Review of Studies on Groundwater Resources in Cà Mau Province

Florian Jenn Hoàng Thị Hạnh Lê Hoài Nam Armin Pechstein Nguyễn Thị Anh Thư

Baseline Study Cà Mau Review of studies on groundwater resources in Cà Mau Province

Technical Report No III-2 June 2017

Center for Water Ministry of Natural Resources Planning Resources and and Investigation Environment

Federal Ministry for Economic Cooperation and Development, BMZ

Authors: Florian Jenn (BGR), Hoàng Thị Hạnh (IGPVN), Lê Hoài Nam (IGPVN), Armin Pechstein (BGR), Nguyễn Thị Anh Thư (IGPVN)

Commissioned by: Federal Ministry for Economic Cooperation and Development (Bundesministerium für wirtschaftliche Zusammenarbeit und Entwicklung, BMZ)

Project: Improvement of Groundwater Protection in Vietnam (IGPVN, 2015–2017)

BMZ Project No.: 2013.2212.2

BGR Project No.: 05‐2374

ELVIS No.:

Date: 06 June 2017 Baseline Study Cà Mau Review of studies on groundwater resources in Ca Mau Province

IGPVN Technical Report III‐2

Bundesanstalt für Geowissenschaften und Rohstoffe / German Federal Institute for Geoscience and Natural Resources Hannover, Germany

Project “Improvement of Groundwater Protection in the Mekong Delta” (IPGVN)

Authors: Florian Jenn Hoàng Thị Hạnh Lê Hoài Nam Armin Pechstein Nguyễn Thị Anh Thư

Hanoi, 06 June 2017 (revised edition) 03 June 2016 (first edition)

German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Contents 1 Introduction ...... 1 1.1 Aim and scope of the baseline study ...... 1 1.2 Overview of data sources...... 1 2 Overview of the study area ...... 4 2.1 Location and administrative divisions ...... 4 2.2 Topography, land use, and hydrography ...... 5 2.3 Climate ...... 9 3 Geology of the study area ...... 10 3.1 Miocene (N1) ...... 12 3 3.1.1 Upper Miocene, Phụng Hiệp formation (N1 ph) ...... 12 3.2 Pliocene (N2) ...... 13 1 3.2.1 Lower Pliocene, Cần Thơ formation (N2 ct) ...... 13 2 3.2.2 Middle Pliocene, Năm Căn formation (N2 nc) ...... 15 3.3 Pleistocene (Q1) ...... 16 1 3.3.1 Lower Pleistocene, Cà Mau formation (Q1 cm) ...... 16 2–3 3.3.2 Middle – Upper Pleistocene, Long Toàn formation (Q1 lt)...... 17 3 3.3.3 Upper Pleistocene, Long Mỹ formation (Q1 lm) ...... 18 3.4 Holocene (Q2)...... 19 1–2 3.4.1 Lower to middle Holocene (Q2 ) ...... 19 2–3 3.4.2 Middle to upper Holocene (Q2 ) ...... 19 3 3.4.3 Upper Holocene (Q2 ) ...... 19 4 Hydrogeology of the study area ...... 21 4.1 Hydrogeological setting ...... 21 4.1.1 Aquifers ...... 22 4.1.2 Aquitards ...... 30 4.1.3 Cross-sections and hydraulic connections ...... 31 4.2 Aquifer characterisation and testing ...... 33 4.2.1 Available data ...... 34 4.2.2 Pumping test performance and evaluation ...... 35 4.2.3 Discussion of the pumping test results ...... 37 4.2.4 Results of test pumping revision ...... 39 4.3 Groundwater chemistry ...... 40 4.3.1 Overview of available analyses ...... 40 4.3.2 Overall hydogeochemistry...... 43 4.3.3 Analysis of possible saltwater interaction processes ...... 49 4.3.4 Goundwater quality ...... 53 4.3.5 Spatial variation ...... 54 4.3.6 Temporal variation ...... 56 4.4 Isotope investigations and groundwater dating ...... 57 5 Groundwater abstraction and resource estimation ...... 60 5.1 Data for larger wells in the Mekong Delta ...... 60 5.2 Investigation in Cà Mau 2009 ...... 61 5.2.1 Overview of results for Cà Mau Province ...... 62 5.2.2 Cà Mau City ...... 64 5.2.3 U Minh District...... 65 5.2.4 Dam Doi District ...... 66 5.2.5 Phu Tan District ...... 67 5.2.6 Thoi Binh District ...... 68 5.2.7 Tran Van Thoi District ...... 69 5.2.8 Cai Nuoc District ...... 70 5.2.9 Năm Căn District ...... 72

Baseline Study Cà Mau i German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

5.2.10 Ngoc Hien District ...... 72 5.3 Exploitable groundwater resources estimation ...... 73 5.4 Conclusions ...... 74 6 Groundwater dynamics and morphology ...... 76 6.1 Groundwater dynamics in the Mekong Delta ...... 76 6.2 Groundwater dynamics in Cà Mau Province ...... 82 6.2.1 Holocene (qh) ...... 82 6.2.2 Upper Pleistocene (qp3) ...... 83 6.2.3 Middle – Upper Pleistocene (qp2–3) ...... 84 6.2.4 Lower Pleistocene (qp1) ...... 84 2 6.2.5 Upper Pliocene (n2 ) ...... 84 1 6.2.6 Lower Pliocene (n2 ) ...... 84 6.3 Groundwater contour maps for Cà Mau Province ...... 86 7 Conclusion and Recommendations ...... 91 7.1 Conclusions ...... 91 7.1.1 Summary assessment of the available data and information ...... 91 7.1.2 Spatial distribution of data ...... 91 7.1.3 Pumping tests ...... 91 7.1.4 Groundwater abstraction ...... 92 7.1.5 Groundwater level ...... 92 7.1.6 Water quality...... 92 7.1.7 Groundwater salinisation ...... 93 7.1.8 Groundwater abstraction and estimation of resources ...... 93 7.2 Recommendations ...... 93 7.2.1 Data storage and availability ...... 93 7.2.2 Improving spatial distribution of data ...... 93 7.2.3 Hydrogeological Characterization ...... 94 7.2.4 Groundwater abstraction and resource estimation ...... 94 7.2.5 Groundwater level ...... 94 7.2.6 Water quality...... 95 8 Acknowledgements ...... 96 9 Bibliography ...... 97

Baseline Study Cà Mau ii German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

List of Figures Figure 2.1. Cà Mau administrative map with districts...... 5 Figure 2.2. Map of surface water bodies (rivers, canals)...... 6 Figure 2.3. Surface water salinization for some intrusion events in the Mekong Delta...... 8 Figure 3.1. Boreholes and wells collected from the evaluated reports...... 11 Figure 3.2. Schematic profile of the sedimentary succession in Cà Mau Province...... 12

Figure 4.1. Map of the distribution of aquifer qp2–3 in Cà Mau Province, including saline groundwater areas ...... 24

Figure 4.2. Map of the distribution of aquifer qp1 in Cà Mau Province, including saline groundwater areas ...... 26 2 Figure 4.3. Map of the distribution of aquifer n2 in Cà Mau Province, including saline groundwater areas ...... 27 1 Figure 4.4. Map of the distribution of aquifer n2 in Cà Mau Province, including saline groundwater areas ...... 29 Figure 4.5. Hydrogeological cross-sections in Cà Mau Province...... 32 Figure 4.6. Extracts from cross-sections I–I′ (left) and II–II′ (right) for borehole Q17704Z...... 33 Figure 4.7. Locations of pump-tested wells in report DWRPIS (2004) where raw data is available...... 35 Figure 4.8. Assessing applicability of Cooper-Jacob method: drawdown at investigation wells in percentage of the total drawdown at the end of the pumping period, as function of logarithmic time...... 38 Figure 4.9. Assessing applicability of Cooper-Jacob method: recovery at investigation wells in percentage of the total drawdown at the end of the pumping period, as function of logarithmic time since pumping stopped...... 38 Figure 4.10. Locations of available water samples, with corresponding aquifers...... 42 Figure 4.11. Histogram of ion balances of the available samples...... 42 Figure 4.12. Box plots of hydrochemical composition of groundwater of Pleistocene and Pliocene aquifers in Cà Mau Province. Data from DWRPIS (2004 and 2014). The box shows 1st and 3rd quartile, the whiskers extend to last data point within 1.5 times the box width...... 44 Figure 4.13. Piper diagram of available groundwater analyses for the Holocene and Pleistocene aquifers...... 48 Figure 4.14. Piper diagram of available groundwater analyses for the Pliocene aquifers...... 49

Figure 4.15. Ratio of alkaline ions to Chloride versus seawater mixing ratio fsea...... 50 Figure 4.16. Equivalent concentrations of alkaline versus earth-alkaline ions that are the result of reactions other than conventional mixing ...... 52 Figure 4.17. Equivalent concentrations of Alkalinity versus alkaline ions that are the result of reactions other than conventional mixing ...... 52 Figure 4.18. Map of TDS in aquifers around Cà Mau City...... 55 Figure 4.19. Time series of major chemical components in groundwater from LK81-II...... 57 Figure 4.20. Stable isotope data from Sóc Trăng in dry season (filled symbols) and rainy season (unfilled symbols) of 2013...... 58 Figure 5.1. Groundwater abstraction in the Mekong Delta ...... 60 Figure 5.2. Level of groundwater extraction in Cà Mau City...... 65 Figure 5.3. The level of groundwater extraction in U Minh District...... 66

Baseline Study Cà Mau iii German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Figure 5.4. The level of groundwater extraction in Dam Doi District...... 67 Figure 5.5.Level of groundwater extraction in Phu Tan District...... 68 Figure 5.6. Level of groundwater extraction in Thoi Binh District...... 69 Figure 5.7. Level of groundwater extraction in Tran Van Thoi District...... 70 Figure 5.8. Level of groundwater extraction in Cai Nuoc District...... 71 Figure 5.9. Level of groundwater extraction in Năm Căn District...... 72 Figure 5.10. Level of groundwater extraction in Ngoc Hien District...... 73 Figure 6.1. The national monitoring network in the Mekong Delta, Vietnam...... 76 Figure 6.2. Time series of mean water level in aquifers of the Mekong Delta...... 78 Figure 6.3. Monthly averages of groundwater level (m asl) of the qh aquifer in the Mekong Delta for the years 1995 (blue), 2005 (green), and 2016 (red)...... 79

Figure 6.4. Monthly averages of groundwater level (m asl) of the qp3 aquifer in the Mekong Delta for the years 1995 (blue), 2005 (green), and 2016 (red)...... 79

Figure 6.5. Monthly averages of groundwater level (m msl) of the qp2–3 aquifer in the Mekong Delta Delta for the years 1995 (blue), 2005 (green), and 2016 (red)...... 80

Figure 6.6. Monthly averages of groundwater level (m msl) of the qp1 aquifer in the Mekong Delta Delta for the years 1995 (blue), 2005 (green), and 2016 (red)...... 80 2 Figure 6.7. Monthly averages of groundwater level (m msl) of the n2 aquifer in the Mekong Delta Delta for the years 1995 (blue), 2005 (green), and 2016 (red)...... 81 1 Figure 6.8. Monthly averages of groundwater level (m msl) of the n2 aquifer in the Mekong Delta Delta for the years 1995 (blue), 2005 (green), and 2016 (red)...... 81 3 Figure 6.9. Monthly averages of groundwater level (m msl) of the n1 aquifer in the Mekong Delta Delta for the years 1995 (blue), 2005 (green), and 2016 (red)...... 82 Figure 6.10. Time series of groundwater levels in the qh aquifer and precipitation in Cà Mau City (Q17701T) and Năm Căn Town (Q199010)...... 83

Figure 6.11. Time series of groundwater levels in the qp3 aquifer and precipitation in Cà Mau City...... 83

Figure 6.12. Time series of groundwater levels in the qp2–3 aquifer and precipitation in Cà Mau City (Q177020) and Năm Căn Town (Q199020)...... 84

Figure 6.13. Time series of groundwater levels in the qp1 aquifer and precipitation in Cà Mau City...... 85 2 Figure 6.14. Time series of groundwater levels in the n2 aquifer and precipitation in Năm Căn Town...... 85 1 Figure 6.15. Time series of groundwater levels in the n2 aquifer and precipitation in Cà Mau City (top, Q17704Z) and Năm Căn Town (bottom, Q19904Z)...... 86

Figure 6.16. Groundwater level contours (m msl) of the qp2–3 aquifer...... 88

Figure 6.17. Groundwater level contours (m msl) of the qp1 aquifer...... 89 2 Figure 6.18 Groundwater level contours (m msl) of the n2 aquifer...... 90

Baseline Study Cà Mau iv German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

List of Tables Table 2.1. Area and population of the district-level divisions of Cà Mau Province...... 4 Table 2.2. Overview of main rivers and canals in Cà Mau...... 7 Table 2.3. Salinity data (g/l) along Ong Doc River for the late dry season of 2015...... 8 Table 2.4. Climatological parameters in Cà Mau...... 9 Table 3.1. Summaries of grainsize statistics for some stratigraphic units...... 14 Table 4.1. Classification of well productivities in Vietnam...... 21 Table 4.2. Stratigraphy and related hydrogeological units...... 22 Table 4.3. Results of pumping tests in boreholes of the Middle Pliocene aquifer ...... 28 Table 4.4. Results of pumping tests in boreholes of the Lower Pliocene aquifer...... 29 Table 4.5. Average hydrogeological parameters for aquifers in Cà Mau Province as used in resource assessment for Cà Mau City...... 34 Table 4.6. Characteristics of pumping wells and testing from DWRPIS (2004) ...... 36 Table 4.7. Results of pumping tests for transmissivity and hydraulic conductivity as stated by DWRPIS (2004) ...... 36 Table 4.8. Revised hydraulic parameters from pumping tests...... 40 Table 5.1. Summary of abstraction wells >200 m³/d in the Mekong Delta ...... 61 Table 5.2. Summary of groundwater extraction in Cà Mau Province...... 63 Table 5.3. Total of groundwater extraction in the districts of Cà Mau Province by aquifer...... 63 Table 5.4. Categories and map colours for specific abstraction rate of administrative units...... 63 Table 5.5. Exploitable groundwater resources (in m³/d) for Cà Mau City and Province from different sources, and actual extraction rates ...... 74 Table 6.1. Number of national monitoring wells in the aquifers of the Mekong Delta...... 77 Table 6.2. Averages of decrease of groundwater levels in wells of the Mekong Delta...... 78

Baseline Study Cà Mau v German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

List of Appendices (CD‐ROM only) 1. Locations (wells, boreholes) in evaluated data sources 2. Land use map of the Mekong Delta 3. Hydrogeological cross-sections of Cà Mau province 4. Borehole logs 5. Pumping test data 6. Water analyses 7. Groundwater abstraction data for the Mekong Delta 8. Groundwater abstraction data for Cà Mau province

Baseline Study Cà Mau vi German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Abbreviations BGR German Federal Institute for Geosciences and Natural Resources (Bundesanstalt für Geowissenschaften und Rohstoffe)

DONRE Department of Natural Resources and Environment

DWPRIS Division for Water Resource Planning and Investigation for the South of Vietnam (formerly Division for Hydrogeology – Geoengineering)

EC electrical conductivity

IGPVN Project “Improvement of Groundwater Protection in Vietnam”

NAWAPI National Centre for Water Resources Planning and Investigation

MD Mekong Delta

MARD Ministry of Agriculture

MOIT Ministry of Industry and Trade

MONRE Ministry of Natural Resources and Environment

ORP oxidation-reduction potential msl mean sea level (standard datum for elevations and water levels)

SD “South Division”, brief form for DWPRIS

TDS Total dissolved solids

Baseline Study Cà Mau vii German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

1 Introduction

1.1 Aim and scope of the baseline study This baseline study provides an overview of the existing hydrogeological information on the province Cà Mau in the Mekong Delta of Vietnam. In this regard, it serves as a basis for the BGR project “Improvement of Groundwater Protection in Vietnam”. The current project phase focuses on Cà Mau Province, where – unlike e.g. Sóc Trăng Province – available data on groundwater resources is rather scarce, and no major international projects have been carried out in recent time. There- fore it is important to summarize and assess the available data and information for prospective hydrogeological investigations and analyses, as groundwater resources in the region are already under stress and the demand for groundwater is continuously increasing. Furthermore, this baseline study intends to make hydrogeological information on Cà Mau Province accessible to a broader scientific community.

This baseline study report starts by briefly presenting the geography of the province, but mainly focuses on compiling relevant data and information about its hydrogeology. These have been retrieved from the archive of the Division for Water Resource Planning and Investigation for the South of Vietnam (DWRPIS, also called “South Division”). Available reports and maps are in Viet- namese, thus key parts have been translated for further use.

Subsequently, the information is critically assessed to identify open questions and gaps in knowledge which may need further clarification and investigations.

1.2 Overview of data sources Because this study is based on existing material, the main sources (hydrogeological reports) are briefly described here in the beginning, in addition to their bibliography references. This section outlines the scope and objectives of evaluated hydrogeological investigations, maps, and reports. The results are described in detail in the corresponding chapters of this study. The investigated locations (coordinates, type of investigation, depth, etc.) have been compiled into an Excel table (Appendix 1) as an overview of the evaluated data.

Report of evaluation groundwater resources Cà Mau Town. DWPRIS, 2000–2004, by request of MONRE and Ministry of Industry and Trade (MOIT). Main author: Eng. Tống Đức Liêm.

Objective: Assessment of hydrogeological conditions of the aquifers in Cà Mau Town area (distribution, lithology, water storage capacity); hydrogeological mapping of Cà Mau Town area 1:50000.

Investigations: Survey of an area of 452km², including data from a 1997 DWPRIS report covering 126 km², totalling 324 existing wells (UNICEF and private). 9 new boreholes were drilled and completed as monitoring wells. Pumping tests including water sampling were performed in these new wells. 10 already existing abstraction wells were used for pump rate monitoring. 102 existing borehole logs collected. 14 geophysical logs. 326 soil samples for granulometry, 360 samples for paleontology and pollen, 72 samples for algae. 203 new groundwater samples (analyses: 132 for

Baseline Study Cà Mau 1 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam major ions, 69 for iron; 9 for germs; 8 for heavy metals, cyanide, and phenol), 15 existing samples (major ions), 6 surface water samples (6 major ions). No isotope analyses. Water-level measurements 5/2003–4/2004 conducted every 5 days.

Results: 19 maps, incl. hydrogeological map 1 : 50 000 and 2 cross-sections. Report (150 p., 9 app.) with estimation of exploitable groundwater reserves for the Pleistocene and Pliocene aquifers. No detailed information about groundwater abstraction.

Report of survey of abstraction and use of groundwater, quality assessment and remedial measures for groundwater pollution in Cà Mau Province. DWPRIS, 2009, by request of Cà Mau DONRE. Main author: Eng. Nguyễn Kim Quyên.

Objective: Survey of abstraction and use of groundwater; quality assessment of groundwater. Iden- tify sources and the risk of groundwater contamination caused by the activities of living, industry, agriculture and fisheries, and especially unused wells, in order to develop measures to treat pol- luted groundwater resources.

Investigations: Survey of 5 295 km2, totalling 140 828 wells. No boreholes drilled and completed as monitoring wells; no pump rate monitoring of abstraction wells; 57 existing borehole logs collected. No pumping tests. 25 groundwater samples (iron, heavy metals, cyanide, germs), no surface water samples. No isotope analyses.

Results: 2 maps for the abstraction and use of groundwater 1 : 50 000 and 2 cross-sections. Report (133 p., 3 app.). No estimation of exploitable groundwater. Detailed information about groundwater abstraction of Cà Mau Province (141 148 wells, abstraction rate 361 604 m³/d, 3 238 broken wells without abstraction).

Report of Investigation and assessment to define restricted areas and limited areas for the new construction of groundwater extraction in the Province of Cà Mau. Services for Natural Resources and Environment Ltd., Cà Mau. 2010, by request of Cà Mau DONRE. Main author: Eng. Nguyễn Văn Thành.

Objective: Assessment of natural conditions (quality and potential reserves of underground water) compared to the current state of the utilization in the province (level of exploitation of the aquifers). Delineation of restricted areas and limited areas for the construction of new wells.

Investigations: Survey of 5 295 km2, totalling 514 existing wells. No boreholes drilled and completed as monitoring wells. No existing abstraction wells for pump rate monitoring. No existing borehole logs and geophysical logs collected. 75 groundwater analyses (60 samples for major ions, 15 heavy metals); no existing groundwater and surface water samples. No isotope analyses. Results: 14 maps, incl. hydrogeological map 1 : 100 000 and 2 cross-sections, 4 maps defining the restricted areas and limited areas for construction of new wells. Report (92p. and app.) estimates exploitable groundwater reserves (2 583 311 m³/d in Pleistocene and Pliocene porous aquifers), and includes the detailed information about groundwater abstraction in Cà Mau Province from the previous study (DWRPIS, 2009).

Baseline Study Cà Mau 2 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Report of Assessment of the impacts of groundwater abstraction and climate change on groundwater resources in Mekong Delta, Viet Nam. DWPRIS, 2011–2014, by request of MONRE and NAWAPI. Main author: Dr. Bùi Trần Vượng.

Objective: Assess the impact of climate change (change in precipitation, temperature and sea level rise) and groundwater abstraction on the Mekong Delta. Propose solutions to cope with the impacts of climate change on groundwater to serve effectively the sustainable economic and social development.

Investigations: Survey of 39 700 km² of the Mekong Delta including the entire area of Cà Mau, Kiên Giang and Bạc Liêu Provinces, totalling 59+78+34 (Cà Mau + Kiên Giang + Bạc Liêu) wells. No boreholes drilled and completed as monitoring wells; no existing abstraction wells for pump rate monitoring; 59+78+34 existing borehole logs collected. 13+21+21 geophysical logs collected. 9+30+14 existing pumping tests collected. 26+53+30 existing groundwater samples (major ions) collected, no surface water samples. No isotope analyses. Water-level measurements of 11 wells, 4/1995– 12/2010, monthly.

Results: 68 maps, incl. hydrogeological map 1 : 200 000 and 2+2+3 cross-sections. Report (248p., 8 app.) with estimation of exploitable groundwater reserves for Cà Mau Province (1 860 561 m³/d in Pleistocene, Pliocene, and Miocene porous aquifers), Kiên Giang Province (1 930 757 m³/d) and Bạc Liêu Province (3 403 710 m³/d), detailed information about groundwater abstraction in Cà Mau Province (67 328 wells, extraction rate 159 118 m³/d), Kiên Giang Province (93 130 wells, extraction rate 197 441 m³/d), Bạc Liêu Province (93 369 wells, extraction rate 248 728 m³/d).

Additional data sources

Polygons for administrative units (country, province, district, and commune levels) have been downloaded as ESRI Shapefiles from the “Global Administrative Areas” website (http://www.gadm.org). Some of the data sources mentioned above contain administrative boundaries, but only as MapInfo files which are not as convenient to use in the systems used in the IGPVN project (ESRI ArcGIS and QGIS). Additionally, they are sometimes outdated or using different, unknown coordinate systems.

Baseline Study Cà Mau 3 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

2 Overview of the study area

2.1 Location and administrative divisions Cà Mau is a coastal province located at the southern tip of Vietnam, in the Mekong delta region. It is about 370 km southwest of Ho Chi Minh City and 180 km from Cần Thơ City. The province has an extension of ≈100 km from north to south and is bounded by these coordinates:

 8°34′ and 9°10′ north

 104°43′ and 105°25′ east

Cà Mau Province is located on a peninsula which is surrounded by the sea. It is bordered by:

 Kiên Giang Province to the north

 Bạc Liêu Province to the northeast

 the South China Sea (Vietnamese East Sea) to the east and southeast

 the Gulf of Thailand (Vietnamese West Sea) to the west.

Figure 2.1 shows a map of the districts of Cà Mau Province.

The total natural area of Cà Mau Province is about 5 332 km² (General Statistics Office of Vietnam, 2016). It is subdivided in 9 district-level units: 1 provincial city (Thành phố Cà Mau) and 8 rural districts (huyện: Thới Bình, U Minh, Trần Văn Thời, Phú Tân, Cái Nước, Đầm Dơi, Năm Căn, Ngọc Hiển). Their areas and populations are summarised in Table 2.1. From 2010 to 2015 the total population of Cà Mau Province in total increased from 1.2085 million to 1.2189 million (General Sta- tistics Office of Vietnam, 2016). Accordingly, the population in most districts has continuously grown, esp. in Cà Mau City and U Minh District.

Table 2.1. Area and population of the district‐level divisions of Cà Mau Province. Data 2007–2009 from Cà Mau Statistical Yearbook 2009, reproduced in SNRE (2010); data 2010–2014 from National Statistical Yearbook (General Statistics Office of Vietnam, 2016), which does not include district level data; data for 2015 from Cà Mau Statistical Yearbook 2015, reproduced in DONRE Cà Mau (2017).

Administrative Area Population division (km²) 2007 2008 2009 2010 2012 2013 2014 2015 Cà Mau City 250 210 837 213 930 215 990 222 991 Thới Bình District 640 136 580 134 351 134 656 135 681 U Minh District 775 93 383 98 991 100 048 101 815 Trần Văn Thời 716 187 440 186 505 186 570 189 126 District Cái Nước District 417 137 530 137 653 137 878 138 444 Phú Tân District 464 102 993 103 639 104 284 103 894 Đầm Dơi District 826 181 736 181 776 182 403 183 332 Năm Căn District 509 66 509 66 515 66 541 65 719 Ngọc Hiển Dis- 733 78 153 78 332 78 610 77 819 trict Total 5332 1 195 161 1 201 692 1 206 980 1 208 500 1 212 100 1 214 200 1 216 400 1 218 821

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Figure 2.1. Cà Mau administrative map with districts.

2.2 Topography, land use, and hydrography As Cà Mau is located in the Mekong Delta region, the terrain is low and flat with many rivers and canals. Most of the area has a lower elevation than the high tide water level and is frequently inun- dated. The average elevation is about 0.4 – 0.6 m msl; about 0.2 m msl in the lowland and 0.8 – 1.1 m msl in the “higher” areas. The terrain gradually slopes from north to south and from the northeast to the southwest.

The Southern Institute for Water Resources Planning compiled the most recent (2011–2011) land use data of the Mekong Delta (SIWRP, 2015), see Appendix 2. The map shows the predominant land-use on a commune1-level. In the east of and in the southern centre of the province, land is mainly used for intensive and semi‐intensive shrimp as well as improved extensive shrimp production. The predominant use in the areas directly north and west of Cà Mau City is two crop rice / vegetable and rice / fresh‐water agriculture. In the north of the province (Thoi Binh District) the main land use class is rice / shrimp. In U Minh District (north-west) and Ngoc Hien District (far south) there are large areas of natural special use forest and production forest, and (in U Minh)

1 Administrative unit below district level.

Baseline Study Cà Mau 5 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam some rice / fresh‐water aquaculture. However, as the map shows only the major land-use, there still is agriculture and aquaculture even in the “forest” areas.

Cà Mau has an interlacing river and canal system, accounting for 3.02% of its natural area, with a total length of water courses of about 7 000 km, see Figure 2.2. There are 8 main rivers and 3 pri- mary canals, which are summarised in Table 2.2. The interlacing river and canal network facilitates the exchange between surface water and groundwater of the shallow aquifers.

Figure 2.2. Map of surface water bodies (rivers, canals). Data from SNRE (2010)

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Table 2.2. Overview of main rivers and canals in Cà Mau. (DWRPIS, 2014)

River, canal Year of con‐ Length Width Depth Geometry measured at struction (km) (m) (m)

Ganh Hao river – 56 60 – 200 5 – 14 Cà Mau – river mouth Cua Lon river – 58 600 – 1 800 19 – 5 Bo Đe – O. Trang Đan Doi river – 45 200 12 Tam Giang Bay Hap river – 50 250 3 river mouth Đong Cung river – 24 140 3 average over whole length of river Ong Đoc river – 60 300 4 river mouth Cai Tau river – 45 45 3.5 river mouth Trem Trem river – 33 80 3.5 river mouth Cà Mau – Bạc Liêu canal 1914 12 70 5 average over whole length of canal Phung Hiep canal 1917 18 70 4.5 average over whole length of canal Huyen Su canal unknown 11 35 4 average over whole length of canal

The tidal system in Cà Mau is affected by the irregular semi-diurnal tidal regime of the East Sea (South China Sea) and the irregular diurnal tidal system of the West Sea (Gulf of Thailand). The tidal amplitude of the East Sea is relatively high, about 3.0 – 3.5 m during spring tide, and about 1.8 – 2.2 m during neap tide. The tide of the West Sea is lower than the East Sea; the highest tidal amplitude is about 1.0 m.

The hydrologic regime is directly affected by the tidal system with broad river mouths leading to the sea. Moving inland, the effect of the tides reduces gradually, which causes the tidal amplitude and the propagation speed in the river to decrease. The systems of interlaced rivers and canals form a wetland that is suitable for agriculture.

Saltwater intrusion into surface water of the Mekong Delta occurs during dry season due to reduced freshwater flow in the canals and is also directly related to the sea tides. Figure 2.3 shows the intrusion for some severe intrusion events in the Delta in the last two decades. In these events, all of Cà Mau surface water was saline (based on the threshold of 4 g/l). However, even in typical yeers, one of Cà Mau’s main rivers, Ông Đốc River, which flows through the province from the centre to the Gulf of Thailand, usually has a salinity (TDS) greater than 25 – 27 g/l, even far inland (Table 2.3). As a comparison, sea water has a TDS of 35.8 mg/l. In southern Cà Mau, salinity in farm canals can exceed sea water TDS because of the high evaporation. In addition, under the impact of climate change induced sea level rise, saltwater intrusion is expected to increase.

Baseline Study Cà Mau 7 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Figure 2.3. Surface water salinization for some intrusion events in the Mekong Delta. (DWRPIS, un‐ published, pers. comm.)

Table 2.3. Salinity data (g/l) along Ong Doc River for the late dry season of 2015. (SIWRR, 2015)

Station / distance from Highest salinity (TDS in g/l) Note river mouth (km) March April May Tran Hoi (10) 28 – 30 28 – 30 28 – 30 Saline intrusion during the dry season. Tran Van Thoi (20) 28 – 30 28 – 30 28 – 30 No freshwater even at low tide. Khanh Binh (30) 27 - 29 27 - 29 27 – 29 No freshwater even at low tide. Tac Thu (40) 26 - 28 26 - 28 26 – 28 No freshwater even at low tide. Khanh Hoa (50) 25 - 27 25 - 27 25 – 27 No freshwater even at low tide.

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2.3 Climate Cà Mau Province is located in the subequatorial zone which is characterized by a tropical monsoon climate with two distinct seasons: rainy season and dry season. The rainy season lasts from May to December, the dry season lasts from December to April. The climatological parameters are shown in Table 2.4and summarised below.

Sunshine duration. The average annual sunshine duration is about 2 269 hours/year. The daily sunshine duration is about 6–8 hours with a maximum of 10–11 hours/day.

Temperature. The annual average temperature is about 26.6 °C to 27.7 °C, the highest monthly average temperature is 28.6 °C (in April and May), and the lowest monthly average temperature is 25.6 °C (in January). The difference between the hottest and the coldest month is about 3 °C.

Humidity. The annual average humidity is about 83 %. It is lower in the dry season, especially in March, when humidity often reaches just about 50 %.

Precipitation. The annual rainfall is about 2 360 mm, mostly in the rainy season (May–December). The number of rainy days is about 170–200 per year. In the west and southwest of the province, the rainy season usually begins earlier and lasts longer than elsewhere.

Evaporation. The average annual potential evaporation is nearly 1 000 mm. In the driest months the evaporation can reach nearly 130 mm, thus significantly exceeding rainfall from December to March.

Wind. Annually, there are two main monsoons: the winter monsoon (northeast monsoon) starts in early November lasting until April. The summer monsoon (southwest monsoon) is from May to October. The average wind speed is low, only at about 1–2 m/s inland and 2.5–3.5 m/s offshore.

Table 2.4. Climatological parameters in Cà Mau. Means over unknown time period, before 2010. (SNRE, 2010)

Month I II III IV V VI VII VIII IX X XI XII Rainfall (mm) 17.6 101.3 1.6 201.4 345.5 173.6 398.5 206.7 488.3 208.6 65.3 19.6 Pot. Evaporation (mm) 97.6 108.7 119.0 108.5 85.4 71.9 69.0 74.3 70.5 67.4 48.4 44.8 Humidity (%) 80 82 78 81 84 81 87 85 87 85 81 89 Temperature (°C) 25.1 26.7 28.6 26.7 28.2 28.8 27.2 28.3 27.0 27.5 27.4 26.7

Baseline Study Cà Mau 9 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

3 Geology of the study area In the Mekong Delta a series of unconsolidated Cenozoic sediments (marine transgression cycles) unconformably cover a Mesozoic basement. In the central part of the delta, the Cenozoic deposits reach a thickness of up to 700m. The depth of the basement and thickness of Cenozoic sediments decrease to the eastern and western margins of this sedimentary basin. In Cà Mau Province the Cenozoic deposits are 300 m thick on average. However, in the northwest (in Kiên Giang Province) the Mesozoic basement even crops out at the surface (Lap Nguyen et al., 2000), e.g. the mountain- ous islands in Rach Gia Bay.

The subsidence of the Mekong Delta basin in the Neogene was caused by the uplift of the Himalaya orgenesis, which was accompanied by high erosion rates in the mountains which provided large amounts of material that formed the Mekong Delta sediments. Repeated cycles of marine transgression and regression during the Neogene and Pleistocene lead to a sequence of marine and ter- restrial/alluvial facies. Glacial and events in the Pleistocene caused particularly strong regression and erosion. High sea levels led to saltwater intrusion into the Mekong Delta sediments, whereas the low sea levels, especially during the glacial events, resulted in flushing with fresh water. These processes resulted in a complicated pattern of fresh and saline water in the Mekong Delta sediments. The mostly fine-grained Holocene sediments protected the deeper layers against recent salinization.

The stratigraphy, lithology, and facies of the unconsolidated sediments, are briefly described from oldest to youngest in the following sections. This is based mainly on the findings of DWRPIS (2004) and includes extracts from borehole logs (mainly borehole LK82) with typical lithology and micro- paleontological evidence. The microfossil, pollen and algae samples were analysed and strati- graphically interpreted at the Institute for Geology (Hanoi). Because that report focused on Cà Mau City, most boreholes presented here are close to the city. Some of them are located in the western part of the neighboring province Bạc Liêu. Figure 3.1 shows the investigation boreholes and abstraction wells collected from the four main data sources; boreholes mentioned in the following sections have been labelled. The full list is presented in Appendix 1.

The abbreviations (stratigraphic symbols) consist of a capital letter for the period, a subscript for the epoch, a superscript for the sub-epoch/age and possibly further italic lower-case letters for the formation. Facies symbols (lower-case letters) are put in the front.

Figure 3.2 presents a schematic geologic profile for Cà Mau Province based on the findings presented in this chapter. The absolute ages for the stratigraphic units in the profile are taken from Wagner et al. (2012), based on the International Stratigraphic Chart 2009 (ICS, 2009). Please note that the Upper Pliocene in the stratigraphy used here corresponds to the Gelasium stage of the Pleistocene in the International Stratigraphic Chart. As there are no Upper Pliocene sediments in the Mekong Delta, this difference is of minor concern.

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Figure 3.1. Boreholes and wells collected from the evaluated reports.

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Figure 3.2. Schematic profile of the sedimentary succession in Cà Mau Province. Absolute ages accord‐ ing to Wagner et al. (2012).

3.1 Miocene (N1)

3 3.1.1 Upper Miocene, Phụng Hiệp formation (N1 ph) The Phụng Hiệp formation is the deepest unconsolidated sediment and therefore not as well investigated as the shallower formations. Its thickness varies from 60m (LK216, Năm Căn) to 100m (LK9596, Gia Rai). The bottom of these deep deposits, which corresponds to the top of the basement of the basin, dips gradually from the northwest to the southeast. The formation is uncon- 1 formably covered by sediments of the Cần Thơ formation (N2 ct).

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The micro-paleontological analysis at the depth interval 300–302m in borehole LK83 detected ferns (Acrostichum sp., Cystopteris sp., Lygodium sp., Polypodium sp.), gymnosperms (Pinus sp.), angiosperms (Aralia sp., Sonneratia sp.), and others. According to Nguyễn Hữu Dần (VIGMR, Vi- etnam Institute of Geosciences and Mineral Resources / Viện Khoa học Địa chất và Khoáng sản) and Nguyễn Huy Dũng (DWRPIS), cited in DWRPIS (2004), these species indicate a Late Miocene 3 age (N1 ).

There are two facies of this formation: alluvial-marine deposits are overlain by marine deposits.

3 3.1.1.1 Alluvial‐marine deposits (amN1 ph) Because deep boreholes have been drilled in the area only in the fine-grained sediments (silt, clayey silt, silty clay, sandy silt) on the upper part of the formation, the deeper coarse sediments (sand, silty sand) have yet to be studied, including their degree of cementation. At 2 wells (215B and LK82), a fine-sand layer with a thickness between 2.5 and 24 m was found. Inside this fine- sand many thin brown silty clay layers are intercalated, which are between 0.5 and 1.5 cm thick.

3 3.1.1.2 Marine deposits (mN1 ph) In Cà Mau Province, 9 boreholes encountered the top of the Phụng Hiệp formation in depths between 271m (LK86) and 344m (LK82). It consists of clayey silt, silty clay, sandy clay sediments and contains weathered laterite with many different colors from grey, ash grey, grey, yellow- brown to reddish-brown. The fine-grained sediments sometimes contain thin beds with plant remains and black coal. The sandy clay layers are intercalated with brown silt (thicknesses from 0.5 to 1.5cm).

3.2 Pliocene (N2)

1 3.2.1 Lower Pliocene, Cần Thơ formation (N2 ct) This formation is between 24 and 89m thick in Cà Mau Province, with an average of 41m. It is unconformably covered by the younger Năm Căn formation.

This sedimentary formation consists of 2 to 3 sedimentary cycles, each starting with coarse- grained sediments (sand, sandy silt) at the bottom and fining upward (silt, clay, clayey silt, silty clay, and silty sand). Grain size analysis statistics have been summarised in DWRPIS (2004) and are reproduced in Table 3.1.

The micro-paleontological analyses at the depth intervals 314–320m (LK 82), 260–263m and 294– 298m (LK83) detected ferns (Acrostichum sp., Cystopteris, sp., Lygodium sp., Osmunda sp., Poly- podiaceae, Stenochlaena palustris, Sphagnum sp.), gymnosperms (Pinus sp., Taxodiaceae), angiosperms (Aralia sp., Rhizophora sp., Rhus sp., Meliaceae, Quercus sp., Salix sp., Sonneratia sp.), foraminifera (Ammonia annectens aff.).

Based on the characteristics of the sediments and micro-paleontology, Cần Thơ formation was formed in a delta landscape with bays, estuaries, and shallow sea.

Le Duc An et al. (1981, unpublished, in DWRPIS (2004)) have divided the Cần Thơ formation into two facies. Again, alluvial-marine deposits are covered by marine deposits.

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Table 3.1. Summaries of grainsize statistics for some stratigraphic units. (DWRPIS, 2004) The text of the report is not clear if these results are for the whole formation or only the marine facies.

Stratigraphic unit Average diameter Md (mm) Sorting coeff. S0 Skewness coeff. Sk

N21ct 0.01 – 0.4 0.5 – 2 0.5 – 2

N22nc 0.01 – 0.4 0.5 – 2 0.5 – 2

Q11cm 0.06 – 0.3 1 – 2 0.5 – 1

Q12–3lt 0.04 – 0.7 1 – 4 0.5 – 1.5

1 3.2.1.1 Alluvial‐marine deposits (amN2 ct) 1 In Cà Mau Province area, the top of amN2 ct deposits is located at depths from 247m to 269m (average 259m). The thickness of the whole alluvial-marine deposits varies from 35m to 71m. They are mainly composed of relatively coarse-grained sediments, varying from fine sand to medium and coarse sand. Sometimes the sand layers are intercalated with clayey silt containing gravel and pebbles (size 0.2 – 1.5 cm). The coarse-grained sediments show variable colours, like ash-grey, grey-green, light-grey to dark-grey.

Coarse unconsolidated sediments occupy the main volume of the formation, they are interbedded with thinly laminated silt lenses. Their thickness varies; they reach maximum thickness in the Southeast with 64m (LK82), toward the centre, north, and southwest they gradually get thinner, with values of 14m (CM1), 17m (LK85) and 48m (LK83). In the LK80 borehole in the northwest no coarse-grained sediments have been found.

The lithology in borehole LK82 is particularly well described and therefore used as a reference; it includes (from bottom to top):

Stratum 1 (344 – 308m): The lower part of stratum is slightly gravelly sand, fine-sand interbedded with grey-green silt. The upper part consists of dark grey silty clay with grey-brown, grey, and white lenses of fine-sand which contain black plant remains.

Stratum 2 (308 – 274m): The lower part is sands and gravel, sometimes interbedded with thin layers of grey-green sandy silt. The upper part is pale yellow, grey, and white silty clay containing a little sand.

1 3.2.1.2 Marine deposits (mN2 ct) 1 In Cà Mau Province area, the top of mN2 ct deposits is located at depths from 237m to 255m or lower. Thickness varies from 7m to 21m.

Sediment composition includes fine-grained sediments (clay, silt, clayey silt, sandy silt) deposited throughout the region. Sometimes, the surface of the layer has been weathered, containing com- pacted hard laterite. The thickness of the fine-grained layer is between 7m and 21m.

Lithology in borehole LK82:

The lower part is greyish green, fine-sand interbedded with a few thin silt layers. The upper part is greyish green, clayey silt, sometimes interbedded with a few thin fine sand layers.

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2 3.2.2 Middle Pliocene, Năm Căn formation (N2 nc) These are the youngest Neogene sediments; there are no Upper Pliocene deposits. The thickness 2 of the whole Năm Căn formation (N2 nc) varies from 55 to 96 m, the average thickness is 79 m. It dips from the northwest to the southeast, and covers the sediments of the Cần Thơ formation 1 (N2 ct) unconformably.

Grain size analysis statistics have been summarised in DWRPIS (2004) and are reproduced in Ta- ble 3.1. The values are identical to those for the Cần Thơ formation. It is unclear if this is a coinci- dence or a mistake in the original report.

The micro-paleontological analysis at the depth intervals 200–226m (LK82) and 185–202m (LK83), detected ferns (Acrostichum sp.; Cystopteris. sp., Lygodium sp., Osmunda sp., Polypodi- aceae), gymnosperms (Pinus sp., Taxodiaceae; Polypodium sp.), angiosperms (Magnolia sp., Mal- vaceae, Poaceae, Rhizophora sp., Quercus sp.) foraminifera (Ammonia sp., Asterorotalia sp., Pseu- dorotalia sp., Textularia sp., Eponides sp., Quinqueloculina aff. vulgaris, Spiraloculina sp., Globig- erinoides sp.). According to DWRPIS (2004), the analysis indicates Middle Pliocene age.

Again, there are two facies of this formation: alluvial-marine overlain by marine deposits.

2 3.2.2.1 Alluvial‐marine deposits (amN2 nc) 2 In Cà Mau Province area, the top of amN2 nc deposits is located at depths from 166m to 218m (average 184m). Thickness varies from 38m to 79m. The formation is composed of 2 to 3 sedimentary cycles; each cycle with coarser sediments at the bottom and finer sediments at the top.

Coarse-grained sediments dominate the formation, continuing throughout the region with varying thickness. They reach maximum thickness in the centre: 71m (boreholes Q17704Z); toward the north, northwest, southwest and southeast they get thinner: 56m (LK85), 46m (LK80), 48m (LK83) and 38m (LK82). The coarse sediments gradually dip towards the southeast and rise to the northwest. They are composed mainly of fine sand to medium and coarse sand, sometimes intercalated with silty sand and blue, yellow, grey, brown clayey sand layers with a thickness of 0.2 – 1.0cm. The sand is composed mainly of quartz and silica, sometimes containing quartz gravels (0.2 – 0.5cm).

Lithology in borehole LK83 includes (from bottom to top):

Stratum 1 (240 – 218m): the lower part is grey clay, grey-brown clayey, slightly silty. The upper part is light yellow clayey silt containing a little sand and fine gravel.

Stratum 2 (218 – 197m): the lower part is fine-sand interbedded with many thin silt layers, which are 2 – 4 mm thick. The upper part is blue silty sand, sometimes containing gravel and plant remains. At the top is light brown clayey silt containing a few thin sand layers, thickness 2 – 4 mm, sometimes interbedded with thin layers of fine sand and plant remains.

The coarse-grained sediments are interbedded with laminated fine-grained lenticular sediments which are widely distributed throughout the area and found at varying depth.

2 3.2.2.2 Marine deposits (mN2 nc) These are fine-grained sediments (clay, silt, clayey silt) distributed throughout the region. Some- times at the top weathered laterite is found. This layer’s thickness varies from 1m to 30m.

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3.3 Pleistocene (Q1)

1 3.3.1 Lower Pleistocene, Cà Mau formation (Q1 cm) Nguyễn Ngọc Hoa et al. (1990, unpublished, contained in DWRPIS (2004)) established this formation in Cà Mau Province area, on the basis of a cross-sectional study in boreholes LK215B, CM3, CM4, LK80, LK81, LK83, LK85, LK87. Thickness changes from 36m to 87m, average thickness is 1 65m. The bottom of the Cà Mau formation (Q1 cm) dips gradually from northwest to southeast, 2 and unconformably overlays the sediments of Năm Căn formation (N2 nc).

Grain size analysis statistics have been summarised in DWRPIS (2004) and are reproduced in Ta- ble 3.1. The values are similar to the Pliocene formations, but they are spread over a slightly smaller range.

The micro-paleontological analysis at the depth intervals 146–186m (LK82) and 102–169 m (LK83), detected ferns (Acrostichum sp., Cyctopteris sp., Lygodium sp., Polypodiaceae), gymnosperms (Pinus sp., Taxodiaceae, Polypodium sp.), angiosperms (Malvaceae, Poaceae, Rhizophora sp., Sonneratia sp.), foraminifera (Ammonia sp., Asterorotalia aff. pulchella, Pseudorotalia aff. Schroeteriana, Spiraloculina sp., Globigerinoides sp., Gyroidinoides sp.). According to DWRPIS 1 (2004), these indicate an Early Pleistocene (Q1 ) age.

Based on the characteristics of the sediments and micro-paleontology, the Cà Mau formation was formed in shallow marine environments of coastal mangroves and estuaries.

Nguyễn Ngọc Hoa et al. (1990, unpublished, contained in DWRPIS (2004)) divided the formation into two facies: alluvial-marine, covered by marine.

1 3.3.1.1 Alluvial‐marine deposits (amQ1 cm) 1 In Cà Mau Province area, the top of the amQ1 cm deposits is located at depths from 84m to 154m (average 120m). Thickness varies from 19m to 77m. Sediment composition includes mainly fine to medium sand; coarse and silty fine-sand; grey-brown silty sand containing a little gravel. The sediment is interbedded with silt layers (thickness from 0.5 to 3.0cm) containing humus.

The coarse sediments have large thickness in the areas to the east, south, and north-east, achieving greatest thicknesses of 56m (borehole CM4) and 44m (LK85). The thickness decreases gradually toward the centre, northwest, and southwest and finally thins out: 35m (LK80) and 23m (LK83), 20 m (LK215B). In borehole LK87 in the east, only the silt layer is present, with a thickness of 21m.

Lithology in borehole LK215B includes (from bottom to top):

Stratum 1 (171 – 135m): The lower part is clayey sand alternating with silt layers and humus. Its colour is dark grey, grey, grey brown black. In the upper part, silt is clayey containing a little sand. Its colour is grey black.

Stratum 2 (135 – 97m): At the bottom is loamy sand containing silt and humus. At the top is silty clay containing a little sand and humus. It is blue, ash grey, or yellowish grey.

Lithology in borehole LK82 includes (from bottom to top):

Stratum 1 (200 – 158m): The lower part is green grey silty fine-sand. In the upper part, clayey silt is interbedded with thin sand layers. It is greyish-green or grey-brown.

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Stratum 2 (158 – 146m): At the bottom is dark grey fine sand, grey-brown fine sand, sometimes interbedded with thin silt layers, at the top is clayey silt containing thin lenses of brown, grey- green sand.

1 3.3.1.2 Marine deposits (mQ1 cm) In the study area, the top layer of the Cà Mau formation consists of fine-grained sediments, they maintain throughout the region and their thickness varies from a few meters to 30 – 35m.

2–3 3.3.2 Middle – Upper Pleistocene, Long Toàn formation (Q1 lt) 2–3 In Cà Mau Province area, the top of the Q1 lt deposits is located at depth from 42 m to 70 m (average 54 m). Thickness varies between 21 m and 94 m, with an average of 49 m. The bottom of the formation slopes from northwest to southeast, and unconformably overlays the sediments of the 1 Cà Mau formation (Q1 cm).

Grain size analysis statistics have been summarised in DWRPIS (2004) and are reproduced in Ta- ble 3.1. The values are similar to the Pliocene and Cà Mau formations, however average diameter and sorting coefficients tend to be higher (meaning less sorting).

The micro-paleontological analysis at the depth intervals 52–117 m (LK82) and 46–90 m (LK83) detected ferns (Acrostichum sp, Cystopteris. sp., Lycopodium sp., Polypodiaceae, Lygodium sp.), gymnosperms (Taxodiaceae), angiosperms (Rhus sp., Poaceae, Microlepia sp., Rhizophora sp.), foraminifera (Asterorotalia sp., Pseudorotalia sp., P. papuanensis, P. schroenthariana), mollusca, marine algae (Actinocyclus sp. at depth 99 m in LK82, Thalassiosira sp. at depth 89 m in LK82), brackish algae (Rhizosolenia sp. at depth 89 m in LK82, Aulacosira Granu, Eunotia morodo), freshwater algae (Synedra sp. at depth 71 m in LK82). According to DWRPIS (2004), they indicate Mid- 2–3 dle to Upper Pleistocene (Q1 ) age. Based on the characteristics of the sediments and micro-paleontology, the Long Toàn formation was formed in brackish and marine environments in a shallow coastal area.

Bùi Thế Định (1990), Nguyễn Ngọc Hoa (1997), Đỗ Tiến Hùng (1997) (all unpublished, cited in DWRPIS (2004)) divided the formation in two facies: alluvial-marine and marine.

2–3 3.3.2.1 Alluvial‐marine deposits (amQ1 lt) Coarse and fine sediments are alternating. The coarse-grained sediments lie at the bottom of the section, having greater thickness in the west, northwest, southwest and southeast with values of 19 m (LK87), 18 m (LK80), 31 m (LK83) and 26 m (LK82). They thin out toward the centre and the north, with thicknesses of 4.0 m (CM2) and 5.0 m (LK85).

The sediments are composed mainly of fine-medium sand; coarse grey silt; brown, grey-blue, yellowish-grey sands, sometimes interbedded with thin quartz gravel and sand, and thin silt layers.

Lithology in borehole LK82 includes (from bottom to top):

Stratum 1 (146 – 86m): sand with gravel; grey-green, dark grey fine sand, interbedded with silt; sandy clay; silty clay interbedded with thin fine sand layers.

Stratum 2 (86 - 77m): at the bottom is green-grey and yellow silty sand. At the top is green-grey silty clay, sometimes interbedded with fine sand layers.

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2–3 3.3.2.2 Marine deposits (mQ1 lt) The lower part consists of fine-grained sediments, at the surface sometimes weathered laterite is found. The fine-grained sediments maintain over a wide area and their thickness varies from a few meters to 40 – 60m. The thin weathered surface is the main basis for the demarcation between the two parts. The upper part is clayey silt, silt, silty sand, ash grey, grey-green, grey-white to light brown. The upper part in some places contains aggregations of red laterite, with a thickness of 2.0 – 3.0 mm, and gravel (size 0.2 – 1.0 cm).

Lithology in borehole LK82 includes (from bottom to top):

Stratum 1 (86 – 77 m): The lower part is greyish green, light yellow, silty sand. The upper part is greyish green, greyish brown, silty clay interbedded with thin fine sand layers.

Stratum 2 (77 – 52.5 m): The lower part is greyish green, silty sand. The upper part is greyish yellow, greyish light, silty clay.

3 3.3.3 Upper Pleistocene, Long Mỹ formation (Q1 lm) 3 In Cà Mau Province area, the top of Q1 lm deposits is located at depths from 12.5 m to 45 m (average 26 m). Thickness ranges from 15 m to 41 m, average thickness is 27 m. The sediments of the 2–3 Long Mỹ formation unconformably overlay the Long Toàn formation (Q1 lt) and are overlaid by 1–2 Holocene (Q2 ). In borehole LK215B the following foraminifera have been found: Pararotalia minura, Sigmoiioides sp., Bolovina sp. Pollen are mainly Labitaceae sp. and Poaceae sp. Indications of wooden vegetation are Rhizophoza sp, gene Paimeae sp. Spores have been found of Cystopteris sp., Lygodium sp., Pol- 3 ypodiaceae. According to DWRPIS (2004), they indicate the Upper Pleistocene (Q1 ) age.

Based on the characteristics of the sediments and micro-paleontology the formation was formed in shallow marine environments.

Nguyễn Ngọc Hoa et al. (1990, unpublished, cited in DWRPIS (2004)) divided it in two facies: alluvial-marine and marine.

3 3.3.3.1 Alluvial‐marine deposits (amQ1 lm) These coarse sediments are intermittently distributed with variable thickness from a few meters to 15 – 20 m. In the west they reach maximum thickness with 19 m (LK81), and thin out toward the centre, south-east of Cà Mau City: 12 m (LK82), 8 m (CM2); 12 m (CM1). In the west (LK87), northwest (LK80), southwest (LK83) and north (LK85) coarse-grained sediments are not found.

These coarse-grained sediments are fine sand to medium sand; grey-blue, light yellow silty sand; sometimes gravelly and sandy clay interbedded with thin silt layers.

3 3.3.3.2 Marine deposits (mQ1 lm) These deposits are mainly composed of fine-grained sediments: clay; silty clay; clayey silt; silt; grey light yellow to golden brown silty sand; the upper parts have been weathered, forming patchy reddish brown laterite. The layers of sandy silt and silt have thin horizontal layering (0.5 – 1.0 cm thick), interbedded with thin layers of fine sand. Also, the top of the layer contains iron oxide, black brown shells, mussels, and poorly decomposed plant remains. Fine sediments occupy the main

Baseline Study Cà Mau 18 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam volume of the marine deposits, maintaining throughout the region and having a thickness between 5 – 10 m and 30 – 40 m. Sometimes they contain laterite.

3.4 Holocene (Q2)

1–2 3.4.1 Lower to middle Holocene (Q2 ) 1–2 In Cà Mau Province area, the top of Q2 deposits is located at depths from 1.0 m to 6.0 m (average 3.5 m). The thickness varies from 10 to 40 m. The sediments unconformably overlay Long Mỹ formation.

The formation consists mainly of blue-grey fine-grained soils and contain humus.

The micro-paleontological analysis at the depth intervals 6–29 m (LK215B) and 2.5–25 m (LK83) detected ferns (Acrostichum sp., Cystopteris. sp., Lygodium, Gleichenia sp., Polypodium sp., Vit- taria sp.), gymnosperms (Pinus sp., Taxodiaceae), angiosperms (Magnolia sp., Poaceae sp., Rhi- zophora sp.), saltwater algae (Coscinodiscus sp, Thalassiosira sp., Angstit, Noduliper; Subtilis), brackish algae (Rizosolema sp., Melosira sp.), Foraminifera (Pararotalia minura, Asterorotalia sp., Asterorotalia aff. pulchella, Textularia sp., Spiraloculina sp.). According to DWRPIS (2004), they indicate the Holocene (Q2) age. Based on the characteristics of the sediments and micro-paleontology, the sediments formed in a coastal-marine environment.

The formation is divided into two facies: mixed alluvial-marine and marine.

1–2 3.4.1.1 Mixed alluvial‐marine deposits (amQ2 ) This sediment is distributed only in a few places: 10 m (LKQ17704Z), 3m (LK80), 5 m (LK82) and 1.5 m (CM1). The deposits consist of fine sand and green-grey silty sand containing humus.

1–2 3.4.1.2 Marine deposits (mQ2 ) They are distributed throughout the study area at depths from 1.0 m to 6.0 m, consisting mainly of mud; clay; silty sand; silt; blue-grey, black- grey clayey silt containing humus.

2–3 3.4.2 Middle to upper Holocene (Q2 ) There is only one facies in this sub-epoch.

2–3 3.4.2.1 Alluvial‐marine deposits (amQ2 ) These deposits are exposed at the surface in almost all areas and are mainly composed of clay and grey-golden fine sand interbedded with thin dark brown clay layers, which contains iron concre- tions of gravel size. They are easily compressible sediments, flexible, and very plastic. At some places good quality clay is mined. Sediment thickness varies between 1.0 m and 5.0 – 6.0 m.

3 3.4.3 Upper Holocene (Q2 ) There are two facies of these most recent sediments.

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3 3.4.3.1 Alluvial‐marine‐swamp deposits (ambQ2 ) These are distributed mainly in the northeast and the northwest region, the major components are clay, silt, brown-grey fine sand, humus, and poorly decomposed plants. Sediment thickness varies between 1.0 and 1.5 m.

3 3.4.3.2 Alluvial deposits (aQ2 ) They are distributed along the banks of the rivers and canals in the form of narrow strips of alluvial sediment. Sediment composition is mainly silt; clay; little fine sand and humus. Sediments are grey- brown, dark grey; when saturated with water they form a slurry; dehydrated they are quite firm.

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4 Hydrogeology of the study area

4.1 Hydrogeological setting The Vietnamese classification of geological formations as aquifers or aquitards is based on their water bearing and production capacity, as follows:

Aquifers are defined as geological formations with sufficient permeability to store and transmit water. That is, a water volume of economic significance can be extracted from the aquifer by springs, dug wells, or drilled wells. Moreover, the productivity of wells in l/s (not considering screen length) is classified according to Table 4.1.

Aquitards are defined as geological formations with low storage and permeability, thus being insufficient for extracting a water amount of economic significance. Accordingly, aquitards have a low hydraulic conductivity.

The division between fresh and salty groundwater in Vietnam is commonly based on total dissolved solids (TDS)2: fresh water has TDS below 1 g/l; water above 1 g/l is considered salty.

Table 4.1. Classification of well productivities in Vietnam.

Productivity Highly productive Moderately productive Poorly productive Well discharge (l/s) >5 1 – 5 <1

In Cà Mau Province seven aquifers and seven aquitards have been documented and studied, corresponding to the formations described in the previous chapter. Each formation usually has an aquitard at the top (marine facies) and an aquifer (alluvial-marine facies) at the bottom.

The commonly used abbreviations (hydrostratigraphic symbols) for the aquifers in the Mekong Delta (and thus for Cà Mau Province) consist of a lower-case letter which describes the geologic period, a subscript for the epoch, and a superscript for the sub-epoch/age. In maps and cross-section, blue colour is used for intergranular (porous) aquifers. The abbreviations which describe the aquitards consist of a capital letter for the period, and again of a subscript for the epoch and a superscript for the sub-epoch/age. In maps and cross-sections, a light brown colour is used for aquitards. Table 4.2 contains an overview of the nomenclature and sequence of hydrogeological units, the schematic geological profile in Figure 3.2 also includes the aquifer designations. A description of each aquifer, including its salinity status, is presented in the following section. Note that the presented nomenclature refers to the unconsolidated sedimentary formations only. The bedrock basement below the Mekong Delta is not considered here, as it does not provide significant amounts of groundwater, being rather regarded as aquitard.

2 In Vietnamese reports and maps, TDS is usually abbreviated by the symbol M (mineralisation).

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Table 4.2. Stratigraphy and related hydrogeological units.

No. Stratigraphy / Facies Hydrogeologic Units Formation Symbols Aquifers Aquitards

1 Multi-origin Holocene de- mQ21-2, amQ21-2, Q2

posits amQ22–3, ambQ23, aQ23 qh

2 Long Mỹ Formation mQ13lm Q13

amQ13lm qp3

3 Long Toàn Formation mQ12–3lt Q12–3

amQ12–3lt qp2–3

4 Cà Mau Formation mQ11cm Q11

amQ11cm qp1

5 Năm Căn Formation mN22nc N22

amN22nc n22

6 Cần Thơ Formation mN21ct N21

amN21ct n21

7 Phụng Hiệp Formation mN13ph N13

amN13ph n13

4.1.1 Aquifers The following sections provide brief characterisations of the aquifers, based on the information from the Report for the Delineation of Restricted Zones (SNRE, 2010). It also contains maps of the aquifers which combine the aquifer’s distribution and the occurrence of fresh and brackish/salty water from this report.

The Holocene intergranular aquifer (qh) and Upper Pleistocene intergranular aquifer (qp3) are distributed over the entire region of Cà Mau, with bottom depths of less than 45m. However, these aquifers are practically no longer used for water supply, and are considered as “water-scarce”. Thus, they are mostly not taken into consideration for hydrogeological investigations in SNRE (2010).

4.1.1.1 Middle ‐ Upper Pleistocene aquifer (qp2–3) 2–3 The aquifer qp2–3 is the basal part of the Long Toàn formation (Q1 lt). It is distributed across the 2–3 entire study area, and is not exposed on the surface. It is covered by the Q1 and Holocene (Q2) aquitards. Depth of the top of the aquifer ranges from 60 m to 115 m (borehole Q199 in the south of Cà Mau), with an average of 89 m. The depth of the aquifer bottom varies from 80 m to 146 m, with an average of 104 m. Its thickness varies from 2.0 m (LK81) to 31 m (LK83, Q199), average 14 m.

Lithological composition is fine - medium sand, with gravel. Sometimes it is interbedded with silt layers.

Productivity: Results of pumping tests in boreholes Q177020 and Q188020 in October 1998 sug- gest that aquifer qp2–3 is a moderately productive aquifer according to the Vietnamese classification (Table 4.1). Discharge Q of the wells was 3.41 l/s and 2.17 l/s, drawdown s was 21.60 m and

Baseline Study Cà Mau 22 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

28.92 m, and specific capacity3 Q/s at the two wells amounted to 0.16 l/sm and 0.075 l/sm, respectively. The static water levels were 8.15 m for Q177020 and 1.73 m for Q18820 (DWRPIS, 2004, Appendix 4.4). This large difference is surprising, as the wells are both in Cà Mau City and quite close to each other. However, their filter screens are at different depths (82–87 m for Q177020, 103–106 m for Q188020), so it might be conceivable that they tap two different layers of a locally subdivided aquifer (refer to the discussion in Section 4.1.3).

Report SNRE (2010) contains the most recent maps of the distribution of aquifers, combined with the saltwater-freshwater boundary (isolines of concentration of dissolved solids TDS = 1g/l). Fig- ure 4.1 shows this map for the qp2–3 aquifer. The investigation data points used for delineating the aquifer distribution are not indicated in the map.

The 1 g/l isoline as the “saltwater interface” has been interpolated using data from various sources: conductivity measurements and water samples in wells and boreholes, borehole geophysics (“Karota”), and several profile lines of vertical electrical sounding (VES) that run through the Mekong Delta. It could not be clearly determined from the reports how these isolines have been constructed. The general procedure is outlined in DWRPIS (2004): analyses of TDS and resistivity from corresponding borehole logs are plotted in a log-log diagram and a straight line is fitted. (In the case of DWRPIS (2004), TDS ranged between 0.37 and 11 g/l, resistivity between 2.5 and 46 Ωm.) This fit line is then used to convert resistivities from VES measurements to TDS at the investigated profiles. This indirect approach seems susceptible to inaccuracies. It is also questionable how much information on deeper aquifers can be extracted from VES, esp. if they are already capped by another saline aquifer. Furthermore, the density of available boreholes is not known, especially in the rural areas. The saltwater interface maps do not show the interpolation points and method used to derive the isolines from the investigation data.

According to the assessment in SNRE (2010), fresh water (TDS < 1 g/l) in qp2–3 is distributed in Cà Mau Province over a total area of 3 600 km². (Figure 4.1) The report mentions resistivity measurements from borehole logging in LK85 and 215-III-NB have values from 12 to 21 Ωm; the exact method (electrode spacing, averaging, etc.) is not mentioned.

Saline water area (TDS > 1 g/l): According to the assessment in SNRE (2010) (Figure 4.1), saline water in qp2–3 is distributed in Cà Mau Province over a total area of 1 700 km². The resistivity measurements from borehole logging in LK83 are reported to have values from 1 to 2 Ωm.

3 Please note that the symbol q is used in the original reports for specific capacity.

Baseline Study Cà Mau 23 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Figure 4.1. Map of the distribution of aquifer qp2–3 in Cà Mau Province, including saline groundwater areas, as presented in SNRE (2010). Please consult the text on the reliability of the information pre‐ sented here.

4.1.1.2 Lower Pleistocene aquifer (qp1)

The aquifer qp1 is the basal part of the Cà Mau formation. It is distributed across the entire study 1 area and covered by the aquitard Q1 . Depth of the top of the aquifer changes from 84 m to 154 m, with an average of 120 m. Depth of the aquifer bottom varies from 155 m to 200m, with an average of 169 m. Thickness varies from 19m (CM2) to 71 m (LK83), with an average of 23m. Lithological composition is fine sand, medium – coarse sand, silty sand, with interbedded layers of silt.

Productivity: Results of pumping tests in boreholes LK81II, Q188030, and GR show that this is a poorly to highly productive aquifer (according to Vietnamese classification, see Table 4.1).

In the area of Cà Mau City, the aquifer is poorly to moderately productive. During pumping tests, discharge Q of the wells LK81II and Q188030 was 1.20 l/s and 0.54 l/s, drawdown s was 31.12 m and 43.90 m, and specific capacity Q/s at the two wells amounted to 0.039 l/sm and 0.012 l/sm, respectively. The static water levels were 9.30 m for LK81II and 4.10 m for Q18830 (DWRPIS, 2004, Appendix 4.4).

In the other districts of the province, this is a highly productive aquifer. During the pumping test in borehole GR the discharge was Q = 21.0 l/s, the drawdown s = 7.90 m, and the specific capacity Q/s = 2.66 l/sm. The static water level was 7.60 m. (DWRPIS, 2004)

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According to the pumping tests results in the multi-aquifer well group LK81, it could not be clarified whether a hydraulic connection exists (e.g. by leakage) between the Lower Pleistocene aquifer 2 1 qp1 and the deeper aquifers n2 and n2 . During pumping of any of these wells, the other wells did not react, but each pumping test lasted only for 2 to 4 days.

Fresh water area (TDS < 1 g/l): According to SNRE (2010) (Figure 4.2), fresh water in qp1 is distributed in Cà Mau Province in a total area of 2 900 km2. The resistivity measurements in boreholes LK83 and LK85, have values ranging from 8 to 28 Ωm.

Saline water area (TDS > 1 g/l): According to SNRE (2010) (Figure 4.2), saline water in qp1 is distributed in Cà Mau Province in a total area of 2 400 km². The resistivity measurements in borehole 215-III-NB have a values around 10 Ωm.

Please refer to the previous section on aquifer qp2–3 for a discussion on the saltwater distribution maps and resistivity measurements.

2 4.1.1.3 Middle Pliocene aquifer (n2 ) 2 The aquifer n2 is also distributed across the entire study area, forming the basal part of the Năm 2 Căn formation. It is covered by the Middle Pliocene aquitard (N2 ). Top depth of the aquifer changes from 166 m to 218 m, with an average of 184 m. Bottom depth varies from 237 m to 255 m, with an average of 245 m. Thickness varies from 38 m (LK215B) to 79 m (LK80), with an average of 62 m.

Lithological composition is fine sand, medium – coarse sand, silty sand; with sandy clay layers (0.2 to 10.0cm thick).

Productivity: Results of pumping tests in boreholes (DWRPIS, 2004) show that this is a highly productive aquifer. Discharge ranged from Q = 5.1 to 33.9 l/s, causing drawdowns s = 5.20 – 22.40m, specific discharges were Q/s = 0.23 – 4.70 l/sm. The static water level were 1.27 – 9.50m. A sum- 2 mary of the pumping tests in n2 is provided in Table 4.3. According to results of pumping tests in the multi-aquifer well group LK82, it could not be resolved whether a hydraulic connection (e.g. 2 by leakage) exists between the Middle Pliocene aquifer n2 and the top and bottom aquifers qp1 1 and n2 , respectively. During pumping of any of these wells, the other wells did not react, but each pumping test lasted only for 2 to 4 days.

2 Fresh water area (TDS < 1 g/l): According to Figure 4.3, fresh water in n2 is distributed in Cà Mau Province in a total area of 3 800 km². The resistivity measurements in boreholes LK80 and S147 have values from 15 to 23 Ωm.

2 Saline water area (TDS > 1 g/l): According to SNRE (2010) (Figure 4.3), saline water in n2 is distributed in Cà Mau Province in a total area of 1400 km². The resistivity measurement in borehole LK83 has values around 3 Ωm.

Please refer to the previous section on aquifer qp2–3 for a discussion on the saltwater distribution maps and resistivity measurements.

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Figure 4.2. Map of the distribution of aquifer qp1 in Cà Mau Province, including saline groundwater areas, as presented in SNRE (2010). Please consult the text on the reliability of the information pre‐ sented here.

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2 Figure 4.3. Map of the distribution of aquifer n2 in Cà Mau Province, including saline groundwater areas, as presented in SNRE (2010). Please consult the text on the reliability of the information pre‐ sented here.

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Table 4.3. Results of pumping tests in boreholes of the Middle Pliocene aquifer, as presented in DWRPIS (2004) Median added as a more suitable way of averaging.

Borehole Welldepth(m) Static water Drawdown Discharge Specific capacity level (m) s (m) Q (l/s) Q/s (l/sm) LK80 236 6.03 22.40 5.12 0.23 LK81I 238 8.50 8.67 8.70 1.00 LK82 258 5.60 14.00 10.35 0.74 LK83 242 6.25 21.29 7.33 0.34 LK85 250 5.80 11.36 10.08 0.89 LK86 236 5.44 12.41 9.72 0.78 LK87 233 6.22 11.43 12.47 1.09 CM1 241 4.84 6.63 26.37 4.00 CM2 244 4.46 9.20 23.99 2.61 CM3 252 6.59 6.36 29.90 4.70 CM4 234 3.83 12.47 23.30 1.87 LK215A 248 1.27 9.19 7.33 0.80 Q17704T 225 3.98 5.39 5.39 1.36 LK1 230 4.10 10.00 13.89 1.39 LK4 260 9.50 8.30 33.89 0.63 LK15 252 1.30 15.70 14.00 0.89 LK19 250 1.30 18.00 16.00 3.85 LK22 245 2.00 5.20 20.00 0.89 LKSOS 237 6.70 22.30 11.11 0.48 Max 260 9.50 22.40 33.89 4.70 Min 225 1.27 5.20 5.12 0.23 Mean 243 4.91 12.12 15.21 1.50 Median 242 5.44 11.36 12.47 0.89

1 4.1.1.4 Lower Pliocene aquifer (n2 ) 1 1 The aquifer n2 is the basal part of the Cần Thơ formation (amN2 ct ). It is widely distributed in 1 most of the study area, except in the northwest where it adjoins bedrock. Aquifer n2 is covered by 1 the Lower Pliocene (N2 ) aquitard. Top depth of the aquifer ranges from 247m to 269m, average 259m. Bottom depth varies from 270m to 344m, average 285m. Thickness varies from 2.0m (LK81) to 82m (LK82), average 21m.

Lithological composition is fine sand, medium – coarse sand, silty sand, with interbedded layers of silty sand and silt.

Productivity: Results of pumping tests (DWRPIS, 2004) show that this is a moderately to highly productive aquifer. Discharges were Q = 1.90 – 11.67 l/s, causing drawdowns s = 6.28 – 37.91 m, specific capacities were Q/s = 0.050 – 1.608 l/sm. Static water levels were 0.69 – 8.05 m. A summary 1 of the pumping tests in n2 is provided in Table 4.4. According to the pumping tests results in the multi-aquifer well group LK81, it could not be clarified whether a hydraulic connection exists (e.g. 1 2 by leakage) between the Lower Pliocene aquifer n2 and the overlying aquifers n2 and qp1. During

Baseline Study Cà Mau 28 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam pumping of any of these wells, the other wells did not react, but each pumping test lasted only for 2 to 4 days.

1 Fresh water area (TDS < 1 g/l): According to SNRE (2010) (Figure 4.4), fresh water in n2 is distributed in Cà Mau Province in a total area of 700 km². The resistivity measurements in boreholes Q17704Z and LK251B have values from 15 to 40Ωm (DWRPIS, 2014).

1 Saline water area (TDS > 1 g/l): According to SNRE (2010) (Figure 4.4), saline water in n2 is distributed in Cà Mau Province in a total area of 4 100km².

1 Figure 4.4. Map of the distribution of aquifer n2 in Cà Mau Province, including saline groundwater areas, as presented in SNRE (2010). Please consult the text on the reliability of the information pre‐ sented here.

Table 4.4. Results of pumping tests in boreholes of the Lower Pliocene aquifer. (DWRPIS, 2004)

Borehole Well depth Static water Drawdown Discharge Q Specific capacity (m) level (m) s (m) (l/s) Q/s (l/sm) LK81 282 8.05 6.28 10.08 1.61 LK215B 281 0.69 37.91 1.90 0.05 Q17704Z 271 3.98 31.13 2.64 0.09

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3 4.1.1.5 Upper Miocene aquifer (n1 ) 3 The aquifer n1 is distributed in most parts of the study area. It is the bottom part of the Phụng 3 Hiệp formation and is covered by the Upper Miocene (N1 ) aquitard. Top depth of the aquifer 3 changes from 318 m to 329 m, with an average of 323 m. The aquifer n1 is deep, and not enough research boreholes have been drilled to determine accurately the bottom and thickness of this aquifer.

Lithological composition is fine sand, medium – coarse sand, silty sand, sandy clay.

Report SNRE (2010) does not contain a map for this aquifer. Report DWRPIS (2014), however, gives sizes of fresh and saline water areas:

3 Fresh water area (TDS < 1 g/l): fresh water in n1 is distributed in Cà Mau Province in a total area of 20 km².

3 Saline water area (TDS > 1 g/l): saline water in n1 is distributed in Cà Mau Province in a total area of 4 000 km².

Please refer to Section 4.1.1.1 for a discussion on the saltwater distribution. For this aquifer, the given areas should be especially taken with care, as the aquifer has not enough boreholes (see above) and the saltwater distribution is likely based mainly on surface geophysics, with which salinity of deep layers is difficult to determine.

4.1.2 Aquitards 2–3 3 The Middle – Upper Pleistocene aquitard (Q1 ), Upper Pleistocene aquitard (Q1 ), and Holocene aquitard (Q2) are continuously distributed in the entire province of Cà Mau. The bottom depth of 2–3 the Q1 aquitard ranges from 60 m to 118 m, with an average of 89 m.

Lithological composition is clayey silt, silty clay, silt, sometimes with fine sand and humus, and laterite gravel.

1 4.1.2.1 Lower Pleistocene aquitard (Q1 ) 1 The Lower Pleistocene aquitard Q1 is distributed across the entire study area and covered by aquifer qp2–3. Top depth of the aquitard ranges from 80 m to 146 m, with an average of 104 m. Bottom depth varies from 96 m to 154 m, with an average of 121 m. Thickness varies from 4.0m (CM4) to 75 m (LK81), with an average 28 m.

2 4.1.2.2 Middle Pliocene aquitard (N2 ) 2 The aquitard N2 is distributed across the entire study area as well, and is covered by the aquifer qp1. Top depth of the aquitard varies from 154 m to 200 m, with an average of 167 m. Bottom depth varies from 166 m to 218 m, with an average of 184 m. Thickness varies from 3.0 m (CM4) to 33 m (LK81),with an average of 14 m.

Lithological composition is silty clay, clayey silt, silt, sandy silt. Sometimes layers of sand and fine sand are interbedded, which are layered horizontally. Sometimes a slightly weathered silty clay containing laterite gravel and plant remains is observed.

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1 4.1.2.3 Lower Pliocene aquitard (N2 ) 1 2 The Lower Pliocene aquitard N2 is distributed across the entire study area, covered by aquifer n2 . Top depth of the aquitard ranges from 237 m to 255 m, with an average of 249 m. Bottom depth varies from 247 m to 269 m, with an average of 259 m. Thickness varies from 7.0 m (LK82) to 30 m (LK85), with an average of 14 m. Lithological composition is clay, clayey silt, silty clay.

3 4.1.2.4 Upper Miocene aquitard (N1 ) 3 The Upper Miocene aquitard N1 is distributed across most of the study area, and covered by the 1 aquifer n2 . Top depth of the aquitard ranges from 271 m (LK86) to 344 m (LK82), with an average of 287 m. Lithological composition is silty clay, clayey silt, fine sand.

There are no further, in-depth studies of this aquitard.

4.1.3 Cross‐sections and hydraulic connections In Cà Mau Province, two hydrogeological cross sections have been constructed as part of the project “Investigation and assessment to define restricted areas and limited areas for the new construction of groundwater extraction in the province of Cà Mau” (SNRE, 2010), see Figure 4.5. Full- size versions are included in Appendix 3.

As already mentioned above, it can be seen that the upper 40 to 100m mainly consist of an aquitard 3 2–3 complex of Q2, Q1 and Q1 . All aquifers in greater depths are therefore confined. The first aquifer of considerable extent and interest is aquifer qp2–3, which seems to be hydraulically connected to 2–3 aquifer qp3 in the north of Cà Mau Province, but separated by aquitard Q1 in the rest of the province. The thickness of aquifer qp2–3 decreases to the south and west.

Around Cà Mau City, all aquifers tend to be split up into sub-aquifers. This is most obvious in sec- 2 tion I–I′. From the cross-sections, it can be seen that e.g. the n2 aquifer is split into several sub- 2 aquifers, as well as that the N2 aquitard is split into several sub-aquitards, especially in the area around LK81. However, it has to be noted that the density of documented boreholes is higher around Cà Mau City, thus the cross-sections are more detailed there, in similar splits may be present in other, less investigated areas. In this regard, the cross section suggests the existence of hydraulic connections between sub-aquifers within the definition of main aquifers. Furthermore 2 it can be seen that in the northern area the aquitard which separates n2 from qp1 is rather thin. Not much information on hydraulic connections between main aquifers is available yet; pump- 1 2 testing each well of the multi-aquifer well group LK81 (n2 , n2 , qp1) did not show a reaction in the other wells. However, each pumping test lasted only for 2 to 4 days.

The borehole log of Q17704Z in Cà Mau City, where the two cross-section lines intersect, was interpreted differently in the two cross-sections, see Figure 4.6. Cross-section I–I′ shows a locally subdivided aquifer qp3 at 30–55 m depth, whereas II–II′ shows only a sequence of aquitards. The 2–3 top of aquifer qp2–3 is at 82 m in both cross-sections, but I–I′ interrupts it with a thick Q1 aquitard 1 from 90–102 m, whereas in II–II′ qp2–3 extends down to 96 m, followed by the Q1 aquitard and qp1 aquifer. This could explain the different static water levels from pumping tests mentioned in Sec- tion 4.1.1.1. The well Q177020 is screened at 82–87 m, which is qp2–3 in both cross-sections, but the nearby well Q188020 is screened at 103–106 m, which could either still be qp2–3 or already qp1.

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Figure 4.5. Hydrogeological cross‐sections in Cà Mau Province. (SNRE, 2010) Top: line I–I′, north–south. Bottom: line II–II′: west–east.

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Figure 4.6. Extracts from cross‐sections I–I′ (left) and II–II′ (right) for borehole Q17704Z.

4.2 Aquifer characterisation and testing This section concentrates on the description of aquifers by means of their hydraulic parameters, i.e. their ability to conduct and store water. The corresponding physical properties are the hydraulic conductivity K or transmissivity , respectively, and the storativity . The diffusivity of an aquifer is then defined as . The most common field experiment to estimate hydraulic aquifer parameters in situ are pumping tests. The evaluation of pumping tests generally provides the transmissivity T of the aquifer, which is related to conductivity by , where is aquifer thickness.

Conventional pumping test evaluation is based on the Theis equation (Theis, 1935) which describes groundwater flow to a well in a homogeneous confined aquifer of infinite extent by

, , with ; where is the drawdown at distance from the pumping well, is the time since pumping started, is the constant discharge rate at the well, and is the exponential integral function, also known as the well function.

Baseline Study Cà Mau 33 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Other techniques investigate rock or soil properties in the laboratory to relate corresponding characteristics to the hydraulic conductivity, e.g., relating the grain size distribution of a soil sample to K by empirical equations (for example Hazen and Beyer estimation formulas).

Available estimates of transmissivity and the hydraulic conductivity for Cà Mau Province are briefly reviewed in the following.

4.2.1 Available data Estimates of hydraulic parameters in Cà Mau Province are rare. To the knowledge of the authors of this study, existing estimates of the hydraulic conductivity and aquifer storativity mainly rely on the evaluation of single-well pumping tests , for which raw data is not always available. The pumping tests have been documented and evaluated in the “Report of evaluation groundwater resources Cà Mau Town” (DWRPIS, 2004) and are listed in Section 4.1.1 (esp. Table 4.3 and Table 4.4) for the various aquifers. The corresponding project was realized by the Subdivision for Water Re- sources Planning and Investigation for Hau River and DWRPIS. They conducted pumping tests in 9 investigation wells, all located close to Cà Mau City (see location map in Figure 4.7) between 2001 and 2003. Additional pumping tests have been carried out earlier, but the corresponding reports do not include the raw data of the tests and hydraulic parameters of investigated aquifers; instead, they mainly concentrate on estimating well performance.

Furthermore, the literature review did not find many laboratory analysis of hydraulic parameters. Grain-size analyses from 25 shallow drillings (less than 30 m deep) for geotechnical investigations are available (DWRPIS, 1996), but they have not been evaluated with a hydrogeological scope.

Also, these drillings just reached the Holocene (qh) and the Upper Pleistocene aquifers (qp3). Due to salinization and groundwater pollution these aquifers are only of minor interest for water use.

DWRPIS (2004) provides average values of hydraulic aquifer parameters as summarized in Table 4.5. From the report it does not always become clear how these values were determined. By utiliz- ing the equations given in the report, it was for example not possible to reproduce the stated values for storativity. Please note also that in the report Sy and S are termed gravitational and elastic storativity, respectively, which are considered to relate to the internationally more recognized terms of specific yield and storativity.

Table 4.5. Average hydrogeological parameters for aquifers in Cà Mau Province as used in resource assessment for Cà Mau City. (DWRPIS, 2004) Sample sizes for averaging are not known for all aquifers.

Aquifer N K (m/s) m (m) T (m²/s) Sy D (m²/s) S

qp3 ? 4.94 · 10−4 29.14 1.44 · 10−2 0.200 3.06 4.70 · 10−3

qp2–3 ? 3.32 · 10−4 13.70 4.55 · 10−3 0.189 148.21 3.07 · 10−5

qp1 1 1.94 · 10−5 23.30 4.53 · 10−4 0.126 8.68 5.22 · 10−5

n22 7 1.75 · 10−4 47.60 8.34 · 10−3 0.172 77.94 1.07 · 10−4

n21 1 4.36 · 10−4 21.00 9.15 · 10−3 0.196 193.45 4.73 · 10−5

n13 ? 6.6 · 10−5 58.80 3.88 · 10−3 0.150 77.6 5.00 · 10−5

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Figure 4.7. Locations of pump‐tested wells in report DWRPIS (2004) where raw data is available.

4.2.2 Pumping test performance and evaluation Eight out of nine pumping tests in DWRPIS (2004) were conducted in Cà Mau Province, one pump- 2 ing test (LK82, n2 aquifer) was conducted in Dinh Thanh Commune, Dong Hai District, Bạc Liêu Province, close to the border of Cà Mau City. Seven pumping tests were performed in the Middle 2 1 Pliocene aquifer (n2 ); one was performed in the Lower Pliocene aquifer (n2 ), and another one in the Lower Pleistocene aquifer (qp1). Corresponding data is included in Appendix 5.

The execution of pumping tests followed the decision 46/2000/QĐ-BCN of the Ministry of Industry on “Guidelines about pumping test in hydrogeological investigation”, issued August 7th, 2000. Groundwater was abstracted with an electric submersible pump, and water levels were measured manually. All pumping tests were carried out as single-well test, i.e., the drawdown was recorded at the pumped well only. The tests consisted of a single drawdown phase while pumping at a constant rate until the water levels reached steady state.

Baseline Study Cà Mau 35 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

The main characteristics of pumped wells and performed tests are given in Table 4.6. Pumping tests lasted from 48 to 96 hours, whereas observation of recovery was rather short (10 – 2 24 hours). Whereas the majority of investigation wells tap aquifer n2 , the well cluster LK81, LK81- I, LK81-II was designed to investigate the multi-layered aquifer system as a whole and the hydrau- 1 2 lic relation between Lower Pliocene aquifer (n2 ), Middle Pliocene aquifer (n2 ) and Lower Pleis- tocene aquifer (qp1), respectively.

The estimation of the hydraulic conductivity or the transmissivity, was performed by evaluating drawdown data by means of the Cooper-Jacob method and recovery data by the Theis-Jacob method. Pumping test evaluation was performed by the Division for Water Resources Planning and Investigation for the South of Viet Nam using the software “AquiferTest” (by Waterloo Hydro- geologic). The estimated values for transmissivity and hydraulic conductivity are presented in Ta- ble 4.7.

Table 4.6. Characteristics of pumping wells and testing from DWRPIS (2004)

Static Coordinates (Hanoi 72) Duration of Draw Depth water Q Well Aquifer down (m) pumping recovery level (l/s) X Y s (m) (h) (h) (m)

LK80 1022109.40 18507462.23 270.0 n22 48 24 6.03 5.12 22.40

LK81 1018047.79 18513593.23 283.0 n21 96 20 8.05 10.08 6.27

LK81-I 1018048.94 18513595.17 244.0 n22 48 19 8.50 8.70 8.67

LK81-II 1018049.88 18513597.11 175.0 qp1 48 14 9.30 1.20 31.12

LK82 1009886.89 18531044.76 370.0 n22 48 12 5.60 10.35 14.00

LK83 1008393.53 18511759.72 302.0 n22 48 18 6.25 7.33 21.29

LK85 1025917.22 18522441.91 287.0 n22 48 11 5.80 10.08 11.36

LK86 1025044.81 18530896.17 300.0 n22 48 13 5.44 9.72 12.41

LK87 1015655.61 18508992.01 279.6 n22 48 10 6.22 12.47 12.47

Table 4.7. Results of pumping tests for transmissivity and hydraulic conductivity as stated by DWRPIS (2004)

Aquifer Estimated from drawdown Estimated from recovery Well Aquifer thickness Transmissivity Conductivity Transmissivity Conductivity (m) (m²/s) (m/s) (m²/s) (m/s)

LK80 n22 58.50 5.984 · 10−4 1.023 · 10−5 4.965 · 10−3 8.495 · 10−5

LK81 n21 23.00 1.146 · 10−2 4.977 · 10−4 1.157 · 10−2 5.069 · 10−4

LK81-I n22 23.00 1.736 · 10−2 7.558 · 10−4 1.154 · 10−2 5.012 · 10−4

LK81-II qp1 13.00 1.002 · 10−4 7.708 · 10−6 4.537 · 10−4 3.484 · 10−5

LK82 n22 37.50 1.146 · 10−2 3.056 · 10−4 1.354 · 10−2 3.634 · 10−4

LK83 n22 48.30 3.194 · 10−3 6.620 · 10−5 3.090 · 10−3 6.412 · 10−5

LK85 n22 47.00 8.738 · 10−3 1.852 · 10−4 1.667 · 10−2 3.553 · 10−4

LK86 n22 62.00 9.664 · 10−3 1.551 · 10−4 4.340 · 10−3 7.014 · 10−5

LK87 n22 34.50 6.725 · 10−3 1.944 · 10−4 5.475 · 10−3 1.586 · 10−4

Baseline Study Cà Mau 36 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

4.2.3 Discussion of the pumping test results Based on the drawdown/recovery records of the above mentioned nine pumping tests provided in DWRPIS (2004), this section aims to review and discuss the parameter estimation to assess the reliability of estimated hydraulic parameters.

The quality of pumping test interpretation depends on the accuracy of the data, and, on choosing a pumping test interpretation method appropriate for the type of the investigated aquifer, e.g., confined, unconfined, leaky or bounded aquifer. The aquifer type is commonly identified by using the derivative of the drawdown with respect to the natural logarithm of time . This procedure became common practice in hydrogeology only recently and necessitates high quality data, i.e., a high temporal resolution as well as a reasonable accuracy of the hydraulic head data. Manual records often do not meet these requirements.

The pumping test evaluation in DWRPIS (2004) is mainly based on the Cooper-Jacob method, which uses the slope of the drawdown curve when plotted as a function of log10t to estimate hydraulic aquifer parameters (Cooper and Jacob, 1946). In this regard it is noted that applying this method is only valid under certain conditions. The most important conditions are:

1. The aquifer is confined and of infinite extent, i.e. unbounded. 2. The aquifer is homogeneous and isotropic. 3. The pumping test is in transient state. 4. The variable is smaller than 0.01.

For single-well pumping tests the last condition is generally met as is usually small for interme- diate to late pumping times. Nevertheless, it seems questionable whether all the nine available pumping tests fulfil the other three conditions.

In order to assess the plausibility of the pumping test evaluations (esp. regarding criterion 3 above), Figure 4.8 shows the drawdown as a function of the logarithm of time for the considered pumping tests. The drawdown is depicted in percentage of the drawdown attained at the end of the pumping period. It can be seen that just 300 s after pumping started the drawdown at all wells already reached 95% of the drawdown at the end of the pumping period. (See Section 4.2.4 for possible explanations.) The development of a straight line during late pumping times can only be observed for wells 82, 83, 85 (Figure 4.8a). The drawdown at these wells can therefore be regarded appropriate for an interpretation by the Cooper-Jacob method. The drawdown at the other wells (Figure 4.8b) stabilized rather quickly at about 100% of the drawdown at the end of the pumping period (sometimes even exceeding 100%). Accordingly, steady-state is reached at these wells, and the Cooper-Jacob method for parameter estimation cannot be applied. However, for wells 81 and 81-I the Cooper-Jacob method might be applied to the middle part of the drawdown curve; however the resulting hydraulic parameters are then not representative for the entire aquifer volume perturbed by pumping.

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Figure 4.8. Assessing applicability of Cooper‐Jacob method: drawdown at investigation wells in percent‐ age of the total drawdown at the end of the pumping period, as function of logarithmic time.

In general, the same conditions as for the interpretation of drawdown tests apply to the interpretation of recovery data. Though, with regard to single-well pumping tests the recovery is less noisy as pumping does not disturb the water-level recordings. For assessing plausibility in a similar way to the drawdown analysis above, Figure 4.9 shows the recovery (residual drawdown) after the pump was stopped as percentage of the drawdown at the end of the pumping period, as a function of time since pumping stopped. It can be seen that at wells LK82, 83 and 85 a straight line develops during late recovery time, where the corresponding interpretation techniques could be used. This may also apply for the interpretation of the recovery at wells LK81-II and LK86. However, at wells LK80, 81 and 81-I the recovery even exceeds the initial hydraulic head prior to pumping, showing a recovery rate of up to 104% of the initial drawdown. This hints at a disturbance in the flow regime, e.g. by neighbouring abstraction wells, or a long-term trend.

Figure 4.9. Assessing applicability of Cooper‐Jacob method: recovery at investigation wells in percent‐ age of the total drawdown at the end of the pumping period, as function of logarithmic time since pumping stopped.

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Furthermore, it has to be noted that tested wells are mostly not screened over the entire thickness of the aquifers, and aquifers split into different layers. In some cases, well screens are divided by sections of casing with wells being screened in sub-aquifers which are separated by relatively thin layers (several meters) of most probably semi-permeable aquitards consisting of silt or clay (see profiles in Appendix 4). Thus, wells sometimes tap water-bearing layers of several meters thickness and not the entire aquifer. For example, well LK83 is screened over 13 m with the screen being divided into two sections. Both sections are separated by 16 m of casing and several semi- permeable layers, but the aquifer thickness used for calculating K was 48.30 m (Table 4.7). For well LK86 the corresponding ratio is 21 m screened section to 62 m aquifer thickness. Thus, the configuration of screens further complicates the evaluation and interpretation of the pumping tests.

4.2.4 Results of test pumping revision The discussion above shows that both well construction (partially screened sub-aquifers) as well as boundary conditions (influence of neighbouring wells) complicated the pumping test interpretation and the estimation of hydraulic parameters, respectively. Thus, hydraulic parameters estimated from the pumping tests reviewed above are rather considered orienting values for the hydraulic conductivity around Cà Mau Town.

After reviewing the data the following pumping tests are considered not appropriate for the estimation of hydraulic parameters: drawdown data from LK80, 81-II, 87 and recovery data from LK80, 81, 81-I. Furthermore, it is proposed to change the thicknesses of aquifer used for calcula- tions for well LK81-I to 41 m, LK83 to 32m, LK85 to 41 m and LK86 to 45 m. Revising Table 4.7 accordingly results in the improved values shown in Table 4.8. Accordingly, taking the geometric 2 mean of estimated hydraulic conductivities yields a hydraulic conductivity for aquifer n2 of K = −4 1 2.05 · 10 m/s which is slightly more than the corresponding value in Table 4.7. For aquifers n2 and qp1 only one estimate of the hydraulic conductivity is available from Table 4.8.

Moreover, it has to be noted that steady-state conditions during drawdown periods were reached very fast, which coincided with quick recoveries. This could be a result of poor well construction, but also be attributed to the fact that all pumping tests were conducted in an area of extensive groundwater abstraction, i.e. in a region where a regional cone of depression is already developed (see the water-level maps in Section 6.3) and neighbouring wells are effecting the flow regime. However, aquifer leakage cannot be excluded, but pumping test interpretation did not improve by considering aquifers to be leaky. This topic may need further investigations to clarify the hydrogeological status of the aquifer system in Cà Mau Province.

Baseline Study Cà Mau 39 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Table 4.8. Revised hydraulic parameters from pumping tests.

Aquifer Estimated from drawdown Estimated from recovery Well Aquifer thickness Transmissivity Conductivity Transmissivity Conductivity (m) (m²/s) (m/s) (m²/s) (m/s)

LK81 n21 23 1.15 · 10−2 5.0 · 10−4 – –

LK81-I n22 41 1.74 · 10−2 4.2 · 10−4 – –

LK81-II qp1 13 – – 4.55 · 10−4 3.5 · 10−5

LK82 n22 38 1.15 · 10−2 3.1 · 10−4 1.35 · 10−2 3.6 · 10−4

LK83 n22 32 3.20 · 10−3 10.0 · 10−5 3.09 · 10−3 10.0 · 10−5

LK85 n22 41 8.74 · 10−3 2.1 · 10−4 1.67 · 10−2 4.1 · 10−4

LK86 n22 45 9.66 · 10−3 2.2 · 10−4 4.34 · 10−3 9.6 · 10−5

LK87 n22 35 – – 5.48 · 10−3 1.6 · 10−4

4.3 Groundwater chemistry

4.3.1 Overview of available analyses Groundwater chemical analysis all four main hydrogeological reports for Cà Mau Province (DWRPIS, 2004, 2009, 2014; SNRE, 2010) were assessed and raw data retrieved where possible.

2 Report DWRPIS (2004) focused on the n2 aquifer which is intensively tapped in Cà Mau Province. Six single wells (LK80, LK82, LK83, LK85, LK86, LK87) and the well group LK81 consisting of three boreholes (LK81, LK81I, LK81II) were drilled. Groundwater samples were collected from various aquifers in Cà Mau Town area. This includes the following sampling campaigns:

 30 samples were collected from the UNICEF tubewells and private wells for major chemical components analysis.

 Water samples were collected at the end of pumping tests carried out in the 9 newly drilled wells and analysed for major chemical components, iron (Fe2+ and Fe3+) and microbial analysis.

 During the monitoring period from August 2003 to May 2004, groundwater samples were collected every three months from 19 wells: the 9 newly drilled wells of that project, 5 wells be- longing to the Cà Mau water supply company, namely LK1, LK4, LK15, LK22, LKSOS, and 5 other existing wells, namely LK215B, CM1, CM2, CM3, CM4. The samples were analysed for major chemical components and iron.

 Eight more groundwater samples were collected from the four aquifers for trace components II I 2 analysis (LK6, LK306 in qp2–3; LK81 , LKT179 in qp1, LK81 , LK86 in n2 and LK81, LK215B in 1 n2 ).

Groundwater samples were analysed for chemical composition at the laboratory of the DWRPIS. Microbial analysis was conducted at the Preventive Health Centre of Cà Mau Province.

There was no information on chemical analytical instruments used for the water sample analyses. Generally, electrical conductivity (EC) and oxygen-reduction potential (ORP) data are not available.

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In 2008, a field survey was carried out in Cà Mau Province during which 140 820 household tube wells were investigated and their abstraction information was recorded (DWRPIS, 2009). Ground- water samples were taken from 150 selected tube wells for chemical analysis (20 parameters) and microbial analysis (2 parameters). Of these wells, 96, 51, and 3 wells were screened in the qp2–3, 2 qp3, and n2 aquifers, respectively. The original analytical results are available in Excel sheets (each sample on an individual sheet), but the aquifer information was not included for any of the 150 samples. Hence, it is not clear from which aquifer the water sample was collected. The qh, qp3 and 1 n2 aquifers were not investigated in this study.

The subsequent project SNRE (2010) mainly collected and reviewed the chemical analytical results 2 of previous studies (DWRPIS, 2004, 2009) and focused on the qp2–3, qp1 and n2 aquifers. Addition- ally, 60 new water samples were taken from pumping wells and private tube wells for chemical analysis (mainly nitrogen and iron species and some ions; for a subset, trace metals). Of these, 56 2 and 3 tube wells were screened in the qp2–3 and n2 aquifers, respectively. No samples were col- 1 lected from qh, qp3, qp1 and n2 in that study. None of the water samples was analysed for microbial parameters. Unfortunately, the original data is not available, and the report only presents summaries (ranges of concentrations, water types) of all investigated data.

In report DWRPIS (2014), the authors collected the chemical analytical results of groundwater samples from 13 provinces/cities of the Mekong Delta from 1991–2010, and assessed the groundwater quality according to the National Guidelines for domestic use. A large number of groundwater sample analytical results was compiled and reviewed: 641, 382, 506, 333, 435, 463, and 178 2 1 3 analytical results for qh, qp3, qp2–3, qp1, n2 , n2 , and n1 , respectively. The number of groundwater samples newly collected and analysed for Cà Mau Province was not explicitly given in the report. No microbial analysis was conducted in this study.

The chemical analysis results obtained from DWRPIS (2004 and 2014) are collected in Appendix 6. In total, there are 150 samples from 80 different sites. With 91 analyses, by far the most samples 2 are from the n2 aquifer. The samples are unevenly distributed over the province. Most of the samples are from the area around Cà Mau City, and only a few from Năm Căn in the south of the province. Most of the rural areas have not been sampled at all, or data is not available.

Most analyses from DWRPIS (2004) report only the sum of Na + K, whereas all analyses in DWRPIS (2014) have values for both alkali ions separately.

Detection limits are usually not given in the available data sets, and values below detection limit not specially indicated.

Ion balances are good in general. They range from −4 to +7%, with a mean of +0.3%, see Figure 2.1 for a histogram.

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Figure 4.10. Locations of available water samples, with corresponding aquifers.

Frequency 0 20406080100

-4-202468 Ion balance (%) Figure 4.11. Histogram of ion balances of the available samples.

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4.3.2 Overall hydogeochemistry 1 Figure 4.12 shows boxplots of various hydrochemical parameters for the aquifers qp2–3 to n2 , based on the data described in the previous section. In general, values scatter over a wider range 2 in the upper (Pleistocene) aquifers, whereas n2 often has the lowest variability. This could how- 2 ever also be an effect of the rather small number of samples from n2 . There is a significant number of outliers, the most extreme one a sample from well Q188030 (Cà Mau City, qp1 aquifer) in 2010, with TDS = 2 483 mg/l, pH = 14 (clearly a data entry error), Cl = 15 421 mg/l (DWRPIS, 2014). The sample from 1998 of this well was not suspicious, though the mineralisation was higher than that of other qp1 samples. The median of mineralisation (TDS) decreases slightly when looking at deeper aquifers down to 2 1 n2 . It strongly increases for aquifer n2 to almost 1 000 mg/l. The variability of values (interquar- tile range, “box”) is higher in the qp2–3 aquifer than in deeper aquifers. pH values are rather variable, and mostly in the range 8 to 8.5. As with TDS, qp2–3 has much more variability than the deeper aquifers.

− Median HCO3 is similar in the two Pleistocene aquifers, but especially qp2–3 varies widely. Concen- trations in the Pliocene aquifers are similar, but lower than in the Pleistocene, and again quite variable.

Chloride concentration reach the highest values in the qp2–3 aquifer, but on average are similar in 1 the first three aquifers. Again, n2 shows a significant increase.

1 Also, sulphate and alkaline ions (Na+K) are similar in the first three aquifers and increase in n2 . This pattern is not as pronounced with earth-alkaline ions (Ca+Mg), which show the highest values in the qp2–3 aquifer. Sulphate concentrations are quite low in general, e.g. compared to values from Sóc Trăng (Hoàng Thị Hạnh and Bäumle, 2017).

Nitrate generally decreases in deeper aquifers, suggesting increasingly reduced conditions. How- 2 ever, a few very high values occur even in n2 . Another ion indicating reducing conditions, Fe2+, has the highest concentration in qp1, and lower concentrations above and below. However, the iron concentrations are generally low.

In summary, parameters are often most variable in the qp2–3 aquifer. One explanation is that wells in this aquifer are scattered throughout the investigated area, whereas the deeper, esp. Pliocene aquifers tend to cluster around Cà Mau City. The boxplots often indicate trends from the aquifer 2 1 qp2–3 down to n2 , and show a noticeable changing in the deepest aquifer n2 , suggesting that the 1 upper aquifers are somewhat related and that n2 is more strongly separated from them.

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Figure 4.12. Box plots of hydrochemical composition of groundwater of Pleistocene and Pliocene aqui‐ fers in Cà Mau Province. Data from DWRPIS (2004 and 2014). The box shows 1st and 3rd quartile, the whiskers extend to last data point within 1.5 times the box width.

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Figure 4.12 (continued)

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Figure 4.12 (continued)

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Figure 4.12 (continued)

In order to investigate hydrochemical groups and processes, Piper diagrams (Figure 4.13 for Hol- ocene and Pleistocene, Figure 4.14 for Pliocene) were created with the software “Diagrammes”

(University of Avignon) for all available aquifers, including the 3 samples from qh and qp3. The points are labelled according to stratigraphy, and samples from the south of the province are shown in different symbols. The line of conservative mixing between Ca–HCO3 freshwater and seawater is indicated by a purple arrow.

Groundwater in the qh and the qp3 aquifer is of the Na–Cl type; composition of qh is similar to the 2 1 relative composition of seawater. Groundwater in qp2–3, qp1, n2 and n2 is mainly Na–HCO3 or Na–

HCO3–Cl type. Groundwater composition in qp2–3 is more varied (as already seen in the boxplots) 2 1 − than in n2 and n2 , which cluster quite closely to a line of 85% alkali, between 30% and 80% HCO3 . 1 This cluster stretches towards seawater, with n2 closest to seawater in general.

There strong outlier in qp1 (well Q188030, sampling in 2010) already mentioned above has a composition similar to the qh and qp3 samples. This might indicate intrusion of saltwater into this par- ticular well. Nearby wells are inconspicuous, however. The previous sample from Q188030 groups with the other qp1 samples in the Piper diagram.

Samples from wells LK85 and LK86 in the northeast of the investigation area are somewhat differ- 2 2− ent from the other wells in n2 . The percentage of alkali is lower and SO4 higher.

2 1 2− In the south of the province, samples from n2 and n2 have a higher share of SO4 and correspond- − ingly reduced HCO3 . The cation composition is similar to samples from the Cà Mau City area. In the qh aquifer, no such differences can be observed.

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qh 100 qp3 qp2-3 qp1 qh (south) 3 qp2-3 (south) O N C + l a C + + M 4 g O S

0 1 0 0 0 0 0 1

3 O N C S a H O g + + M K 3 4 O C

0 1 100 0 0 0 0 0 1 1000 0 100 Ca Cl+NO3 Figure 4.13. Piper diagram of available groundwater analyses for the Holocene and Pleistocene aqui‐ fers. Samples from the south of the province are indicated separately. The purple arrow indicates the mixing line between freshwater and seawater.

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n22 100 n21 n22 (south) n21 (south) n22 (LK85, LK86) 3 O N C + l a C + + M 4 g

0 1 0 0 0 0 0 1

3 O N C S a H g O + + M K 3 4 O C

0 1 100 0 0 0 0 0 1 1000 0 100 Ca Cl+NO3 Figure 4.14. Piper diagram of available groundwater analyses for the Pliocene aquifers. Samples from the south of the province are indicated separately. The purple arrow indicates the mixing line between freshwater and seawater.

4.3.3 Analysis of possible saltwater interaction processes When seawater intrudes into a freshwater aquifer, the waters usually do not only simple mix (“conservative mixing”) but cation exchange also occurs: Na+ from the seawater is adsorbed and Ca2+ is released (Appelo and Postma, 2005, p. 242):

Na+ + 0.5Ca–X2 → Na–X + 0.5Ca2+

X denotes the cation exchanger.

When saltwater is flushed out by freshwater (“freshening”), the reverse process occurs:

2+ + 0.5Ca + Na–X → 0.5Ca–X2 + Na

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When exchanging Ca for Na, a NaHCO3 water type results (lower corner of the diamond in the Piper diagram). Also, water may become undersaturated with respect to calcite and calcite dissolution − can occur, which in turn can increase HCO3 concentration and raise pH to over 8.

Chloride can be assumed to be conservative regarding the ion exchanges. If other geogenic sources (rock salt deposits) can be disregarded, seawater is the dominant source of chloride in groundwater, and the mixing ratio of a water sample with seawater can be estimated as

− fsea = [Cl ] / 566 meq/l where the brackets denote molar concentration.

The ratio of Na and Cl molar concentrations could be used to assess salinization: seawater has a ratio of 0.86. With the available dataset, the (Na+K)/Cl ratio has been used instead, because a large part of analyses don’t give separate Na and K values. The (Na+K)/Cl ratio for seawater is 0.88. Figure 4.15 shows the relationship of this ratio versus the seawater mixing ratio. For most samples, (Na+K)/Cl is significantly greater than 1, and increasing for lower seawater mixing ratios. Samples from LK85 and LK86 form a separate group in this diagram, too (above and to the left of the main 2 group of n2 samples). Only samples with fsea > 0.03 are close to the (Na+K)/Cl ratio for seawater.

These are all samples from qh and qp3 aquifers, as well as some from qp2–3 (national monitoring well Q188020 in Cà Mau City and household wells scattered around the city without any spatial 2 pattern). Of the deeper aquifer, samples from n2 get closest to the seawater line.

qh qp3 qp2-3 qp1 n22 n21 (Na+K)/Cl

seawater 0.5 1.0 2.0 5.0 10.0 20.0

5e-04 1e-03 5e-03 1e-02 5e-02 1e-01 5e-01 1e+00 f_sea Figure 4.15. Ratio of alkaline ions to Chloride versus seawater mixing ratio fsea.

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The processes that occur in addition to conservative mixing can be accounted for by subtracting the hypothetical mixed sample concentrations from the actual concentrations:

[Alk]react = [Alk]sample – fsea · 2.4 meq/l

[Na+K]react = [Na]sample – fsea · 485 meq/l − [K]sample – fsea · 10.6 meq/l

[Ca+Mg]react = [Ca]sample – fsea · 21.4 meq/l − [Mg]sample – fsea · 110.2 meq/l The subscript “react” means “reactions in addition to conservative mixing”. The numbers are the equivalent concentrations of the respective ions in seawater (Appelo and Postma, 2005, p. 246). Again, the sum Na+K has to be used with the dataset for Cà Mau Province, as many analyses do not report these ions separately.

In the plot of [Na+K]react over [Ca+Mg]react (Figure 4.16), few samples follow the line of slope −1 which would characterise ion exchange as the major process (in addition to conservative mixing). Most points cluster around the origin (no additional reactions, or effects cancelling each other) or in a dispersed group with [Na+K]react between 5 and 15 meq/l and [Ca+Mg]react ≈ 0. This second group consists of the respective first samplings of the wells LK80–LK87. Subsequent samples of the same wells lie close to the origin.

Five samples from the qp2–3 aquifer scatter towards the lower right, i.e. Na is exchanged for Ca, indicating an intrusion process. These are the samples with high mixing ratios and close to the seawater line in Figure 4.15. As these are scattered throughout the area, and neighbouring wells are inconspicuous, it seems likely that this does not indicate a general salinity intrusion into the whole aquifer but contamination of these specific wells.

Plotting [Alk]reac versus [Na+K]react (Figure 4.17) shows that the samples have about 5–10 meq/l Alkalinity more than of what to expect from conservative mixing, and mostly up to 10 meq/l Na+K less. Samples with an excess of Na+K lie close to a line with slope 1, which represents exchange of Ca for Na and subsequent calcite dissolution (which produces Alkalinity).

The samples in these diagrams do not tend to cluster by aquifer only. Instead, groups of similar samples usually contain samples from several aquifers and the groups are distinguished by location (e.g. LK85 and LK86) or by other circumstances (first vs. subsequent samplings).

In contrast to the analysis of hydrogeochemistry in the IPGVN Final Report for Sóc Trăng (Hoàng Thị Hạnh and Bäumle, 2017), no obvious patterns concerning sulphate could be identified in plots 2− for [SO4 ]react.

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qh qp3 qp2-3 qp1 n22 n21 (Na+K)_react -40 -30 -20 -10 0 10 20

-20 -10 0 10 20 (Ca+Mg)_react Figure 4.16. Equivalent concentrations of alkaline versus earth‐alkaline ions that are the result of reac‐ tions other than conventional mixing (subscript “react”). Cyan: line of slope −1 (ion exchange).

qh qp3 qp2-3 qp1 n22 n21 react.Alk 4 6 8 10121416

-80 -60 -40 -20 0 react.NaK Figure 4.17. Equivalent concentrations of Alkalinity versus alkaline ions that are the result of reactions other than conventional mixing (subscript “react”). Cyan: line of slope +1 (exchange of Na for Ca with subsequent calcite dissolution).

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No indications for widespread salinization have been found in the deeper aquifers (qp2–3 and below). However there are indications for localised saltwater contamination in Q188020 and 5 household wells in qp2–3, and a quite sudden increase in Q188030 in 2010. The trends in the Piper diagram and the “react” plots indicate an intrusion + ion exchange processes in qp2–3 for these wells.

However, the data suggests a progression of already freshened (Na-HCO3-tpye) water in deeper, esp. Pliocene, aquifers towards seawater composition: the points stretch from the lower to the right corner of the diamond in the Piper diagram, and their (Na+K)/Cl decreases with higher fsea.

4.3.4 Goundwater quality In the following sections, the chemistry and suitability for use (according to the Vietnamese water quality guideline) is briefly characterised based on the collected data (Appendix 6).

4.3.4.1 Holocence (qh)

The qh aquifer in Cà Mau Province was saline according to the groundwater quality monitoring data from 1998–2000 of the two national monitoring wells Q17701T (in Cà Mau City) and Q199010 (in Năm Căn Town). The more recent chemical analytical results in 2010 (DWRPIS, 2014) + - also confirmed this. Very high concentrations of NH4 and NO2 were observed at Q17701T. Generally, groundwater in the qh aquifer is not suitable for domestic use.

4.3.4.2 Upper Pleistocene (qp3)

Similar to the qh aquifer, in the area of Cà Mau City, the qp3 aquifer was saline and showed elevated + 2− levels of NH4 and SO4 , as seen in the analytical results for well Q17701Z (DWRPIS, 2014).

4.3.4.3 Middle–Upper Pleistocene (qp2–3)

The qp2–3 aquifer is one of the main abstraction aquifers in the study area. This aquifer was investigated and assessed in all four studies in Cà Mau mentioned above. Groundwater in the qp2–3 aq- - uifer generally meets the Vietnamese Guideline for domestic use. Some components (Na, NO2 , + NH4 ) need to be removed before consumption due to elevated concentrations observed in many - - + samples. Nitrogen compounds (NO3 , NO2 , NH4 ) were found in many groundwater samples at various, but not problematic, levels. It is likely that vertical contamination of the qp2–3 aquifer oc- curred due to abandoned wells that were not (suitably) filled up, or newly drilled boreholes whose annulus was not sealed properly.

The chemical analytical results of groundwater samples collected from the household tube wells (DWRPIS, 2009) showed one sample of Pb level (0.018 mg/l) and three samples of Hg levels (0.005 – 0.009 mg/l) exceeding the Vietnamese Guidelines. However, data from other references (DWRPIS, 2004; SNRE, 2010) released both before and after DWRPIS (2009) showed that groundwater from qp2–3 aquifer was not contaminated by these two heavy metals. There was no information about the analytical instruments used for Pb and Hg analysis in DWRPIS (2009). It should be noted that the number of samples with exceeding values of Pb and Hg in DWRPIS (2009) is too small for any conclusive assessment. Other chemical components (Fe, Zn, Cu) were detected in the groundwater samples at low values. Chloride concentration varied in a narrow range (88 – 133 mg/l) and did not exceed the Guideline. No As was detected in the 150 samples collected in

Baseline Study Cà Mau 53 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

DWRPIS (2009). Microbial analytical results of 96 samples collected from this aquifer in DWRPIS (2009) showed that 1 sample (in Tran Van Thoi District) was contaminated by E. coli and another sample (from Năm Căn District) was contaminated by coliform bacteria.

4.3.4.4 Lower Pleistocene (qp1) + 2− As already mentioned in the previous section, the concentrations of Na, Cl, NH4 and SO4 in groundwater samples collected from the Q188030 national monitoring well in 2010 (DWRPIS, 2014) were much higher than reported in the previous study (DWRPIS, 2004) and exceeded the Vietnamese Guideline requirements.

- - + Generally, nitrogen compounds (NO3 , NO2 , NH4 ) were found at various levels.

This aquifer has the highest observed concentrations of Fe, but they still are well within Guideline requirements.

Microbial analytical results of 51 samples collected from this aquifer in DWRPIS (2009) showed that 4 samples were contaminated by E.coli and other coliform bacteria.

2 4.3.4.5 Upper Pliocene (n2 ) - - + 2- 2 The concentrations of nitrogen compounds (NO3 , NO2 , NH4 ) and SO4 in the n2 aquifer varied in wide ranges, but did not exceed Guideline requirements, according to the monitoring data in DWRPIS (2004). The chemical analytical results of 3 water samples collected from this aquifer in DWRPIS (2009) showed that no parameter exceeded the Guideline values for domestic use. No microbial contamination was observed for those 3 samples. However, elevated levels of the above components were observed in the sample collected from national monitoring well Q19904T in 2010 (DWRPIS, 2014). The Na and Cl concentrations of some samples exceeded the Guideline values. Observed Fe concentrations were generally low.

1 4.3.4.6 Lower Pliocene (n2 ) - + The concentrations of NO2 and NH4 spread over wide ranges according to the monitoring data in DWRPIS (2004), but did not exceed Vietnamese Guideline values. Elevated levels of Na, Cl, Ca, Mg, 2− − SO4 , NO2 were observed in the sample collected from the national monitoring well Q19904Z in 2010 (DWRPIS, 2014).

4.3.5 Spatial variation The available data does not exhibit strong spatial patterns, as illustrated by the example of TDS in Figure 4.18. TDS tends to be a bit higher in the city area, and exceptionally high values are limited to the shallow aquifers qh – qp2–3 and scattered throughout the area.

Baseline Study Cà Mau 54 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

Figure 4.18. Map of TDS in aquifers around Cà Mau City. Colour: aquifer, size: TDS. Text labels are names of the communes/wards.

A hydrochemical zonation of Cà Mau City area was suggested in DWRPIS (2004) based on the total mineralization (TDS) of groundwater. A map is not available, however. With the data available in this baseline study, the zonation can neither be clearly confirmed nor clearly rejected. The zonation is summarised here as follows (see Figure 4.18 for the names of communes):

“Total mineralization increases with depth”: This zone covered about ¾ of the study area, including the centre of Cà Mau City, a part of the An Xuyên, Tân Thành and Đinh Bình Communes of Cà Mau City, and a part of Hồ Thị Kỷ Commune.

From top to bottom, there was a fresh water zone of qp2–3: TDS = 0.42 – 0.86 g/l; qp1 with TDS = 2 1 0.4 – 0.72 g/l; n2 with TDS = 0.36 – 0.84 g/l; n2 with TDS = 0.8 – 1.03 g/l. In this zone, water quality of the upper aquifers was better than that of the deeper ones.

“Total mineralization decreases with depth”: This zone is distributed in the southwest of the study area, located in Lương Thế Trân Commune with an area of 40 km².

From top to bottom, there was a brackish water zone in qp2–3 with TDS = 1.24 – 2.95 g/l, and fresh 2 1 water zones in qp1 with TDS = 0.67 g/l, n2 with TDS = 0.59 – 0.84 g/l, and n2 with TDS < 1 g/l.

“Complicated hydrochemical zonation”: This zone included the northwest and northeast of the study area, covering about 160 km², located in Hồ Thị Kỷ, Tân Lộc, and An Xuyên Communes of Cà Mau Province and Phong Thạnh Tây Com- mune of Bạc Liêu Province).

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From top to bottom, there was a fresh-to-brackish water in qp2–3 with TDS = 0.51 – 1.59 g/l, fresh 2 water in qp1 with TDS = 0.52 – 0.98 g/l, fresh water in n2 with TDS = 0.51 – 0.53 g/l, and slightly 1 brackish water in n2 with TDS = 1.01 g/l. The fresh – brackish/saline interfaces of various aquifers were close to each other but did not overlap. Therefore, DWRPIS (2004) recommends that abstraction of groundwater in this zone should be carried out with caution. Appropriate sealing and iso- lation techniques for drilling are needed.

Both in SNRE (2010) (for Cà Mau Province) and DWRPIS (2014) (for the whole Mekong Delta), the authors developed groundwater quality maps for the aquifers in which two different groundwater bodies (fresh and saline) were distinguished according to the total mineralization of < 1 g/l and ≥ 1 g/l. These maps for Cà Mau are shown and discussed in Section 4.1.1.

4.3.6 Temporal variation Groundwater quality monitoring data from 1996 to 2008 of the National monitoring wells (Q17701T, Q199010, Q17701Z, Q177020, Q188020, Q199020, Q188030, Q17704T, Q19904T, Q17704Z, Q19904Z) were studied in DWRPIS (2009) and SNRE (2010). Graphs showing the contents of various chemical components over time were found in both documents but no original - - - 2- data were available. The graphs show the variation of Cl , HCO3 , NO3 , SO4 , Fe, and Al. For evaluation in this baseline study, the original chemical analytical data for wells in DWRPIS (2004) and DWRPIS (2014) for which repeated samplings are available were selected and ar- ranged in time series for each observation point. Then, graphs were plotted to present the variation over time. However, most of these rather short time series did not reveal noteworthy patterns.

- 2- In LK81-II in the qp1 aquifer, Cl and SO4 contents varied strongly during the monitoring period - 2- 2000 – 2004, but do not exhibit any obvious trend. (Figure 4.19) The Cl and SO4 contents of 2 groundwater of the n2 aquifer(not shown)varied to different degree depending on borehole. LK80 - 2- and LK81-I showed strong variation in Cl and SO4 contents while many other boreholes (LK82, LK83, LK85, LK86, CM3, CM4, LK1, LK4, LK15, LK22, LKSOS) only showed slight to moderate variation in those parameters, and LK87, CM1, CM2 showed almost stable concentrations of Cl- and 2 - 2- 1 SO4 . The Cl and SO4 contents of groundwater of the n2 aquifer also varied but not strongly (LK81-II and LK215B). In summary, nor clear trends in chemical composition could be detected from available data in the period from 1998–2008.

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LK81-II (qp1)

1000

100 Concentration (mg/L)

10-8-2000 17-4-2003 1-11-2003 8-2-2004 6-5-2004

pH HCO3-(mg/L) Cl-(mg/L) SO4 (mg/L) Na (mg/L) Ca Mg(mg/L)

Figure 4.19. Time series of major chemical components in groundwater from LK81‐II.

In report DWRPIS (2014), the authors studied the chemical composition of groundwater from the monitoring wells in the Mekong Delta between 1991 and 2010 and reported the differences in Fe, + - Al, NH4 , NO3 contents between dry seasons and rainy seasons. Generally, the contents of chemical components in groundwater were higher in dry seasons than in rainy seasons. However, the graphs found in this report only show the average values of the chemical contents over the whole period. The report also describes the variation of groundwater composition of the 7 aquifers during 1991 – 2010. However, it was not mentioned from which wells the chemical data was obtained.

4.4 Isotope investigations and groundwater dating An isotopic and chemical study of the Mekong Delta aquifers was initiated in 1982 by the Centre of Nuclear Techniques of Vietnam (now the Institute for Nuclear Science and Technology) in cooperation with the International Atomic Energy Agency (IAEA). From 1982 to 1986, 83 water samples were collected in the Mekong Delta. Based on the initial analytical results of those samples, a complementary sampling was carried out. In total, 137 samples of both surface and groundwater were analysed for both chemical and isotopic contents. The stable isotopes and 3H analyses were performed in the IAEA Isotope Hydrology Laboratory, 14C content of the TDIC was measured at the Centre of Nuclear Techniques in HCM City. The results were summarized in the report “Environ- mental Isotope Study of Mekong Delta Groundwater (Vietnam), 1989” (Louvat and Hồ Hữu Dũng,

Baseline Study Cà Mau 57 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

1989). The study treated all Pleistocene aquifers as a single one; Upper and Lower Pliocene were treated separately. Miocene and Mesozoic were again combined for stable isotope analysis and not analysed for 14C.

The stable isotope analytical results of water samples collected in Cà Mau area showed ranges of δ2H from −48.8 ‰ to −40.8 ‰ and δ18O from −6.88 ‰ to −6.08 ‰. The absolute age of groundwater generated by 14C dating varied between 24700±1900 to >40000 years, which is close to the limits of the method. The study suggests that the Pliocene and Pleistocene aquifers are recharged by precipitation from higher areas in Cambodia. There is no indication of interaction between the aquifers.

For comparison, more information is available for Sóc Trăng Province. The project “Improvement of Groundwater Protection in Vietnam” conducted three samplings in 2013 for stable isotopes, 3H, and 14C analyses. The results are shown in Hoàng Thị Hạnh and Bäumle (2017). Groundwater samples in Sóc Trăng Province do not show a strong variation in stable isotope composition between the dry and rainy season. Groundwater in Sóc Trăng may originate from meteoric water, possibly exposed to evaporation in a different paleoclimate. Salt water from the sea is going upstream Hau River during the dry season but it does not affect the adjacent aquifers. (See the IGPVN Technical Reports 44 and 45 (Hoàng Thị Hạnh, 2014a, 2014b) for more details). The 14C contents and determined ages were overall in good agreement with the regional radioisotope study conducted by the IAEA (Louvat and Hồ Hữu Dũng, 1989).

-10

IAEA Project VIE/8/003 GMWL -20 GNIP Bangkok Bangkok MWL ST1 ST3

H (‰) -30 ST4 2 ST7 d ST11 GW1 -40 GW2 GW3 SW1 SW2 -50 SW3 SW4 rain water -60 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 d18O (‰)

Figure 4.20. Stable isotope data from Sóc Trăng in dry season (filled symbols) and rainy season (unfilled symbols) of 2013. ST and GW: groundwater samples (qp2–3), SW: surface water samples. (Hoàng Thị Hạnh and Bäumle, 2017)

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The stable isotope contents of groundwater and surface water in Cù Lao Dung District (a large island in the mouth of the Hau River in Sóc Trăng Province) was studied by Tran Dang An et al. (2014). Water samples were collected during the driest period of the year including 22 groundwater samples from private tube wells with the depth of 80 – 130 m in the qp2–3 aquifer, 8 river water samples and 7 canal water samples in March 2013. On-site parameters, major ions and stable isotopes were analysed. The results showed the range of δ2H from −47 ‰ to −33.67 ‰ and δ18O from -5.5 ‰ to -4.38 ‰ for groundwater; δ2H from −49.18 ‰ to −34.77 ‰ and δ18O from −6.55 ‰ to −4.81 ‰ for river water; δ2H from −51.74 ‰ to −27.10 ‰ and δ18O from −6.87 ‰ to −3.81 ‰ for canal water.

The authors concluded that the surface water characteristics in this area were affected by complex factors: tidal regime, saline intrusion and water flow from the upper Mekong River Basin, while those of groundwater might not be influenced. Currently no interaction between river/canal water and groundwater in Cù Lao Dung Island could be detected, and therefore, the main recharge areas of groundwater seems to be located outside of the study area.

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5 Groundwater abstraction and resource estimation

5.1 Data for larger wells in the Mekong Delta Wells with an abstraction rate of more than 200m³/d have to be reported to the DONREs. As part of the climate change impact study DWRPIS (2014), information on these larger abstraction wells has been compiled for all Mekong Delta provinces. The coordinates, aquifers, extraction rates and years of construction of these wells are tabulated in Appendix 7. Their locations are shown in Fig- ure 5.1, a summary per aquifer and province is shown in Table 5.1.

There is a apparent absence of wells in the southern half of Cà Mau Province in this map. However, the detailed survey of the province in 2009 (next section) confirms that there is considerable extraction of groundwater in this area.

Figure 5.1. Groundwater abstraction in the Mekong Delta (data from DWRPIS (2014))

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Table 5.1. Summary of abstraction wells >200 m³/d in the Mekong Delta (based on inventory in DWRPIS (2014)). N = number of wells, Q = abstraction rate (m³/d)

Province Aquifer

qh + qp3 qp2–3 qp1 n22 n21 n13 Total N Q N Q N Q N Q N Q N Q N Q An Giang 27 7 968 1 240 34 25 840 2 864 64 34 912 Bến Tre 10 9 160 5 3 300 15 12 460 Bạc Liêu 44 23 540 28 18 295 17 12 247 89 54 082 Cà Mau 1 820 19 19 213 47 45 692 67 65 725 Cần Thơ 150 54 769 99 41 732 249 96 501 Đồng Tháp 68 65 008 68 65 008 Hậu Giang 2 400 9 4 440 8 2 640 19 7 480 Kiên Giang 1 200 50 32 105 6 4 875 3 3 500 60 40 680 Long An 8 11 393 76 97 033 9 9 500 3 2 710 96 120 636 Sóc Trăng 60 40 200 16 10 800 10 11 140 86 62 140 Tiền Giang 25 28 589 25 28 589 Trà Vinh 20 26 490 20 26 490 Vĩnh Long 3 800 5 4 626 8 5426 Total 30 8 568 338 183 404 79 62 341 347 292 427 29 27 650 43 45 739 866 620 129

5.2 Investigation in Cà Mau 2009 The status of groundwater abstraction in Cà Mau Province was investigated in 2009 (DWRPIS, 2009). In this project, the DWPRIS in conjunction with the Cà Mau DONRE has conducted a survey of exploitation and use of groundwater in the province of Cà Mau, as well as mapping of the current status of groundwater exploitation according to district administrative units. The survey consisted of the following steps:

 Building a network of cooperators: at the communes, the staff of Cà Mau DONRE and South Division worked directly with the hamlet chiefs, who understand the geography and know the number of wells in the locality they manage.

 The staff of the Cà Mau DONRE and DWPRIS directly trained the cooperators involved in the investigation.

 The cooperators received the questionnaire from the staff of the Cà Mau DONRE and DWPRIS. The questionnaire inquired mainly about groundwater abstraction: depth of well, well design, well owner's name, location, extraction rate, and the basic properties of groundwater.

 Cooperators went to each household in their respective localities, interviewed and certified the questionnaire. Then the People's Committees of communes/wards confirmed the surveyed households on the investigation list.

 The staff of Cà Mau DONRE and DWPRIS who had direct supervision function in the course of an investigation, checked the results.

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 The survey results were sent to Cà Mau DONRE and DWPRIS as the basis for mapping and writ- ing the thematic reports.

For small wells of rural households, pumping rates are usually not available. The survey assumed that each will extract 2 m³/d, a rather high, “worst case” value which accounts for the fact that some wells not only provide domestic water for a single family but also for small businesses (e.g. restaurants) or small agriculture.

5.2.1 Overview of results for Cà Mau Province According to results of the survey, Cà Mau Province has 137 988 wells for groundwater, with a total abstraction rate of about 373 000 m³/d. Table 5.2 shows numbers and abstraction rates for different operators/owner groups.

In Cà Mau Province, the extraction wells focus on the Middle – Upper Pleistocene aquifer (qp2–3), 2 Lower Pleistocene aquifer (qp1), Middle Pliocene aquifer (n2 ), and to a smaller extent the Lower 1 Pliocene aquifer (n2 ). The report assessed the state of the aquifer exploitation in accordance with the administrative units and the level of groundwater exploitation by region. In general, the higher levels of exploitation are mainly concentrated in the town area of Cà Mau City with a high share of domestic use. The aquifer qp2–3 is exploited the most, with a total of 128 319 wells and an abstrac- 2 tion rate of ≈265 000 m³/d. The aquifer n2 has the least number of wells, but is exploited with a large rate of ≈52 000 m3/d. The reason is the concentration of big wells, with extraction on an in- 1 dustrial scale, with a total of 607 wells. The aquifer n2 has the lowest abstraction rate, with a value of ≈5 500 m³/d. (Table 5.3)

To show the level of extraction in the maps by colour, a specific abstraction rate is calculated as m³/(d · km²) for the administrative units. It is presented at the commune/ward level, categorised in 6 levels (see Table 5.4).

The following sections summarise the results and show the level of exploitation for the districts by aquifer (as totals for the whole district) and by commune/ward (as totals for all aquifers). The numerical data on the commune/ward level is collected in Appendix 8.

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Table 5.2. Summary of groundwater extraction in Cà Mau Province. N = number of wells, Q = extraction rate (m³/d). (DWRPIS, 2009)

District / Company for Factories, Center for po‐ Small wells at rural Total City Water Supply schools, and table water households and Urban En‐ other organi‐ and rural san‐ vironment zations itation N Q N Q N Q N Q N Q Cà Mau 19 26 064 82 15 884 17 830 12 415 24 830 12 533 67 608 City U Minh 8 2 148 21 9 120 5 260 13 534 27 068 13 568 38 596 Dam Doi 3 1 113 13 4 707 16 1 180 20 589 41 178 20 621 48 178 Phu Tan 0 0 8 550 15 1 170 8 391 16 782 8 414 18 502 Thoi Binh 2 864 16 5 085 15 630 21 126 42 252 21 159 48 831 Tran Van 4 2 028 33 8 680 18 970 24 755 49 510 24 810 61 188 Thoi Cai Nuoc 2 924 24 5 205 12 778 20 042 40 084 20 080 46 991 Năm Căn 5 2 792 14 4 250 11 760 8 502 17 004 8 532 24 806 Ngoc 0 0 1 30 34 2 130 8 236 16 472 8 271 18 632 Hien Total 43 35 933 212 53 511 143 8 708 137 590 275 180 137 988 373 332

Table 5.3. Total of groundwater extraction in the districts of Cà Mau Province by aquifer. N = number of wells, Q = extraction rate (m³/d).

Aquifer

District qp2–3 qp1 n22 n21 N Q N Q N Q N Q Cà Mau City 10 166 21 719 2 239 5 533 122 37 866 6 2 490 U Minh 13 108 27 841 452 9 665 7 640 1 450 Dam Doi 19 292 38 916 1 255 5 640 74 3 622 0 0 Thoi Binh 20 717 41 948 439 6 791 3 92 0 0 Tran Van Thoi 24 010 49 735 789 6 331 11 5 122 0 0 Cai Nuoc 18 680 37 640 1 370 7 299 30 2 052 0 0 Phu Tan 8 302 17 770 109 332 3 400 0 0 Năm Căn 6 572 13 414 1 333 6 438 27 2 362 600 2 592 Ngoc Hien 7 472 16 388 789 1 830 10 414 0 0 Total 128 319 265 371 8 775 49 859 287 52 570 607 5 532

Table 5.4. Categories and map colours for specific abstraction rate of administrative units.

Level of extraction Colour Specific abstraction rate range, m³/(d · km²) very high > 1 000 high 500 – 1 000 medium 200 – 500 medium to low 100 – 200 low 50 – 100 very low < 50

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5.2.2 Cà Mau City Cà Mau City has 12 533 wells, with a total abstraction rate of approximately 68 000 m³/d.

Most of the wards (urban subdivisions of districts) of the city of Cà Mau have no wells in the qp2–3 aquifer, it is mainly used in the communes (rural subdivision of districts). An Xuyen Commune has the most wells in this aquifer.

Wells in the aquifer qp1 are also mainly concentrated in the communes and some wards. Tac Van Commune has most wells in this aquifer.

2 The wells in the n2 aquifer are mainly concentrated in the wards of the city of Cà Mau. With the exception of the communes of Tan Thanh and Ly Van Lam, the remaining communes mostly have only small wells with low abstraction rate. The wells of the other wards show a very high abstraction rate, especially in Ward 2, where only 2 wells have been drilled but the total abstraction rate is about 4 800 m³/d.

1 Exploitation of the aquifer n2 is quite low.

Cà Mau City – expressed as total over all aquifers – has a very high level of groundwater extraction with 1 750 m3/(d · km2). The highest level of groundwater extraction is very high with 17 014 m3/(d · km2) in Ward 2 and the lowest is very low with 26 m3/(d · km2) in Ward 7. The 2 3 2 aquifer n2 has the highest level of groundwater extraction with 1,605 m /(d · km ), which is clas- 1 3 sified very high; the aquifer n2 has the lowest level of groundwater extraction with 12 m /(d · km2), which is very low. The map of these results is shown in Figure 5.2.

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Figure 5.2. Level of groundwater extraction in Cà Mau City. (DWRPIS, 2009)

5.2.3 U Minh District U Minh District has 13 568 wells, with an abstraction rate of approximately 38 000 m³/d.

The wells mainly exploit the aquifer qp2–3. Most of them have only a low abstraction rate. Khanh An Commune has the most wells in this aquifer.

Wells in the aquifer qp1 are exploitation wells with large abstraction rates, which provide water for many households. Exceptions are the communes Nguyen Phing, Khanh Tien, and Khanh Lam, which mostly have only small wells with low abstraction rates. Khanh Lam Commune has the most wells in this aquifer.

2 Wells in the aquifer n2 have large abstraction rates, they provide water for many households. Ex- ceptions are the communes Nguyen Phing and Khanh Hoi, which do not have any exploitation wells in this aqufier.

1 The aquifer n2 has only one well, with a large abstraction rate, in Khanh Hoi Commune. U Minh District – expressed as total over all aquifers – has a low level of groundwater extraction (88 m³/(d · km²)). The highest level of groundwater extraction is medium with 204 m3/(d · km2) in U Minh Town and the lowest is very low with 20 m3/(d · km2) in Khanh Hoa Commune. The

Baseline Study Cà Mau 65 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam

3 2 aquifer qp2–3 has the highest level of groundwater extraction (low) with 50 m /(d · km ); the aqui- 1 3 2 fer n2 has the lowest level of groundwater extraction (very low) with 2 m /(d · km ). The map of these results is shown in Figure 5.3.

Figure 5.3. The level of groundwater extraction in U Minh District. (DWRPIS, 2009)

5.2.4 Dam Doi District Dam Doi District has 20.621 wells, with abstraction rates of approximately 48 000 m³/d.

The wells mainly exploit the aquifer qp2–3, and mostly have only low abstraction rates. Tan Duyet Commune has most wells into this aquifer.

Wells in the aquifer qp1 have low to medium abstraction rates, with the exception of the Nguyen Han Commune, which has no wells. Tan Trung Commune has the most wells in this aquifer.

2 Wells in the aquifer n2 have large abstraction rates and supply water for many households.

1 The aquifer n2 is not yet used in this district. Dam Doi District – expressed as total over all aquifers – has a medium to low level of groundwater extraction (106 m3/(d · km2)). The highest level of groundwater extraction (high) is 630 m3/(d ·

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km2) in Dam Doi Town and the lowest (very low) is 1.6 m3/(d · km2) in Nguyen Huan Commune. 3 2 The aquifer qp2–3 has the highest level of groundwater extraction (low) with 65 m /(d · km ); the 2 3 2 aquifer n2 has the lowest level of groundwater extraction (very low) with 17 m /(d · km ). The map of these results is shown in Figure 5.4.

Figure 5.4. The level of groundwater extraction in Dam Doi District. (DWRPIS, 2009)

5.2.5 Phu Tan District Phu Tan District has 8 414 wells, with an abstraction rate of approximately 18 000 m3/day.

The wells mainly exploit the aquifer qp2–3. In most communes there are only small wells with low abstraction rates, with the exception of the communes Phu Thuan, Nguyen Viet Khai, Nguyen Thang, which have wells with large abstraction rates. Tan Hung Tay Commune has the most wells in this aquifer.

Wells in the aquifer qp1 have abstraction rates from medium to small. Rach Cheo Commune has the most wells in this aquifer.

2 Wells in the aquifer n2 have large abstraction rates and supply water for many households. Cai Doi Vam Commune has 3 big wells.

1 The aquifer n2 is not yet used in this district.

Phu Tan District – expressed as total over all aquifers – has a low level of groundwater extraction (54 m³/(d · km²)). The highest level of groundwater extraction (medium to low) is 175 m³/(d ·

Baseline Study Cà Mau 67 German Technical Cooperation with Vietnam Improvement of Groundwater Protection in Vietnam km²) in Cai Doi Town and the lowest (very low) is 1.3 m³/(d · km²) in Viet Thang Commune. The aquifer qp2–3 has the highest level of groundwater extraction (medium to low) with 154 m³/(d · km²); the aquifer qp1 has the lowest level of groundwater extraction (very low) with 1 m³/(d · km²). The map of these results is shown in Figure 5.5.

Figure 5.5.Level of groundwater extraction in Phu Tan District. (DWRPIS, 2009)

5.2.6 Thoi Binh District Thoi Binh District has 21 159 wells, with an abstraction rate of approximately 48 000 m3/d.

The wells mainly exploit the aquifer qp2–3, and usually have low abstraction rates. Ho Thi Ky Com- mune has the most wells into this aquifer.

Wells in the aquifer qp1 usually have low abstraction rates, with the exception of Thoi Binh Town and Tri Phai Commune, which have big wells with large abstraction rates. Thoi Binh Commune has the most wells in this aquifer.

2 Wells in the aquifer n2 have large abstraction rates, and supply water for many households. There are 3 big wells in Ho Thi Ky Commune.

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1 The aquifer is n2 is not yet used in this district.

Thoi Binh District – expressed as total over all aquifers – has a low level of groundwater extraction (93 m3/(d · km2)). The highest level of groundwater extraction is medium with 255 m3/(d · km2) in Thoi Binh Town and the lowest is very low with 25 m3/(d · km2) in Bien Bach Commune. The 3 2 aquifer qp2–3 has the highest level of groundwater extraction (medium to low) with 73 m /(d · km ); 2 3 2 the aquifer n2 has the lowest level of groundwater extraction (very low) with 1 m /(d · km ). The map of these results is shown in Figure 5.6.

Figure 5.6. Level of groundwater extraction in Thoi Binh District. (DWRPIS, 2009)

5.2.7 Tran Van Thoi District Tran Van Thoi District has 24 810 wells, with an abstraction rate of approximately 61 000 m3/d.

The wells mainly exploit the aquifer qp2–3, and usually have low abstraction rates. Song Doc Town has the most wells into this aquifer.

Wells in the aquifer qp1 have low abstraction rates, with the exception of the Tran Van Thoi Town and Song Doc Town, which possess big wells with large abstraction rates. Tran Hoi Commune has the most wells in this aquifer.

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2 Wells in the aquifer n2 have large abstraction rates, and supply water for many households. There are 9 big wells in Song Doc Town.

1 The aquifer n2 is not yet used in this district.

Tran Van Thoi District – expressed as total over all aquifers – has a medium to low level of groundwater extraction (111 m³/(d · km²)). The highest level of groundwater extraction is medium with 408 m³/(d · km²) in Song Doc Town and the lowest is very small with 38 m³/(d · km²) in Khanh

Binh Tay Bac Commune. The aquifer qp2–3 has the highest level of groundwater extraction (low) 2 with 82 m³/(d · km²); the aquifer n2 has the lowest level of groundwater extraction (very low) with 13 m³/(d · km²). The map of these results is shown in Figure 5.7.

Figure 5.7. Level of groundwater extraction in Tran Van Thoi District. (DWRPIS, 2009)

5.2.8 Cai Nuoc District Cai Nuoc District has 20 080 wells, with an abstraction rate of approximately 47 000 m3/d.

The wells mainly exploit the aquifer qp2–3, and usually have only low abstraction rates. Tan Hung Dong has most wells in this aquifer.

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Wells in the aquifer qp1 have low to medium abstraction rates; with the exception of Dong Thoi and Dong Hung Commune, which do not have wells in this aquifer. Thanh Phu Commune has most wells in this aquifer.

2 Wells in the aquifer n2 have large abstraction rates, and provide water for many households. Tan Hung Dong and Hoa My Commune do not have wells in this aquifer.

1 The aquifer n2 is not yet used in this district.

Cai Nuoc District – expressed as total over all aquifers – has a medium to low level of groundwater extraction (121 m3/(d · km2)). The highest level of groundwater extraction is medium with 283 m3/(d · km2) in Cai Nuoc Town and the lowest is low with 63 m3/(d · km2) in Hoa My Commune. 3 2 The aquifer qp2–3 has the highest level of groundwater extraction (low) with 92 m /(d · km ); the 2 3 2 aquifer n2 has the lowest level of groundwater extraction (very low) with 6 m /(d · km ). The map of these results is shown in Figure 5.8.

Figure 5.8. Level of groundwater extraction in Cai Nuoc District. (DWRPIS, 2009)

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5.2.9 Năm Căn District Năm Căn District has 8 532 wells, with an abstraction rate of approximately 25 000 m3/d.

The wells mainly exploit the aquifer qp2–3, and usually are small wells with low abstraction rates. Dat Moi Commune has most wells into this aquifer.

Wells in the aquifer qp1 have small to medium abstraction rates. Hiep Tung and Hang Vi Communes do not have wells in this aquifer.

2 Wells in the aquifer n2 have large abstraction rates and provide water for many households. Năm Căn Town has most wells in this aquifer.

1 The aquifer n2 has 2 wells with large abstraction rates in Hang Vinh Commune, and wells with low abstraction rates in Ham Rong Commune.

Năm Căn District – expressed as total over all aquifers – has a low level of groundwater extraction (75 m3/(d · km2)). The highest level of groundwater extraction (medium) is 268 m3/(d · km2) in Năm Căn Town and the lowest (very low) is 16 m3/(d · km2) in Tam Giang Dong Commune. The 3 2 aquifer qp2–3 has the highest level of groundwater extraction (very low) with 32 m /(d · km ); the 2 3 2 aquifer n2 has the lowest level of groundwater extraction (also very low) with 10 m /(d · km ). The map of these results is shown in Figure 5.9.

Figure 5.9. Level of groundwater extraction in Năm Căn District. (DWRPIS, 2009)

5.2.10 Ngoc Hien District Ngoc Hien District has 8 532 wells, with an abstraction rate of approximately 19 000 m3/d.

The wells mainly exploit the aquifer qp2–3, and usually are only small wells with low abstraction rates. Vien An Dong Commune has most boreholes sunk into this aquifer.

Wells in the aquifer qp1 have small to medium abstraction rates. Tay An Commune does not have wells in this aquifer. Tay An Tay Commune has the most wells in this aquifer.

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2 Wells in the aquifer n2 have large abstraction rates, providing water supply for many households. Vien An Dong and Dat Mui Communes do not have wells in this aquifer.

1 The aquifer n2 is not yet used in this district.

Table 18. Ngoc Hien District – expressed as total over all aquifers – has a very low level of groundwater extraction (25 m3/(d · km2)). The highest level of groundwater extraction is very low with 36 m3/(d · km2) in Viem An Dong Commune and the lowest is also very low with19.5 m3/(d · km2) in Tan An Commune. The aquifer qp2–3 has the highest level of groundwater extraction (very low) 3 2 2 with 22 m /(d · km ); the aquifer n2 has the lowest level of groundwater extraction (very low) with 1 m3/(d · km2). The map of these results is shown in Figure 5.10.

Figure 5.10. Level of groundwater extraction in Ngoc Hien District. (DWRPIS, 2009)

5.3 Exploitable groundwater resources estimation Exploitable groundwater resources had been calculated for the Cà Mau City area (DWRPIS, 2004), Cà Mau Province (SNRE, 2010) and all Mekong Delta provinces (DWRPIS, 2014). The method is summarised here, following the description in DWRPIS (2004): exploitable resources Q are calculated using the following equation: ∗ where

Qd dynamic reserve (m³/d) F area of freshwater (TDS < 1 g/l) in the aquifer (m²) m thickness of the aquifer (m) h head (m) µ “gravitational storativity” = specific yield

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µ* “elastic storativity” = storativity α coefficient (ratio) of static reserve usage; assumed as 0.4 4 tkt extraction time; assumed as 10 d (≈27 years)

Dynamic reserve (lateral inflow into the study area, groundwater recharge in the study area) is estimated based on hydraulic gradient. If no gradient can be found, dynamic reserves are assumed

2 2 as 0. For example, in DWRPIS (2004) this was the case for qp2–3, qp1, and n2 ; for n2 a gradient was detected but for safety Qd was assumed as 0 as well. For this case, the reserves are essentially assumed to be elastic storage and dewatering of the aquifer by lowering the water level. In the Me- kong Delta study (DWRPIS, 2014), values for Qd were determined by balancing the fluxes in the numerical model.

The results of the three studies (DWRPIS, 2004, 2014; SNRE, 2010) are summarised in Table 5.5. The actual extraction rates from the 2009 survey (Table 5.3) are added for comparison. The most recent resources estimation, with the best data basis (DWRPIS, 2014) corresponds to ≈350 m³/(km² · d) = 0.35 mm/d ≈ 130 mm/a.

Table 5.5. Exploitable groundwater resources (in m³/d) for Cà Mau City and Province from different sources, and actual extraction rates from Table 5.3 for comparison.

Aquifer Cà Mau City Cà Mau Province Resources Extraction Resources Resources Extraction (DWRPIS, 2004) rate 2009 (SNRE, 2010) (DWRPIS, 2014) rate 2009

qp3 0

qp2–3 42 141 21 719 869 672 488 952 265 371

qp1 70 391 5 533 422 547 522 460 49 859

n22 190 942 37 866 1 291 092 749 140 52 570

n21 56 650 2 490 94 607 5 532

n13 5 403 Sum 360 124 2 583 311 1 860 562

5.4 Conclusions on abstraction and resource estimation Groundwater abstraction, as documented by the investigation in 2009 (DWRPIS, 2009), is intensive, especially considering the huge number of privately built wells. They usually do not extract much individually (often single households/farms), but the cumulative amounts can be considerable. There are also concerns about hydraulic connections especially between the Pleistocene aquifers due to insufficient construction standards (annular sealing of aquicludes) and abandoned wells.

The classification by the 2009 report relates abstraction rates to area and assigns categories. Alt- hough not many communes and aquifers exceed the medium level (mostly they are around low), even the 200 m³/(km² · d) at the lower end of medium mean 0.2 mm/d or about 70 mm annually!

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Considering the not well known but rather low groundwater recharge in the Mekong Delta, this must be considered a substantial use of groundwater resources. Even compared to the overly op- timistic (see below) estimation of exploitable resources of around 130 mm/d, this medium extraction is rather high.

The survey is already over 6 years old, so changes (very likely an increase of the number of wells and the abstraction rate) should be expected. Additionally, the survey was questionnaire-based, so there are few real measurements of pumping rates. Even for large wells, where operators should report water level and pumping rates to the authorities, data are sometimes scarce.

The reported values of exploitable resources are very high, and to a considerable part include static reserves, i.e. elastic storage and dewatering, applied over a calculation period of 27 years. Conse- quently, these estimates consider “groundwater mining” over the next three decades. Therefore these values should be considered with extreme caution. Sustainable groundwater extraction should be based only on the “dynamic reserves”, i.e. groundwater recharge and lateral flow. De- clining groundwater levels indicate that even the current status of extraction, which is well below the official exploitable resources, is not sustainable anymore. Additionally, this approach does not consider consequences of groundwater extraction like saltwater intrusion.

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6 Groundwater dynamics and morphology

6.1 Groundwater dynamics in the Mekong Delta In the abstraction and climate change impact assessment report (DWRPIS, 2014), groundwater level data from 1995 to 2010 of the 106 monitoring wells in the Mekong Delta was compiled. The monthly average values of each well and the monthly average, minimum, and maximum values of each aquifer are available in the Annex 11 of DWRPIS (2014) and translated in Appendix 9 of this study.

Furthermore, NAWAPI provided IGPVN with monthly water levels for the Mekong Delta national 2 monitoring wells. Some wells have been discontinued, e.g. Q17704T monitoring n2 aquifer in Cà 3 Mau City and Q59704Z monitoring n1 in Bạc Liêu City.

Table 6.1 shows the number of monitoring wells with available data in each aquifer for the dataset in DWRPIS (2014) and for the complete period 1995–2016.

Figure 6.1. The national monitoring network in the Mekong Delta, Vietnam. (DWRPIS, 2014, Annex 11)

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Table 6.1. Number of national monitoring wells in the aquifers of the Mekong Delta.

Aquifer Number of wells in DWRPIS Number of wells with (2014), data 1995–2010 data 1995–2016 qh 21 20

qp3 15 14

qp2–3 19 18

qp1 12 12

n22 15 13

n21 16 15

n13 8 5

Some wells in Cà Mau Province (Q177020, Q17704T, Q17704Z, Q188030) show suspicious peaks where water level sharply increased by more than 5 metres and quickly receded after a few months. These peaks have been excluded from further analysis in this baseline study.

In the study DWRPIS (2014), for each aquifer the average water levels over the whole Mekong for each month were plotted (DWRPIS, 2014, Annex 11); these diagrams have been extended here until 2016 and are presented in Figure 6.3 – Figure 6.9.

The groundwater levels in all seven aquifers showed seasonal fluctuations, though to various degree. In general, the groundwater levels decreased in the dry season and increased in the rainy season, minimum values were observed in May (end of dry season) and maximum values were observed in November (end of rainy season). This pattern is most pronounced in the shallow aquifers (qh, qp3) and attenuated with greater depth. The seasonal amplitude in the shallow aquifers decreased over time, possibly due to insufficient recharge. (The annual maxima decreased more than minima.)

The groundwater levels in all of the seven aquifers show a declining trend from 1995 to 2010 of different extents, as shown in Figure 6.2 and Table 6.2. The average of the decrease of groundwater levels for the wells during the report’s monitoring period 1995 – 2010 is highest for the Pliocene 1 2 aquifers n2 and n2 . The table also shows the average decreases based on the recent data from

NAWAPI for 2011–2016. The rate of decrease became lower for qp2–3, but groundwater decline 1 3 accelerated for qp3 and especially n2 and n1 . Moreover, it is important to note that while the groundwater level in all aquifers fluctuated around mean sea level in 1995/96 it dropped to several meters below mean sea level by 2010, the end of the study DWRPIS (2014), and continued to decline further. This applies to all aquifers, except of aquifer qh which may be directly connected to surface waters.

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h (m asl) h (m

qh qp3 qp23 qp1 n22 n21 n13 -8 -6 -4 -2 0 2

1995 2000 2005 2010 2015

Year Figure 6.2. Time series of mean water level in aquifers of the Mekong Delta. Data from DWRPIS (2014) and NAWAPI.

Table 6.2. Averages of decrease of groundwater levels in wells of the Mekong Delta.

Aquifer Average decrease in groundwater level in m/year 1995–2010 (DWRPIS, 2011–2016 2014) qh 0.064 0.061

qp3 0.150 0.233

qp2–3 0.300 0.251

qp1 0.285 0.339

n22 0.434 0.393

n21, 0.365 0.539

n13 0.266 0.537

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qh h (m asl) h (m 0.5 1.0 1.5

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 6.3. Monthly averages of groundwater level (m asl) of the qh aquifer in the Mekong Delta for the years 1995 (blue), 2005 (green), and 2016 (red). Data from DWRPIS (2014) and NAWAPI.

qp3 h (m asl) h (m -4 -3 -2 -1 0 1