me to finish this work. I am grateful to

Dr. O. Batelaan for his suggestions, practical help and support on my work over many years.

I would like to thank the Directorial Board of the Research Institute of Geology and Mineral

Resources (RIGMR) for the strongly support on my work; to Prof. Duong Duc Kiem,

Pham Binh, Nguyen Tam, Dang My Cung and many senior researchers and colleagues at

RIGRM for the professional advice, supporting data and for their assistance in fieldtrips; to

the local people in the Son La and Tam Duong areas who helped me with spring monitoring,

tracer sampling and other help during the fieldtrips.

I am grateful to jury members: Prof. J. Wastiels, Prof. J. Vereecken, Prof. F. De Smedt,

Prof. W. Bauwens, Prof. E. Keppens, Dr. O. Batelaan (Vrije Universtiteit Brussel),

Dr. N. Goldscheider (Universit de Neuchtel, Switzerland), Prof. R. Swennen (Katholieke

Universiteit Leuven) and Dr. M. Dusar (Belgian Geological Survey) for their willing review,

valuable and helpful suggestions to improve this thesis. I thank to Dr. Michael Whitburn for

his help on English correction.

Special thanks to the Belgian Technical Cooperation, the Vietnamese-Belgian Karst Project,

and the Swiss Commission for Scholarship for partly provided financial support of my work.Thank to Mrs. Daphne Windey, Dr. Paul Verle (Belgian Technical Cooperation in Brussels)

and all people at the Belgian Technical Cooperation in Hanoi for the willing logistic support.

A special thanks goes to Dr. Koen Van Keer for his fully support and practical help on my

work from initial stage and during difficult moments.

I want to thank the professors and assistants working at the Centre of Hydrogeology,

University of Neuchtel, for giving their knowledge on karst hydrogeology and laboratory

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experience to me. Thanks to Mariona, Alessandro and many other friends for the

unforgettable time we spent together in Switzerland.

Many thanks to my Vietnamese friends in VUB and other Universities/cities for the pleasant

time we spent together to study in Belgium. Special thanks to my dear friends, whom I cannot

mention here for understanding and support.

Many thanks to my parents for their patience and give me encouragement during my study.


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Abstract

In NW Vietnam, karst areas cover nearly 18% of the land surface and have substantial socio-

economic importance as groundwater resources, as well as zones for forestry, agriculture andtourism. In many areas, however, both the karst landforms and the groundwater resources

have recently come under pressure in response to urbanisation, economic development and

increase of population. Karst aquifers are particularly vulnerable to contamination resulting

from human activities. Karst groundwater consequently requires special protection. A sound

knowledge of the hydrogeological system is a precondition for any protection strategy. Such

understanding, however, is presently lacking in Vietnam.

This work aims at better understanding the hydrogeological characteristics of the tropicalkarst regions in Vietnam and providing a scientific basis for groundwater protection. The

study focuses on two major mountainous areas that belong to the NW karst belt: Son La and

Tam Duong, which mainly consist of thick Middle Triassic carbonate-rock formations. An

investigation methodology has been applied and adapted to the conditions of the remote areas,

for which little information is available. The employed methods included tracer tests,

hydrodynamic, hydrochemical and microbiological spring monitoring, as well as stable

isotope and rare earth elements studies.

Tracer tests proved underground connections between several swallow holes and springs in

the two test site areas. The NW-SE and SW-NE faults have a great influence on the

underground drainage patterns. The flow paths run either across the folds along the SW-NE

faults or follow the NW-SE faults; these flow paths coincide with the preferential directions

of cave development.

Groundwater mixing effects can be observed in both areas. Hydrochemical data from Son La

show a significant difference in the Mg 2+ and Ca 2+ contents between a swallow hole and a

connected spring, which can be explained by mixing effects. Stable isotope results further

support this observation. The high stability of 18O of karst springs in the Nam La valley (Son

La) compared with meteoric water also indicates that this karst system contains well-mixed

groundwater. The hydrochemical results from the Tam Duong area show a difference in Mg 2+

and Ca 2+content between a swallow hole and a connected spring, which also can be explained

by the mixing effect. The little variation in chemical content along the flow path compared to

the Son La area may reflect the reduced waterrock interaction in this karst system.

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Large karst springs are observed in Son La, while smaller karst springs occur in Tam Duong.

The results obtained from this study suggest that concentrated recharge prevails in the Tam

Duong area, while the recharge processes and groundwater flow in the Son La area appear to

be more complicated. There is evidence for point recharge and conduit flow on one hand, but

also for significant diffuse recharge and flow through small joints and fractures on the other

hand.

Tracer tests in the Son La area gave groundwater flow velocities ranging from 75 to 166 m/h.

These are typical values for karst aquifers and indicate low-resistance flow paths. The flow

velocities in the Tam Duong area are up to 700 m/h, which is one of the highest values

recorded in the literature. The two investigated springs near Tam Duong show a different

hydrological and physical-chemical response on precipitation events. A dilution effect wasobserved at one karst spring, while the other spring displayed a piston effect.

The physical-chemical parameters of all sampled karst water in both areas meet the WHO

standards for drinking water. The REE concentration levels found in spring water from Tam

Duong are higher than those from other karst areas reported in the literature but still safe for

the health of the consumers. In contrast, the microbial investigation revealed that all karst

water contain high levels of thermotolerant coliforms (TTC). The contamination shows high

temporal fluctuations and mainly results from untreated domestic wastewaters, agriculture and

other human activities.

In order to protect the valuable groundwater resources in Vietnamese karst areas, a simplified

methodology for mapping groundwater vulnerability and contamination risk was developed

and first applied in the test sites. It is based on a conceptual framework proposed by the

European COST Action 620. The vulnerability map takes into account the overlying layers

(O) and the flow concentration (C). The risk map is obtained by a combination of the

vulnerability map and a simplified hazard assessment. The maps provide a basis for land-use

planning and groundwater protection zoning. Groundwater protection should be a priority in

vulnerable zones such as swallow holes and along sinking streams.

The work gives details and an insight into the understanding of karst hydrogeological

characterization in the Son La and Tam Duong areas. The methods applied in this work

constitute useful tools for the hydrogeological investigation of remote and mountainous

tropical karst areas in Vietnam and made it possible to provide a scientific basis for

sustainable groundwater management.

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Table of contents

Acknowledgments ...............................................................................................................i

Abstract..............................................................................................................................iii

Table of contents ................................................................................................................v

List of figures ....................................................................................................................ix

List of tables ....................................................................................................................xiii

1 Introduction .................................................................................................. 1

1.1 Karst in tropical regions ..........................................................................................1

1.2 Karst hydrogeological research in Vietnam...........................................................2

1.2.1 Overview of karst in Vietnam ............................................................................2

1.2.2 Importance of karst hydrogeology study............................................................3

1.3 Objectives and structure of the study.....................................................................4

1.4 Research collaboration.............................................................................................6

2 Study area-the NW karst belt ..................................................................... 7

2.1 Geography .................................................................................................................7 2.1.1 Location and topography....................................................................................7

2.1.2 Climate ...............................................................................................................7

2.1.3 Social and economic conditions .........................................................................8

2.2 The geology of the NW karst belt............................................................................9

2.2.1 Overview of geological setting...........................................................................9

2.2.2 Tectonics...........................................................................................................10

2.3 Principles of hydrogeological characterization of karst aquifers ......................11 2.4 Karst landform .......................................................................................................14

2.4.1 Definition of tropical karst landforms ..............................................................14

2.4.2 Karst landscapes in NW Vietnam.....................................................................15

3 Methods and techniques ............................................................................ 19

3.1 Tracing experiment ................................................................................................19

3.1.1 Tracing in karst study .......................................................................................19

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3.1.2 Tracer breakthrough curve ............................................................................... 20

3.1.3 Traci95 Programme.......................................................................................... 22

3.1.4 Tracing tests in Son La and Tam Duong areas ................................................ 23

3.2 Hydrochemical investigation................................................................................. 24

3.2.1 Overview.......................................................................................................... 24

3.2.2 Hydrochemical investigation in the test sites................................................... 25

3.3 Microbiological investigation ................................................................................ 26

3.4 Stable isotope study................................................................................................ 27

3.5 Rare earth elements study..................................................................................... 29

4 Hydrogeology of the Son La karst area....................................................31

4.1 Location, landscape and climate........................................................................... 31

4.2 Overview of previous studies ................................................................................ 32

4.3 Geology.................................................................................................................... 33

4.3.1 Geological framework and stratigraphy........................................................... 33

4.3.2 Stratigraphy...................................................................................................... 34

4.3.3 Tectonics.......................................................................................................... 36

4.3.4 Hydrogeology, spring and surface water ......................................................... 384.4 Tracer tests ............................................................................................................. 41

4.4.1 Tracer tests ....................................................................................................... 41

4.4.2 Tracer sampling and analysis ........................................................................... 43

4.4.3 Results .............................................................................................................. 43

4.4.4 Discussion........................................................................................................ 47

4.5 Hydrochemistry...................................................................................................... 50

4.5.1 Hydrochemistry and karst water quality .......................................................... 504.5.2 Oxygen isotope ................................................................................................ 53

4.6 Conclusion .............................................................................................................. 57

4.6.1 Hydrogeology and underground flow paths..................................................... 57

4.6.2 Hydraulic properties and groundwater quality................................................. 57

4.6.3 Groundwater mixing ........................................................................................ 58

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5 Hydrogeology of Tam Duong karst area.................................................. 61

5.1 Location, topography and climate ........................................................................61

5.2 Overview of previous studies.................................................................................62 5.3 Geology ....................................................................................................................63

5.3.1 Geological framework ......................................................................................63

5.3.2 Stratigraphy ......................................................................................................63

5.3.3 Tectonics...........................................................................................................65

5.3.4 Hydrogeology, spring and surface water ..........................................................67

5.4 Tracer experiment ..................................................................................................69

5.4.1 Overview ..........................................................................................................695.4.2 Injection and sampling points...........................................................................70

5.4.3 Tracer analysis ..................................................................................................71

5.4.4 Results ..............................................................................................................71

5.4.5 Discussion.........................................................................................................74

5.5 Hydrochemistry and microbiology .......................................................................76

5.5.1 Overview ..........................................................................................................76

5.5.2 Sample collection .............................................................................................76

5.5.3 Sample analysis ................................................................................................76

5.5.4 Results ..............................................................................................................77

5.5.5 Discussion.........................................................................................................80

5.6 Rare earth elements (REE) study..........................................................................84

5.6.1 Sampling and analytical techniques .................................................................84

5.6.2 Results and discussion ......................................................................................85

5.7 Conclusion ...............................................................................................................92

5.7.1 Point recharge, fault tectonics and underground flow path ..............................92

5.7.2 Dynamics and interaction of the hydrochemical and microbiological

parameters.........................................................................................................................93

5.7.3 Groundwater quality .........................................................................................94

6 Karst Groundwater Vulnerability and Risk Mapping........................... 97

6.1 The European approach: COST 620 ....................................................................97

6.1.1 Introduction ......................................................................................................97

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6.1.2 Definitions of groundwater vulnerability, hazard and risk .............................. 97

6.1.3 The origin-pathway-target model..................................................................... 99

6.2 Methodology adaptation...................................................................................... 100

6.2.1 (General) proposed methodology................................................................... 100

6.2.2 Groundwater vulnerability ............................................................................. 101

6.2.3 Hazard and risk .............................................................................................. 103

6.3 Application in the Tham Ta Toong area............................................................ 104

6.3.1 Introduction.................................................................................................... 104

6.3.2 Groundwater vulnerability mapping .............................................................. 106

6.4 Application in Tam Duong area ......................................................................... 107

6.4.1 Introduction.................................................................................................... 107

6.4.2 Groundwater vulnerability mapping .............................................................. 108

6.4.3 Hazard assessment, risk mapping and validation .......................................... 110

6.5 Discussion on applicability of the methodology ................................................ 111

7 Conclusions ...............................................................................................113

7.1 Karst hydrogeological characterisation ............................................................. 113

7.1.1 Groundwater flow path and groundwater mixing effect ................................ 1137.1.2 Hydraulic properties....................................................................................... 114

7.1.3 Karst water quality ......................................................................................... 115

7.2 Groundwater protection...................................................................................... 115

7.3 An investigation methodology............................................................................. 116

7.4 Recommendations ................................................................................................ 117

References ........................................................................................................121

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List of figures

Fig. 1.1: Karst areas of Vietnam (modified after Dusar et al. 1994) with location of the test

sites ...............................................................................................................................3

Fig. 2.1: Tectonic framework of NW Vietnam (modified after Tran Van Tri et al, 1979)

and location of the test sites..........................................................................................9

Fig. 2.2: Shallow and deep karst systems with regard to the position of the base level (Bgli,

1980)...........................................................................................................................12

Fig. 2.3: Recharge into carbonate aquifers (Gunn, 1986) ........................................................12

Fig. 2.4: Conductivity scale-effect in karst system (Kiraly, 1975) ..........................................13

Fig. 2.5: Interpretation of a karst spring hydrograph and chemograph (Ford and Williams,1989)...........................................................................................................................14

Fig. 2.6: Geographical location of main cities/towns in Northern Vietnam ............................16

Fig. 2.7: Peak cluster depression karst landscape in NW Vietnam ..........................................17

Fig. 2.8: Peak forest karst landscape in NW Vietnam..............................................................17

Fig. 3.1: Tracer breakthrough curve and residence times.........................................................21

Fig. 3.3: The portable microbial Lab Oxfam-DelAgua with main consumables (Photo by

Oxfam-DelAgua)........................................................................................................27Fig. 4.1: Son La karst landscape, view from Son La pass to the SW.......................................31

Fig. 4.2: Monthly rainfall (mm) in Son La (collected data in Son La station from 1974-1998)

....................................................................................................................................32

Fig. 4.3: Geological map and geological cross sections of the Son La area (modified after

Vibekap, 2003). ..........................................................................................................37

Fig. 4.4: Karst aquifers, springs and surface water in the Son La area. ...................................38

Fig. 4.5: Spring hydrograph of Nam La River measured at Ban Toong village and Hang Doi

spring in 2000 (VIBEKAP data). ...............................................................................40

Fig. 4.6: Tracer location and proven groundwater flow connections.......................................44

Fig. 4.7: Measured tracer concentrations at Ban Sang spring for the February 2000 test and

theoretical breakthrough curves modelled using Traci95 (left uranine, right salt). ...45

Fig. 4.8: Measured tracer concentrations at the Long Ngo spring for the test in October 2000

and theoretical breakthrough curves modelled using Traci95....................................46

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Fig. 4.9: Measured uranine concentrations at the Hang Doi spring during the October 2001

test, and theoretical breakthrough curves modelled by using Traci 95...................... 46

Fig. 4.10: Measured electrical conductivity (EC) at the Long Ngo spring and rainfall recorded

at the Son La station during the October 2000 tracer test.......................................... 49

Fig. 4.11: Piper diagram of karst rivers systems in the Son La area ; the black triangle symbol

presents for the Nam La River water system; the grey cycle symbol is for the Suoi

Muoi River water system. .......................................................................................... 51

Fig. 4.12: Species of dissolved inorganic carbon as function of pH (Fetter, 2001)................. 52

Fig. 4.13: Location of sampling stations for isotope study in the Nam La River area, Son La

province...................................................................................................................... 53

Fig. 4.14: Oxygen isotope composition of rainfall, river and spring water at the Nam La

valley, Son La (July-October, 2002).................................................................... 56

Fig. 4.15: Influence of oxygen isotope composition of rainfall water on the Nam La River

water........................................................................................................................... 56

Fig. 4.16: The Mg 2+ versus Ca 2+ concentrations at swallow holes and connected springs in the

Son La area; the dot lines indicate the existence of underground flow connections,

which was proven by the tracer test. Flow connection 1: Ban Lay-Long Ngo, flow

connection 2: Nha Tu-Hang Doi, flow connection 3: Tham Han-Ban Sang. ............ 58Fig. 5.1: View of the test site from NE (left) and from SW (right). ........................................ 61

Fig. 5.2: Measured precipitation and temperature in the Tam Duong area from 1996 to 2000

(reference data: Japanese Mining Project, 2002)....................................................... 62

Fig. 5.3: Geological map and geological cross section in the Tam Duong area (modified after

VIBEKAP, 2003). The number 1 represents Dau Nguon Sin Ho spring; and number

2 represents Nha May Che spring. The symbols I, II and III represent Nam So, Lan

Nhi Thang-Hong Thu Man and Yen Chau faults respectively. ................................. 65Fig. 5.4: Karst aquifer, springs and surface water at the Tam Duong area (same area as Fig.

5.3); number 1, 2 as on Fig. 5.3; the Lo Gach, Nam Loong, C320 and Lai Chau army

springs are represented by the number 3, 4, 5 and 6 respectively. The Tam Duong

and Nung Nang streams are mapped on the basis of the field observations.............. 68

Fig. 5.5: Tracer location and proven groundwater flow connections (detail from Fig. 5.4)... 72

Fig. 5.6: Measured tracer concentrations at spring 1 (left) and spring 2 (right) and theoretical

breakthrough curves simulated using Traci 95. ......................................................... 73

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Fig. 6.7: Hydrogeological map of the test site. The estimated catchemnt area of two main

karst springs, groundwater flow paths and other karst features are also presented in

the figure. ................................................................................................................. 108

Fig. 6.8: O and C map of the test site. The resulting vulnerability map is shown in Fig. 6.9.

.................................................................................................................................. 109

Fig. 6.9: Vulnerability, hazard and risk maps for the Tam Duong test site. Both the tracer test

results and the high contents of bacteria in spring 1 confirm the vulnerability and risk

assessment near swallow hole 1............................................................................... 111

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List of tables

Table 4.1: Stratigraphic table of the Son La area corresponding to mapsheet in Fig. 4.3........34

Table 4.2: Summary of tracer experiments in the Son La karst area, Son La province ...........42

Table 4.3: Overview obtained tracer results and estimated hydraulic parameters of karst

groundwater flow paths in the Son La area. ...........................................................47

Table 4.4: Physical properties and major ions content (mg/l) of karst water in the Son La area

(VIBEKAP data) ....................................................................................................50

Table 4.5: The 18O of meteoric water, river and karst spring water at the Nam La valley,

Son La (July-October 2002) (location: Fig. 4.13) ..................................................54

Table 4.6: The molar [Mg 2+]/[Ca 2+] ratios for swallow hole and connected spring waters

from the Son La area ..............................................................................................59

Table 5.1: Stratigraphical table of the Tam Duong area..........................................................64

Table 5.2: Tracer results and estimated hydraulic properties from tracer experiments at the

Dau Nguon Sin Ho spring (spring 1) and Nha May Che spring (spring 2)............74

Table 5.3: Microbial contamination and major ions content in 15 karst springs which are used

for drinking water in the Tam Duong area, and the WHO standards. The

bicarbonate was calculated by using AquaChem 4.0. ............................................77Table 5.4: The molar [Mg 2+]/[Ca 2+] ratios for swallow hole and connected spring waters

from the Tam Duong area.......................................................................................81

Table 5.5: REE, Sc and Y concentrations (ppb) of Triassic limestone from the Tam Duong

and Nam Son areas. ................................................................................................86

Table 5.6: Average of 9 rare earth elements concentration (ppb) in Triassic carbonate rocks

from Tam Duong, and Nam Son in compared to other carbonate rocks from

Dinant and southern Nevada; Dinant data are from D. Nuyens (1992), and Nevadadata is from Guo et al .(2005). ................................................................................86

Table 5.7: Field parameters, and Sc, Y and REE concentrations (ppb) of water from

carbonate, granite and conglomerate in the Tam Duong area. ...............................88

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Introduction

1 Introduction

1.1 Karst in tropical regions

Karst is a term used to describe a landscape and/or a type of aquifer made of hard rock and

characterised by surface and underground phenomena of chemical dissolution. Carbonate

terrains cover about 7-12 % of planets dry and ice-free surface. About 25 % of the global

populations water requirements is supplied by karst water (Ford and Williams, 1989).

There are three consistent factors influencing the nature of karst landscape and development.

The first factor is the rocks in which karst landforms are formed, the second factor is climate,

and the third factor is the drainage system or base-flow of the area. Areas of differing climate

produce different landforms or karst topography: e.g., Caribbean karst, temperate karst and

tropical karst (Ford and Williams, 1989). Karst landforms are best developed in the tropical

regions where high rainfall, warm temperatures and thick vegetation result in high

concentration of CO 2, and large quantities of groundwater flows.

In tropical regions, there are others landforms in addition to those found in temperate karst

zones. Features such as dolines, poljes, dry valleys, caves, etc ., are found in all karst regions,

but residual hills as tower karst is specifically characteristic of tropical karst. The tower karst

occurs in Papua New Guinea, Australia, Honduras, Cuba, Jamaica, Puerto Rica and Southeast

Asia including Malaysia, Indonesia, Thailand and Vietnam (Gunn, 2004). The tower karst in

Guilin of southern China is regarded as one of the most spectacular landform in the world.

The group of karstifiable rocks are not restricted to evaporites and carbonates, which are

distributed abundantly in all continents. Under tropical conditions, quartzitic rocks are also

karstifiable (Ford and Williams, 1989). The best silicate karst developments have been

reported from Venezuela, Brazil, northern Australia and southern Africa (Williems et al.,

2002; Gunn, 2004). Quartz sandstone landscape in northern Australia is similar to tower karst

developed on limestone (Young, 1986).

The understanding of karst hydrogeology in tropical regions is generally less common in

comparison to other karst zones. Many previous karst studies in tropical regions have focused

more on origin and evolution of karst towers and on karstifiable rocks than on karst

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Chapter 1

hydrogeology. To date, the number of publications on karst hydrogeology, karst groundwater,

and karst modelling as well as karst groundwater protection in tropical regions are still

relatively limited. It is necessary to understand karst hydrogeology in order to protect the

spectacular karst landscape and its sustainable development in tropical regions.

1.2 Karst hydrogeological research in Vietnam

1.2.1 Overview of karst in Vietnam

Karst is a widespread phenomenon in Southeast Asia. This region contains some of the most

spectacular surface karst in the world. The countries of Myanmar, Thailand, Laos, Cambodia,

Malaysia and Vietnam all have important limestone karst areas. Large areas of sandstone and

buried evaporite karst are also present in this region. The total karst areas cover about 10 % of

the region, around 215, 000 km 2 (Mouret, 2004).

In Vietnam, karst areas cover approximately 18% of the land surface or about 60,000 km 2

(Dao Trong Nang, 1979). Figure 1.1 shows the occurrence of karst areas in Vietnam. The

areas are located on a tropical humid belt and have typical tropical karst characteristics.

Geographically, the Vietnamese karst areas are divided into four main karst regions: the TayBac, the Dong Bac, the Viet Bac and the Centre of the country (Fig. 1.1). The NW karst belt

is stretching over 300 km from the Chinese border to the coastline (Gulf of Bac Bo) and coves

about 8,190 km 2 (Tuyet , 1998). This karst belt is closely related with the well-known tropical

karst regions of South China. This is the karst area studied in this work.

Karst areas have a substantial socio-economical importance as groundwater resources (for

drinking and irrigation water supply and hydropower generation), as well as zones for

forestry, agriculture and aquaculture, for extraction of limestone and mineral resources, and

for tourism. They generally also have a great local and global ecological significance, being

sanctuaries for the last primary forests of Vietnam, as well as for numerous endemic plant and

animal species. Several karst areas in Vietnam are listed as World Heritage Site, such as the

Ha Long Bay and Phong Nha.

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Introduction

Fig. 1.1: Karst areas of Vietnam (modified after Dusar et al. 1994) with location of the test sites

1.2.2 Importance of karst hydrogeology study

In many karst areas of Vietnam, the landform and groundwater recently have come under high

pressure in response to urbanization, economical development and increase of population.

Moreover, karst landscapes and aquifers are extremely fragile. Karst aquifers are particularly

vulnerable to contamination resulting from human activity. Contaminants can easily reach

groundwater through thin soils and via swallow holes where they are rapidly transported over

large distances (Vesper et al., 2001). Unsound management or protection can trigger problems

such as water depletion, water pollution (with sediments and chemical or microbial

contaminants), and accelerated erosion. These problems already manifest themselves in

various karst areas of Vietnam.

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Chapter 1

A sound knowledge of the hydrogeological system is a precondition for any protection

strategy. It is essential to integrated and sustainable management and conservation of the

regions. Such understanding is presently lacking in Vietnam. The existing knowledge on

karst hydrogeology is of a general, descriptive and fragmentary nature. Earlier studies mainly

reported on karst geomorphology or surface karst, and descriptive karst landform and its

classification (Dao Trong Nang, 1979; Rozycki, 1984; Tuyet et al., 1996). Several other karst

projects focused on exploited mines in karst areas. In these projects, different karst landforms

were located on maps (Nguyen Quang My, 1992). Sub-surface karst and karst hydrogeology

were only general mentioned, such as the existence of cave or depth of caves based on

geological observations and theoretical descriptions. Other studies focused on stratigraphical

and paleotological investigation in carbonate rocks.

During the past years, several karst areas in Northern Vietnam have been studied within the

framework of the Vietnamese-Belgian Karst Project (VIBEKAP), which includes

speleological, geomorphological and hydrological investigations, remote sensing, GIS,

flooding prediction and other aspects and methods (Hung et al., 2002; VIBEKAP, 2003; Tam,

2003; Liu et al., 2005; Tam et al., 2005). However, many questions on karst hydrogeology

are still not considered. Information on water quality at karst springs, which are used for

drinking water, is still missing. Groundwater flow on karst and fluctuations in quantity and

quality of groundwater resources, as well as contamination sources have not been

investigated. A karst groundwater protection scheme is still not established and applied in

any of studies Vietnamese karst areas. Hence, through building knowledge on karst

(hydrogeology) the situation and living condition in the karst areas of the country can be

improved.

1.3 Objectives and structure of the study

This work is a contribution to the knowledge on karst hydrogeology and groundwater

protection in the tropical karst regions of NW Vietnam. The objectives of the study are:

To test and adapt chemical-microbiological, stable isotope, and tracing techniques in

karst hydrogeology.

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Introduction

To map underground water flow paths and characterize the properties of underground

water transport.

To characterize the dynamics of karst underground water flow in reaction to precipitation events.

To provide scientific information about groundwater quality and to assess the degree

of pollution as well as to identify the processes of contaminant transport in karst

systems.

To develop and apply an approach for karst groundwater vulnerability mapping in

remote mountainous areas

The study deals with two major areas that belong to the NW karst belt: Tam Duong and Son

La (Fig. 1.1). The economic development, urbanisation and increase of population in those

areas has recently put more and more direct and/or indirect pressure on karst groundwater

demand, groundwater environment, and its related problems. It is, therefore, necessary to have

a comprehensive understanding of karst hydrology and groundwater protection in these areas.

Chapter 2 presents an overview of the regional geography, geology and karst landscape.

Applied methods and their modification for local conditions are discussed in chapter 3. The

next chapters present the two studied areas: Son La (chapter 4) and Tam Duong (chapter 5).

The geology, hydrology and hydrogeology of the area are described and the obtained results

of the tracer tests, hydrochemical and microbial investigation, stable isotope composition and

rare earth elements study are discussed in detail. This is followed by conclusions on

hydrogeological characteristics. Chapter 6 focuses on groundwater vulnerability, hazard and

risk mapping. An overview of the European approach (COST 620) and how it has been

adapted and applied to the Son La and Tam Duong areas are presented in this chapter. The

last chapter 7 gives a discussion on hydrogeology, groundwater protection, and investigated

methods and recommendations for the future research in the areas.

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Chapter 1

1.4 Research collaboration

This thesis was prepared at the Department of Hydrology and Hydraulic Engineering

(HYDR), Vrije Universiteit Brussels, in collaboration with the Research Institute of Geology

and Mineral Resources (RIGMR), Hanoi, Vietnam, and the Centre of Hydrogeology (CHYN),

University of Neuchtel, Switzerland. The Belgian Technical Cooperation (BTC), the

Vietnam Belgian Karst Project (VIBEKAP) and the Swiss Commission for Scholarship partly

provided financial support for these research activities.

The first stage of this study was done in the Son La area. The field work activities here were

directly linked to the VIBEKAP project, except for stable isotope investigation. In a next

stage, our work was focused to the Tam Duong area. The field work activities in this area

were supported by CHYN and RIGMR; senior researchers, colleagues of RIGMR and friends

gave good support to this study in the Tam Duong area.

The fieldtrips would not have been successful without the support and collaboration of local

people. A huge number of observations were needed in the field and in remote area like the

Tam Duong and Son La, where geographic information and infrastructure are limited, it

would take a lot of time to localise and access the swallow holes and springs without the helpof local people. During the tracer tests, water samples were taken manually by the people

from the Thai, HMong and Kinh ethnic groups. All samples were collected at the correct

time and gave good results; only very few samples gave aberrant values. Children helped

spontaneously to do the flow measurements, the water sampling, and the microbiological

colony counting in the area of Tam Duong.

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The NW karst belt

2 Study area-the NW karst belt

2.1 Geography

2.1.1 Location and topography

The NW karst belt is one of four main karst regions in Vietnam. It is located within the

coordinates 20 o00 to 24 o00 north and 102 o15 to 106 o10 east. This karst belt extends in a

NW-SE direction from the Chinese border to the Gulf of Bac Bo (Fig.1.1). The belt is 500 km

long and has an average width of 20 to 30 km, to maximum 50 km in some places.

Topographically, the regional relief decreases from NW to SE. Tuyet et al. (1996) noted that

some places in the NW part have altitudes approximately of 3000 m, and decrease to 2000 m

and 1000 m in the centre part. The altitude steps-down to 500 or even to 200 m in the SE part.

Several studies have described the topography of this region, which is characterized by linear

belt-shaped, strongly folded, bedrocks of the Ma River anticlinorium and Da River

synclinorium. The surface is divided into narrow and elongated mountain belts, separated

from each other by tectonic faults, that are expressed on the ground surface in the form of

streams, river valleys, elongated troughs, etc,. The mountainous relief of the area has both

step-wise characteristics and linear forms, at the same time changing alternately from high

mountain ranges surrounding plateaus to low mountains with valleys and tectonic-denudation

depressions.

On the other hand, the regional relief is strongly dissected with relative height differences

from 30-50 m to 1500-2000 m and drainage density 1-1.2 km/km 2, strongly affected by

exogenous processes (erosion, gravity movement, landslides, rock fall, etc.), whichintensively occur due to the humid tropical climate with high rainfall intensity (Tuyet et al.,

1996; Van et al., 2003).

2.1.2 Climate

The region has a tropical monsoon climate with cold, dry winters and hot, humid summers.

The climate is variable due to the complicated regional variation in relief. The region is

divided into two climate zones: the mountainous climate zone in the NW part and CentralVietnam climate zone in the SE part.

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The first climate zone covers mostly the karst belt including all of the Son La, Lai Chau (Tam

Duong), and Dien Bien provinces and a part of the Lao Cai, Yen Bai and Hoa Binh provinces,

while the second climate zone covers only the low relief area that is close to the coast. The

average rainfall in NW part is 1500 mm, ranging from 1438 mm in Son La to 2145 mm in

Tam Duong. The rainfall is very variable between the years and is unevenly distributed in two

seasons. The rainy period begins in May and usually ends in September or October with

rainfall amounts of about 82% of the total annual rainfall. The greatest monthly rainfall occurs

in June, July or August. The dry period is from October through April. In the SE part the

rainfall period starts usually one month later. The average rainfall there is 1720 mm.

The average annual air temperature is 21.5 oC in the NW part and 23.5 oC in the SE part. The

temperature decreases with altitude and varies about 10o

C between winter and summer. Theaverage relative humidity is often higher than 80%, and even more than 85% in the rainy

period. Data published by the National Meteorological Centre show that the average annual

evaporation for the whole area is 875 mm, and this evaporation increases with decreasing

altitude.

2.1.3 Social and economic conditions

Various ethnic minority groups are living in the NW area, mainly including the Thai,HMong, Kinh, Dao, Muong, and Tay ethnic groups. The HMong and Dao often inhabit the

high mountains above 1000 m, while the Thai and Tay often settle along the rivers, valleys,

and the lowland areas. The Kinh lives along the roads, the town centers and the lowlands. The

Muong lives in the SE of the karst belt.

The area has one of the highest population growths in the country. Agricultural production of

the area is not abundant and consists of mainly rice, maize, tea, while industrial activities are

minimal. This zone has a backward economic development and difficult living conditions. In

recent years the Vietnamese Government focuses on improving the living conditions in this

zone but the living conditions are highly depending on the natural resources and its produce.

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The NW karst belt

2.2 The geology of the NW karst belt

2.2.1 Overview of geological setting

North Vietnam can be divided into two main tectonic units: the Bac Bo Fold Belt and the

Indochina Fold Belt separated spatially and structurally by the Ma River Suture zone or Ma

River fault (Tran Van Tri et al., 1979).

The Bac Bo Fold Belt is composed by three fold systems Tay Bac, Viet Bac and Dong Bac

(Tran Van Tri et al., 1979). The NW karst belt is located within the structural framework of

the Tay Bac fold system. Figure 2.1 shows the tectonic framework of NW Vietnam and the

location of the test sites.

Fig. 2.1: Tectonic framework of NW Vietnam (modified after Tran Van Tri et al, 1979)and location of the test sites

The Tay Bac has general a NW orientation and it is situated between two deep-seated faults,

so-called the Ma River and Hong River (Red River). Structurally, this system forms part of

the Hong River anticlinorium, the Da River rift and the Ma River anticlinorium (Tran Van Triet al., 1979).

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Chapter 2

The NW region consists of different formations formed in periods from Late Proterozoic to

Cainozoic, including terrigenous rocks, carbonate rocks and metamorphic siliceous rocks.

Carbonate formations were formed in a large period from Proterozoic to Cretaceous and

consist mainly of limestone and dolomite (Tuyet et al., 1996). However, only the carbonate

formations of Middle Devonian Ban Pap Formation, Carboniferous-Permian Bac Son

Formation and especially Middle Triassic Dong Giao Formation are widespread and

favourable for karstification.

2.2.2 Tectonics

The mountain range of NW Vietnam is located in an active tectonic region with different

tectonic cycles. The Ma River suture is related to the active Himalaya uplift and South China

Sea rift. This results in ongoing uplift of NW Vietnam and rejuvenation of the karst

landscape. Detailed regional and local tectonic characteristics have been studied by many

geological projects. The following sections only briefly describe the tectonic characteristics of

the area on a regional scale.

2.2.2.1 Folding characteristics

As mentioned above, the Tay Bac mountain ranges in NW Vietnam mainly consist of the

Hong River (Red River) anticlinorium, the Da River rift - formed as syclinorium, and the Ma

River anticlinorium.

From general structural point of view, series of folds, including series of syclinoriums and

alternating anticlinoriums, are described in this region (Tran Van Tri et al., 1979; Tuyet et al.,

1996). The carbonate rocks located in the tectonic units above were affected by different

folding processes at different stages.

The oldest carbonate rocks formed in Early Proterozoic were controlled by the folding process

during the later Proterozoic. Such carbonate rocks metamorphosed to marble, and formed in

the core of the Ma River and Red River anticlinoriums. The slight folds are often observed in

carbonate rocks formed in Middle Devonian. The carbonate formations formed in the later

Paleozoic are generally defined by folds plunge of about 50 o. In contrast, the carbonate rocks

formed in the Middle Triassic are characterized by a series of open anticlinoriums and

syclinoriums trending NW-SE like a wave system.

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The NW karst belt

2.2.2.2 Faulting characteristics

Fault systems in the NW Vietnam are classified into 4 different orientations: the NW-SE, the

NE-SW, E-W and N-S (Tran Van Tri et al., 1979). The Hong River (Red River), the Da River

and the Ma River faults are major fault systems in the area. These deep-seated faults form in a

NW-SE direction, with a 20 o to 40 o dip. Basic magmatic intrusive-effusive formations such as

Vien Nam and Cam Thuy are observed along these faults. Other dominant faults systems in

the upper part of the Ma River and the fault system in the lower part of the Da River are also

developed in a NW-SE direction.

On a regional scale, NE-SW faults are relatively short and discontinuous. Most of the faults

are of thrust type and originated under the compressive state of the regional stress fields and

may be tight; thus not favourable for the development of large valley, sinkhole and cave

systems (Van, 2003). However, due to uplifted erosion, shallow decompression phenomena

are widening fractures and joints, and local stretches of the active fault system may be in

tensional regime.

2.3 Principles of hydrogeological characterization of karst aquifers

A karst aquifer is an aquifer in which the flow of water is or can be appreciable through one

or more of the following: joints, faults, bedding plane partings, and cavities any or all of

which have been enlarged by dissolution (Field, 2002). The karst aquifer may have primary

(intergranular) and secondary (fracture) porosity openings, which are saturated with water

when below the water table.

Karst aquifers are subdivided in two main types depending on their position compared with

the relevant (hydrologic) base level (Bgli, 1980 (Fig. 2.2). Shallow karst aquifers have their

karst basis above the base level of the system. Deep karst is present when the base of karst

aquifer is below the base level of the system. A karst system can be mixed of shallow and

deep karst types.

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Chapter 2

Fig. 2.2: Shallow and deep karst systems with regard to the position of the base level (Bgli, 1980)

The karst aquifers receive recharge through autogenic and allogenic systems. An autogenic

system is one composed entirely of karst rocks and derives its precipitation water through the

soil and unsaturated zone. By contrast, an allogenic system derives water from an adjacent

non-karst area via dolines or swallow holes. Ford and Williams (1989) noted that the mixed

autogenic and allogenic intermediate case is the most common in practice. Figure 2.3

illustrates the recharge into a karst aquifer from both concentrated and diffuse sources.

Fig. 2.3: Recharge into carbonate aquifers (Gunn, 1986)

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The NW karst belt

Karst aquifers are heterogeneous and anisotropy results in variation and directional difference

of hydraulic conductivity. Kiraly (1975) demonstrated that the hydraulic conductivity in karst

systems varies considerably with the scale of estimated samples (Fig. 2.4). For instance, rock

samples (pore and micro-fissures) have hydraulic conductivity values of 10 -9 to 10 -5 m/s,

while the hydraulic conductivity ranges from 10 -4 to 10 0 m/s at aquifer catchment scale. The

highly different conductivities/permeabilities of fissured systems and conduit systems

complicate the hydrogeological characterization of karst aquifers. Groundwater flow in karst

aquifers, consequently, is significantly different from that of other aquifers. In karst aquifers,

flow in conduit networks is fast and often turbulent, while flow through the matrix of the rock

(fissures and pores) can be exceedingly low. The residence times in any karst aquifer vary

considerably with the flow path that the water has followed (Smart and Worthington, 2004a).

Fig. 2.4: Conductivity scale-effect in karst system (Kiraly, 1975)

Variations in discharge at a karst spring are often accompanied by changes in spring water

temperature and electrical conductivity. Figure 2.5 shows an idealized separation of spring

hydrograph and chemograph data (Ford and Williams, 1989). In conduit systems, there is

often first an increase in conductivity together with an increase in discharge followed by a

decrease in conductivity and temperature. This type of reaction on hydrologic events is called

piston effect and will be further discussed in the section 5.5.

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Chapter 2

Fig. 2.5: Interpretation of a karst spring hydrograph and chemograph (Ford and Williams, 1989)

2.4 Karst landform

2.4.1 Definition of tropical karst landforms

Several terms such as cockpit karst, cone karst, kegel karst, tower karst, fengcong and

fengling are usually used in order to describe variant morphology of residual hills of tropical

karst land form. Because the tropical karst terms are used as synonyms in different areas or

countries, it is therefore useful to explain the origin of above terms:

Cockpit karst is a Jamaican term that is used to describe tropical karst topography

containing many closed depressions surrounded by steep conical hills (Field, 2002).

Cone karst is a karst landscape dominated by low conical (or hemispherical) hills that

form only in wet tropical climates (Field, 2002). The generally conical carbonate hills

may be isolated from each other visually or share lower ground surfaces such as

pedestals or ridge remnants (Day and Tang, 2004).

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The NW karst belt

Kegel karst is a German term that is used to describe several types of tropical humid

karst characterized by numerous closely spaced cones, hemispherical or tower-shaped

hills having intervening closed depressions and narrow steep-walled karst valleys or

passageways.

Tower karst is a landscape of residual (carbonate) hills scattered in a plain, even

though the towers may not necessary be steep (Ford and Williams, 1989). The

residual hills display a variety of shapes from tall sheer sided towers to cones or even

hemispheres. Others are asymmetric, reflecting the influence of dip or erosional

processes. Although some rise directly from the plain, many surmount pedestals.

Some towers are isolated, while others are in groups rising from a common base.

Chinese researchers have identified two types of tropical karst landscapes: fenglin

(peak forest) and fengcong (peak cluster). The peak forest consists of individual

isolated residual hills rising from floodplains. The peak cluster comprises a group of

residual hills emerging from a common bedrock basement and incorporating closed

depressions between the clusters of peaks.

There is no definitive distinction between cone karst and tower karst (Day and Tang, 2004)

and the cockpit karst is one kind of kegel karst (or tower karst). To avoid confusing, we preferto use only tower karst to describe the general residual carbonate hills, and peak forest or peak

cluster for the karst landform features in this study.

2.4.2 Karst landscapes in NW Vietnam

The karst landscape in Southeast Asia, particular in Vietnam, is one of the most notable and

spectacular landscapes in the world (Mouret, 2004; Waltham and Hamilton-Smith, 2004). The

NW Vietnamese karst belt is closely related to the karst belt of southern China, but alsorepresents a typical Vietnamese karst landscape due to the distinctive characteristics of

stratigraphy, tectonic, climate and geomorphology in this region.

The karst landscape of peak clusterdepression stretching in the NW-SE alternate with dry,

narrow, and steep valleys are widely developed in the area (Fig. 2.7). Major canyons often cut

through these karst terrains (Mouret, 2004). This type of karst landscape is not only observed

in the high altitude areas of the center part and NW part of the belt, as for instance in Tam

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Chapter 2

Duong and Son La; it also occurs in the SE part in relatively low relief areas such as Cuc

Phuong and Moc Chau (Fig. 2.6).

Fig. 2.6: Geographical location of main cities/towns in Northern Vietnam

The peak forest karst landscape is also observed in this belt (Fig. 2.8). Such peak forest existin relative low and moderate altitude areas, as for instant in Hoa Lu, Son La, Moc Chau

(Fig.2.6); and also in high altitude areas like Tam Duong, Tua Chua, Sin Ho, etc. These peak

forests have many different forms of vertical slope towers, or conical or pyramid towers. The

steep slope towers have only a minimal soil cover; the conical and pyramid towers are

covered by residual soil. Most invidual towers are asymmetrical, reflecting structural or other

influences. It is observed that the invidual towers usually rise from a continuous carbonate

surface covered by alluvium, while other towers rise from a surface of non-carbonate rocks.

The most impressive karst towers rise from the sea in the Ha Long Bay (World Heritage Site).

Several big karst plateaus and karst fields are present in the area. Van et al. (2003) mentioned

that the plateaus can clearly be distinguished from other type of landforms by their high

altitude due to neotectonic uplift. The Sin Ho (Tam Duong) karst plateau is located at an

altitude of about 2000 m, the Moc Chau karst plateau at an altitude of about 1000 m. The Mai

Chau karst erosion valley and Cuc Phuong karst field are beautiful and are high potential

areas for tourism.

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The NW karst belt

Fig. 2.7: Peak cluster depression karst landscape in NW Vietnam

Fig. 2.8: Peak forest karst landscape in NW Vietnam

Cave systems are numerous in this karst belt. Subhorizontal caves are dominant. Vertical

caves, however, are often investigated in the NW part of the belt where relief is uplifted

because of neotectonic activities. In addition, due to the neotectonic uplift, ancient caves with

multilevel systems uplift are found in area. The Cong Nuoc cave, situated in the Tam Duong

area with the depth of -600 m, is known as the deepest cave in Southeast Asia.

High density vegetation and typical tropical forest cover the karst belt. The green karst

landscape is often observed in the SE part and in high mountainous areas of this karst belt.

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Methods and techniques

3 Methods and techniques

Worldwide, karst hydrogeological research is conducted on different scales and using

different methods. Some of the methods - even the essential methods - are less used in low-

income countries and/or remote areas because of the difficult technical and operational

conditions. The test sites are located in one of the most remote and poorest regions of

Vietnam. The following methods are applied to the research areas.

Tracer experiments

Hydrochemical study

Microbiological investigation

Stable isotope study

Rare Earth Elements (REE) study

Karst groundwater vulnerability, hazard and risk mapping

These methods are not employed separately in the concerned areas. A method and its results

are applied and interpreted in combination with those of other techniques to achieve effective

inFormation of the test sites. Details of the vulnerability, hazard and risk mapping applied in

this study are presented in chapter 6.

3.1 Tracing experiment

3.1.1 Tracing in karst study

Smart and Worthington (2004b) defined water tracing (tracing experiment) as the use of

natural or induced properties in the water, allowing detection of that water at some point

downstream and gaining insight in the character of the flow path followed by the water.

Tracer techniques are powerful tools with many applications in hydrological investigations.

The tracing technique in hydrogeology and related issues have been described in detail by

Kss (1998), Smart and Worthington (2004b), Crawford (2004), Divine and McDonnell

(2005), etc. This section only briefly presents the tracing in karst research in accordance with

above references.

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Chapter 3

`The tracing test is a primary tool of the karst hydrogeologists. General speaking, the

technique is often used in karst studies to determine an underground water flow path,

groundwater travel times, catchment boundaries and recharge areas. Tracer tests have also

been applied to define contamination problems and to assess the vulnerability and

determination of protection zones in karst area. The first tracing tests using chaff were applied

to solve problems in karst groundwater in 10 A.D. (Crawford, 2004). Recently, the technique

has been developed and applied in all type of aquifers.

Tracers can be divided into physical, chemical, isotopic and biological tracers; and two

tracing types: natural tracing and artificial tracing. Three tracing methods are applicable:

qualitative, semi-quantitative and quantitative. Qualitative tracing simply detects the tracer in

the water, while semi-quantitative tracing includes defining the concentration of the tracer inthe water over the time. Quantitative tracing includes tracer concentration measurements with

flow determinations. Natural tracing includes any substance naturally occurring in the water

that is used to follow flowing water. Unlike that, artificial tracing deliberately introduced into

the water to follow flowing water. Artificial tracing is widely used in karst hydrology because

it allows control over the magnitude of the tracer concentration and the specificity of the site

to be traced. Fluorescent dyes are the predominant tracers currently being used.

Tracer tests in karst are widely used in point-to-point mode to define the trajectory taken by

underground water. Typically, this enables to identify the destination spring (resurgence) of a

sinking stream. The detection of an injected tracer at the spring means that there is a

connection between the point of injection and the point of recovery. A series of point-to-point

tracer tests can be used to establish a regional network of karst underground flow paths.

Depending upon the natural conditions and the aim of the experiment, tracer(s) can be

injected into swallow holes, boreholes, closed depressions, fissures, or even spread over the

ground surface either by instantaneous or continuous injection. The sampling method isselected on the basis of the tracer used, the application and local conditions. Qualitative

tracing, semi-quantitative and quantitative tracing is applied depending on the same

considerations.

3.1.2 Tracer breakthrough curve

The curve generated by plotting measured tracer concentration versus time (after injection) is

the so-called tracer breakthrough curve. The shape of the tracer breakthrough curve depends

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upon the character of the tracer, flow conditions and structure of the aquifer. The

interpretation of tracing test is based on a detailed evaluation of the tracer breakthrough curve.

It is useful, therefore, to introduce the main parameters that are often used in analysis of a

tracer breakthrough curve (Fig. 3.1).

Time to first arrival (t 1) is the time when the first tracer is detected at the sampling

point

Time to maximum concentration (t 2) is the time when the maximum tracer

concentration is detected at the sampling point

Mean travel time (t 3) is the time when the centroid of the tracer mass traverses the

sampling point

Fig. 3.1: Tracer breakthrough curve and residence times

A simple way to evaluate tracer velocities is to calculate linear velocities, using the distance

between the injection and sampling points. The fastest flow velocity, dominating flow

velocity and median flow velocity are calculated using the time to first arrival, the time to

maximum concentration and the mean travel time respectively. A computer programme to

analyse tracer breakthrough curves, Traci95, for instance, allows more parameters of

breakthrough curves to be determined, as for example: the dispersion coefficient, Peclet

number, etc.

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3.1.3 Traci95 Programme

Traci for Windows 95 (Traci95) is a user-friendly computer program used to evaluate

artificial tracer tests. It contains several analytical solutions for different groundwater aquifer

types and hydraulic situations. Traci95 can be used for tracer analysis in karst aquifers (Kss,

1998; Werner et al., 1997).

The mathematical evaluation of the breakthrough curve from a tracing test is possible using

analytical and numerical processes. The simple explicit form of the analytical solutions allows

quick determination of transport parameters. The analytical solution below applied for tracing

test in conduit system of a karst aquifer (Werner et al., 1997) is given by equation 1.2.

Many karst breakthrough curves, however, are characterized by tailing or multiply peaks.

Such breakthrough curves can be evaluated with the multi-dispersion-model, MDM (Werner

et al., 1997). This model is an extension of the solution in equation 1.2 above. The obtained

breakthrough curve is taken to be a superposition of different flow paths. The parameters of

the individual curve are determined step by step in the evaluation.

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3.1.4 Tracing tests in Son La and Tam Duong areas

As mention above, the tracing test provides a major tool in the hydrogeological

characterization of karst aquifers. However, before this work, tracing tests were not used in

any karst study in NW Vietnam. Within the framework of this thesis , the tracing experiments

were first applied in the Son La and Tam Duong karst areas. A detailed account of the tracing

experiment is presented in section 4.3 and 5.3.

Dyes and common salt (sodium chloride) were selected as tracers. A quantitative tracermethod that combines determination of the tracer concentration over time with flow

(discharge) measurements was mainly employed. This method allows to characterise the

groundwater flow path, and the total mass of tracer collected at the sampling site to be

determined. The discharge was measured either by a water level logger (tests in Suoi Muoi,

Nam La), or by flow velocity multiplied to cross-section area (tests in Bon Phang, Tam

Duong), or by the salt-dilution method (test in Tam Duong). In addition, the qualitative tracer

method that provides point-to-point and catchment information was also applied in the areas.

This method involves only the identification of the injected tracer in water samples. Collected

water samples of the experiment in the Bom Bay area and charcoal bags samples of the

experiment in Tam Duong were analysed to confirm the presence of tracer in the water.

In both the Son La and Tam Duong areas, the selected tracers were injected by instantaneous

injection method. The tracers were dissolved in water prior to injection. The active swallow

holes or caves were selected for injection, and springs and caves were selected for sampling.

The sampling technique, however, was slightly modified. As automatic-sampling equipmentwas not available, local peoples took the samples manually. The water samples were collected

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studied karst aquifers. On the other hand, hydrochemical investigations are also applied to

study contaminant processes for management and protection of karst aquifers.

The hydrochemical investigations in karst studies mainly focus on the variability of spring

water chemistry as a means of characterising the karst aquifer, particularly the interpretation

of plots of chemical variables as a function of time (chemographs) either on seasonal time

scales or during individual storm rainfall events. Simultaneously, the discharge rates as a

function of time (hydrographs) are recorded (Vesper and White, 2004).

3.2.2 Hydrochemical investigation in the test sites

The hydrochemical study in the Son La area was carried out to determine water quality, and to

characterize spatial distribution of karst water in the area. The VIBEKAP data from the

January 2000 campaign is used in the present study. The water samples were collected at most

important springs, swallow holes and streams that are located in different geological

formations. Physical parameters such as pH, temperature, TDS were measured in the field.

Major ions such as Ca 2+, Mg 2+, Na +, K +, SO 42-, Cl -, NO 3- and F - were analysed at the

Laboratory of PhysicalChemical Geology, Catholic University Leuven, Belgium. The

obtained data are interpreted by using the software AquaChem 4.0. Details of results are

presented in section 4.4.

The hydrochemical investigation in the Tam Duong area was carried out in a rainy season of

2004 to determine the water quality, and to understand how chemical parameters vary during

and after precipitation events. This study required continuously sampling over short time

intervals in several karst springs. In order to avoid that a huge volume of water samples

should be transported, the samples were taken in small 13 ml plastic tubes. These samples

were analysed at the Centre of Hydrogeology, University of Neuchtel, Switzerland by ionic

chromatograph method using ionic chromatograph dionex DX-120. The major cations Na +,

K +, NH 4+, Ca 2+, Mg 2+, and anions F -, Cl -, NO 3- and SO 42- were measured. In principle, the

equipment performs isocratic ion analysis applications using conductivity detection. To

measure the conductivity, two different columns of eluents (one for anions and other for

cations) are respectively used. Only 3 ml of sample is injected for measurement of the major

anions; and another 3 ml of acidified water sample is injected for the major cations. The

standard samples and blank samples are first calibrated before starting the analysis of each

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Chapter 3

series of water sample. The ion contents are then calibrated as a function of detected

conductivity. Details of sampling and results are presented in section 5.4.

3.3 Microbiological investigation

Many types of micro-organisms are found in groundwater, including bacteria, archaebacteria

and protozoans as well as viruses. Most of these microorganisms occur naturally and

permanently in groundwater and are harmless while some are pathogenic (Chapelle, 2001).

Microbial contamination often results from human activities. In many studies, bacteria

including Escherichia coli, faecal streptococci and Clostridium are commonly used as an

indicator to know if water is contaminated and may contain other pathogenic micro-

organisms. Thermotolerant coliform analysis carried out at 44C indicate bacteria of faecal

origin.

Bacteriological problems in karst, particular in karst groundwater, are mentioned by many

authors (Ford and Williams, 1989; Drew and Htzl, 1999; Auckenthaler et al., 2003; Pronk et

al., in press). In karst systems, the infiltration and flow conditions favour the transport of

micro-organisms. In recharge periods, contaminants from the land surface can be washed into

aquifer either diffusely by infiltration and subsequent percolation through the soil, epikarst,

unsaturated zone, or concentrated via swallow holes (Goldscheider, 2004). The high velocity

leads to a fast transport of the contaminants through the whole karst aquifer system.

Therefore, using faecal bacteria as contamination indicator in karst aquifers has been applied

in many areas.

The faecal bacteria analysis, particularly thermotolerant coliform, using the culture-counting

techniques was applied for the first time in the Tam Duong area. The samples for

microbiology were collected at all important springs which are used for drinking water in thearea. At the same time, samples were collected at short time intervals, ranging from 8 to 14 h,

at two main springs. In normal conditions, water samples for microbial study should be

analysed in the laboratory on the same sampling day. However, in case of the Tam Duong

area it was impossible to transport the water samples to a microbial laboratory in the required

time. Hence, the analyses were done in situ using the portable microbial Lab OXFAM-

DELAGUA (Fig. 3.3).

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Fig. 3.3: The portable microbial Lab Oxfam-DelAgua with main consumables (Photo by Oxfam-DelAgua)

In principle, the analysis is carried out by passing a measured quantity of water through a

sterile filter (0.45 m pore size). Any bacteria present in the water are caught in the filter. The

filter is then placed onto a paper pad soaked in a liquid growth medium which feeds coliform

bacteria but inhibits the growth of any bacteria caught on the filter. The filter is kept in 44 0C

at the kits incubator during 18 h (1-2 h in room temperature and 15-16 h in incubator).

During that time the coliform bacteria multiply many times to form colonies that can be seen

or counted with the naked eye. To avoid errors during counting, different volumes of the

samples (from 0.1 ml to 100 ml) are analysed depending on the level of contamination. Most

samples were analysed twice with different volumes of water (1 or 2 ml and 10 ml or even

larger volume). The contaminant level is presented by both minimum and maximum counted

colonies per unit quantity of water. Details of sampling and results are presented in section 5.4

3.4 Stable isotope study

Stable isotope ratios of oxygen and hydrogen have been applied extensively in hydrological

investigations and complementing geochemistry over the past few decades. The use of stable

isotopes, in particular the isotope ratios of oxygen and hydrogen as conservative tracer, has

improved our understanding of problems related to catchment and groundwater studies

(Barnes and Allison, 1988; Darling and Bath, 1988; Schramke et al., 1996; Clark and Fritz,

1997; Vitvar and Balderes, 1997; Genereux and Hooper 1998; Katz, 2002). Coplen et al.(2001) have provided some relevant examples using stable isotopes to solve practical

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Chapter 3

hydrologic problems of recharge and discharge processes, mean transit time, mixing fraction

between surface-groundwater interactions, and climatic conditions during the recharge

process

Several researchers addressed the problems on karst hydrology investigations by using stable

isotopes. The studies of 18O and 2D seepage water of karst caves (Yonge et al., 1985;

Caballero et al., 1995) show that the isotopic compositions of those waters, just as in

groundwater, can be considered as the mean isotopic value for precipitation water in the

surrounding area. Other studies of 18O and 13C in speleotherms found that the isotopic

behavior strongly depended on the cave environment (Verheyden, 2000). Results from

isotopic and hydrochemical studies to evaluate the regional recharge as well as the importance

of point-source recharge to karst aquifers in sub-humid to semi-arid regions with lowtopography are presented by Leaney and Herczeg (1995) and Herczeg et al. (1997).

Researchers have also used stable isotopes as tracers for determining the component mixing in

groundwater in karst area (Lakey and Krothe, 1996; Nativ et al., 1999; Lee and Krothe, 2001;

Long, 2002).

Nguyen Tam (2004) presented the use of oxygen and carbon isotopes measured in a

stalagmite from the Ta Chinh cave (NW Vietnam) as palaeo-climatic indicators to provide

preliminary paleoenviromental information of the area. However, the application of stable

isotopes in hydrogeologial study has not been carried out in this NW karst belt. This study

reports stable isotope measurements, particular oxygen isotopes, of meteoric and surface and

spring water in attempt to use as natural tracer for determining hydrogeological functioning of

the area. The study also provides the information of 18O as valuable reference data for any

further stable isotope application in hydrogeological, geological and speleological studies in

Vietnam.

The oxygen isotope samples (96 samples) were prepared and analyzed at Laboratory of Stable

Isotopes, Department of Geology, VUB, by isotope ratio mass spectrometry using the

CO 2/H2O equilibration method. In principle, a small quantity of CO 2-gas is brought in contact

with the water sample so that isotopic exchange can take place (C 16O2 + H 218O C 16O18O +

H216O). If isotopic equilibrium is attained at a constant known temperature, the oxygen

isotope composition ( 18O) of the water can be calculated from the 18O of the CO 2 measured

in a mass spectrometer (after Epstein and Mayeda, 1953). The 18O values then are reported

in units of parts per mil ( 0/00) with reference to Standard Mean Ocean Water (Equation 1.3).

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8/11/2019 Nguyet, Vu T. M._hydrogeological Char

Documents

Nguyet, Vu T. M._hydrogeological Characterisation and Groundwater Protection of Tropical Mountainous Karst Areas in NW Vietnam