Báo cáo Hydrogeological Characterisation and Groundwater Protection of Tropical Mountainous Karst areas in NW Vietnam

Hydrogeological Characterisation and Groundwater Protection of Tropical Mountainous Karst areas in NW Vietnam by Vu Thi Minh Nguyet Department of Hydrology and Hydraulic Engineering

Hydrogeological Characterisation

and Groundwater Protection

of Tropical Mountainous Karst areas

in NW Vietnam

by

Vu Thi Minh Nguyet

Department of Hydrology and Hydraulic Engineering

V U B – HYDROLOGIE (48)

2006

This publication is Nr 48 of the series “V U B – Hydrologie” edited by the Department

of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel

Orders should be sent by letter to the Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels

c 2006 Dienst Uitgaven VUB

Wettelijk depot 1885

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

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 (Université de Neuchâtel, 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 Daphnée 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 Neuchâtel, for giving their knowledge on karst hydrogeology and laboratory

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

Abstract

In NW Vietnam, karst areas cover nearly 18% of the land surface and have substantial economic importance as groundwater resources, as well as zones for forestry, agriculture and tourism 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

socio-This work aims at better understanding the hydrogeological characteristics of the tropical karst 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 Mg2+ and Ca2+ 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 δ18O 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 Mg2+and Ca2+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 water–rock interaction in this karst system

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 was observed 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

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

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 38

4.4 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 50

4.5.2 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

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 69

5.4.2 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

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 113

7.1.2 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

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 (Bögli, 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) 27

Fig 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

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 Mg2+ versus Ca2+ 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 58

Fig 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 65

Fig 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

Fig 5.7: Plan view and vertical profile of Suoi Thau cave (Belgian–Vietnamese Caving

Expedition, 2002) .75 Fig 5.8: Piper diagram of karst springs in the Tam Duong area; the cycle symbol represents

the Dau Nguon Sin Ho spring, the cross symbol is the Nha May Che spring The triangle symbol represent other karst springs, which are used for drinking water in the area 79 Fig 5.9: Precipitation, conductivity, water level and hydrochemical and microbiological

parameters at the Dau Nguon Sin Ho spring (spring 1) The large symbols and the bold underlined numbers represent a sample taken at the Nung Nang cave at the 29.08.04 .82 Fig 5.10: Precipitation, conductivity, water level, hydrochemical and microbiological

parameters at the Nha May Che spring The large symbols and bold underlined numbers represent a sample taken at the Suoi Thau swallow hole at the 23.08.04 83 Fig 5.11: Location of the various sampling sites in the Tam Duong area 85 Fig 5.12: Shale – normalized REE patterns of carbonate rocks from the Tam Duong and

Nam Son areas 90 Fig 5.13: Shale-normalized REE patterns for water from the Tam Duong area .91 Fig 5.14: The Mg2+ and Ca2+ concentrations measured in all water samples from the Tam

Duong area; the dot symbol represents Dau Nguon Sin Ho spring; the cross symbol is Nha May Che spring and triangle symbol is other springs .94 Fig 6.1: The intrinsic vulnerability mapping is based on the origin-pathway-target model

(Goldscheider and Popescu, 2004) .100 Fig 6.2: Proposed methodology for groundwater vulnerability and risk mapping (for

explanation see the text) .101 Fig 6.3: Illustration of the proposed method of groundwater vulnerability mapping The O

factor takes into account the protectiveness of the overlying layers, the C factor considers the concentration of flow towards swallow holes (allogenic recharge), the vulnerability map is created by overlying the O and C maps (Nguyet and Goldscheider, in press) .103 Fig 6.5: Geology of the Tham Ta Toong area, Son La province (Nguyet et al., 2004b) .105Fig 6.6: O map, C map and vulnerability map of the Son La karst area, and legend for the

three maps (Nguyet et al., 2004b) .107

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.

109Fig 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

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 δ18O 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 [Mg2+]/[Ca2+] 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 .77

Table 5.4: The molar [Mg2+]/[Ca2+] 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 Nevada

data 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

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 planet’s dry and ice-free surface About 25 % of the global

population’s 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 CO2, 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

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 km2 (Mouret, 2004)

In Vietnam, karst areas cover approximately 18% of the land surface or about 60,000 km2(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 Tay Bac, 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 km2 (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

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

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

• 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

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 Neuchâtel, 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 help

of local people During the tracer tests, water samples were taken manually by the people from the Thai, H’Mong 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

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 20o00’ to 24o00’ north and 102o15’ to 106o10’ 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/km2, strongly affected by exogenous processes (erosion, gravity movement, landslides, rock fall, etc.), which intensively 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 Central Vietnam climate zone in the SE part

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 21.5oC in the NW part and 23.5oC in the SE part The temperature decreases with altitude and varies about 10oC between winter and summer The average 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, H’Mong, Kinh, Dao, Muong, and Tay ethnic groups The H’Mong 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

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 Tri

et al., 1979)

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 50o 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

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 20o to 40o 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 (Bögli, 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

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

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 100 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

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)

• 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 prefer

to 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 also represents a typical Vietnamese karst landscape due to the distinctive characteristics of stratigraphy, tectonic, climate and geomorphology in this region

The karst landscape of peak cluster–depression 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

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 exist

in 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

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

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 Käss (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

`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 in the 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 is selected 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

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 (t1) is the time when the first tracer is detected at the sampling point

• Time to maximum concentration (t2) is the time when the maximum tracer concentration is detected at the sampling point

• Mean travel time (t3) 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

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 (Käss, 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