Can groundwater secure drinking-water supply and supplementary irrigation in new settlements of North-West Cambodia?
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Since the end of the Cambodian Civil War in 1998, the population of the Oddar Meanchey province has drastically increased despite the lack of adequate infrastructure, including basic amenities such as drinking-water supply. To improve the access to drinking water, governmental and aid agencies have focussed on drilling shallow boreholes. The use of groundwater for irrigation is also a growing concern to cope with the occasional late arrival of the rainy season or to produce food during the dry season. Since the groundwater resource in the province has not been documented, a 4-year study was undertaken (2011–2014), aiming to estimate the capability of groundwater to supply domestic needs and supplementary irrigation for rice production. Aquifer properties were estimated by combined use of hydrogeological techniques with the geophysical magnetic resonance sounding method. Groundwater storage and recharge were estimated based on new developments in the application of the geophysical method for quantifying specific yield. The median groundwater storage of the targeted sandstone aquifer is 173 mm, the recharge is diffuse and annually ranges from 10 to 70 mm, and the transmissivity is low to medium. Simulations of pumping indicate that the aquifer can easily supply 100 L of drinking water per capita daily, even considering the estimated population in 2030. However, the shallow aquifer can generally not deliver enough water to irrigate paddy fields of several hectares during a 2-month delay in the onset of the monsoon.
KeywordsCambodia Groundwater development Groundwater recharge Irrigation Geophysics
Les eaux souterraines peuvent-elles sécuriser l’alimentation en eau potable et un complément d’irrigation dans les nouveaux villages du Nord-Ouest du Cambodge?
Depuis la fin de la guerre civile au Cambodge en 1998, la population de la province d’Oddar Meanchey a augmenté très fortement, malgré l’absence d’infrastructures adéquates, y compris les équipements de base nécessaires à l’alimentation en eau potable. Pour améliorer l’accès à l’eau potable, des agences gouvernementales et d’aide se focalisent sur la réalisation de forages peu profonds. L’utilisation de l’eau souterraine pour l’irrigation est aussi une préoccupation croissante pour faire face à l’arrivée occasionnellement tardive de la saison des pluies ou pour produire de la nourriture pendant la saison sèche. Comme la ressource en eau souterraine de la province est peu connue, une étude de quatre ans a été entreprise (2011–2014), avec l’objectif d’estimer l’aptitude des eaux souterraines à satisfaire les besoins domestiques et un complément d’irrigation pour la production de riz. Les propriétés aquifères ont été estimées par la mise en œuvre combinée de techniques hydrogéologiques et de la méthode géophysique de sondage par résonnance magnétique protonique. Le stock d’eau souterraine et la recharge ont été estimés sur la base de nouveaux développements dans l’application de cette méthode géophysique pour quantifier la porosité de drainage. Le stock médian d’eau souterraine de l’aquifère gréseux ciblé est de 173 mm, la recharge est diffuse et varie annuellement entre 10 et 70 mm et la transmissivité est faible à moyenne. Des simulations de pompage indiquent que l’aquifère peut aisément fournir quotidiennement 100 L d’eau potable par habitant, même en considérant la population estimée en 2030. Cependant, l’aquifère superficiel ne peut généralement pas produire suffisamment d’eau pour irriguer les champs de riz de plusieurs hectares pendant une période de 2 mois en début de mousson.
Puede el agua subterránea asegurar el abastecimiento de agua potable y el riego suplementario en los nuevos asentamientos del Noroeste de Camboya?
Desde el final de la guerra civil de Camboya en 1998, la población ha aumentado drásticamente en la provincia Oddar Meanchey, a pesar de la carencia de una adecuada infraestructura, incluyendo los servicios básicos tal como el abastecimiento de agua potable. Para mejorar el acceso al agua potable, los organismos gubernamentales y de ayuda se enfocaron en la perforación de pozos poco profundos. El uso del aguas subterránea para el riego es también una preocupación cada vez mayor para hacer frente a la ocasional llegada tardía de la temporada de lluvias o para producir alimentos durante la estación seca. Dado que el recurso de agua subterránea en la provincia no ha sido documentado, se llevó a cabo un estudio de cuatro años (2011–2014), con el objetivo de estimar la capacidad de las aguas subterráneas para abastecer las necesidades domésticas y al riego suplementario para la producción de arroz. Las propiedades del acuífero se estimaron mediante el uso combinado de técnicas hidrogeológicas con el método geofísico de resonancia magnética. Se estimó el almacenamiento del agua subterránea y la recarga sobre la base del nuevo desarrollo en la aplicación del método geofísico para cuantificar el rendimiento específico. El almacenamiento medio de agua subterránea en el acuífero de arenisca, tomado como objetivo, es de 173 mm, la recarga es difusa y anualmente oscila entre 10 y 70 mm, y la transmisividad es de baja a media. Las simulaciones de bombeo indican que el acuífero puede fácilmente suministrar 100 L diarios de agua potable por habitante, incluso teniendo en cuenta la población estimada para 2030. Sin embargo, el acuífero somero no puede, en general, suministrar suficiente cantidad de agua para regar los arrozales de varias hectáreas durante el retraso de 2 meses en el inicio del monzón.
自从1998年柬埔寨内战结束后,基础设施缺乏,就连基本的设施诸如饮用水供给同样匮乏,但Oddar Meanchey省的人口却大量增加。为了提高饮用水使用率,政府和援助机构加大了开凿浅层井的力度。利用地下水灌溉也日益受到关注,以应对偶尔迟到的雨季或者在旱季种植农作物。由于该省没有地下水资源相关资料,为此进行了4年的研究,目的就是评估地下水满足家庭用水和满足水稻种植灌溉的能力。通过利用水文地质技术和地球物理磁共振探测方法评估了含水层特性。根据量化单位出水量的地球物理方法应用的新进展,估算了地下水储存量和补给量。所估算的砂岩含水层的地下水储存量中间值为173 mm,补给量是弥散的,每年为10 至 70 mm,导水系数从 低到中。抽水模拟表明,即使按照2030年估算的人口算,含水层也可以很轻松地每天向每人提供100升的饮用水。然而,浅层含水层在季风开始时两个月的延期中通常不能提供足够的水灌溉几公顷的稻田。
A água subterrânea pode assegurar o abastecimento de água potável e a irrigação suplementar nos novos assentamentos no Noroeste do Camboja?
Desde o fim da guerra civil cambojana em 1998, a população da província de Oddar Meanchey cresceu drasticamente apesar da falta de infraestrutura adequada, incluindo amenidades básicas como abastecimento de água potável. Para melhorar o acesso a água potável, agências governamentais e de assistência humanitária estão focadas em perfuração de poços rasos. A utilização da água subterrânea para a irrigação é também uma preocupação crescente para lidar com a ocasional chegada atrasada da temporada chuvosa ou para produzir comida durante o período seco. Como as águas subterrâneas na província não foram documentadas, um estudo de 4 anos foi realizado (2011–2014), com o objetivo de estimar a capacidade das águas subterrâneas para abastecer necessidades domesticas e irrigação suplementar para a produção de arroz. As propriedades do aquífero foram estimadas combinando o uso de técnicas hidrogeológicas e o método de sondagem por ressonância magnética geofísica. A recarga e o armazenamento subterrâneo foram estimados baseados no desenvolvimento da aplicação do método geofísico para quantificar a capacidade especifica. O armazenamento subterrâneo mediano do aquífero sedimentar escolhido é de 173 mm, a recarga é difusa e anualmente varia entre 10 e 70 mm, e a transmissividade é de baixa a média. Simulações de bombeamento indicam que o aquífero pode facilmente abastecer diariamente 100 L de agua potável per capita, mesmo considerando a população estimada em 2030. Entretanto, o aquífero livre e raso pode geralmente não fornecer água suficiente para irrigar os campos de arroz de alguns hectares durante um atraso de 2 meses no começo da monção.
At the moment, about 34 % of rural inhabitants of Cambodia do not have access to safe drinking water (WHO and UNICEF 2014 ) and 15–25 % of the population is undernourished (WFP 2014). National averages mask inequalities: vulnerable people living in the recently developing Oddar Meanchey province cannot access safe drinking water and face periodic food shortage.
Almost 90 % of the families living in Oddar Meachey province are farmers and their livelihood mainly relies on a single rain-fed rice harvest per year (Nesbitt 1997). A food security study conducted in 2008 indicated that about 27 % of the people living in several regions of Cambodia, including Oddar Meanchey province, were potentially food insecure during the lean season (WFP 2008). To update and specify this figure, aid organizations (the Cambodian and the French Red Cross, CRC/FRC) carried out a survey in August 2010 among 200 villagers located in two districts of Oddar Meanchey province (Fig. 1). The assessment revealed that 80 % of the people have already faced drought, i.e. a late onset of the monsoon-causing water shortage at the first growing stage of the rice production. This temporary water shortage has a direct impact on food production and availability for 87 % of the families and also on income generation for 66 % of them. A large majority of the interviewed people (84 %) stated that they were not prepared to face delay in the onset of the monsoon, and 86 % of the people recommended improving their access to water and their storage capacity to be prepared for drought.
To supply communities with water all year round, governmental and aid agencies are supporting drilling projects. However, no quantitative assessment on groundwater resources was available in Oddar Meanchey province because of the recent history (i.e. civil war). Moreover, Oddar Meanchey is located in a geological context whose potential groundwater has rarely been investigated (e.g. Rasmussen and Bradford 1977), since most of the published works focus on the alluvial sediments of the Mekong River that are vulnerable to arsenic contamination (e.g. Berg et al. 2007). A preliminary compilation and review of information on groundwater has been completed by Landon (2011) who confirms that most of the works carried out in Cambodia concerned the alluvium units and that there is a large uncertainty in groundwater recharge estimates (e.g. JICA and MRD 2002). As mentioned by Johnston et al. (2013), the limited use of groundwater for irrigation in many areas of Cambodia is attributed to several reasons such as poor knowledge of the resource and its sustainability.
This report aims at improving the knowledge on groundwater potential for both the drinking-water supply and supplementary irrigation in Oddar Meanchey Province, in order to support the implementation of activities of governmental and aid agencies. A study has been implemented over a 4-year period using hydrogeological techniques—e.g. water-table mapping, drilling boreholes, setting up pumping tests, monitoring groundwater levels and rainfall, analysis of water isotopes—together with the geophysical magnetic resonance sounding (MRS) method. Based on new developments in the application of MRS for assessing aquifer specific yield (Vouillamoz et al. 2012a, 2014a), a sizable MRS survey has been conducted for quantifying aquifer storage and recharge. From the unrivalled data set obtained, the groundwater resource of shallow sandstones that are targeted by local drillers has been estimated for the first time in the Oddar Meanchey province.
Area of investigation
The investigated area (3,375 km2), located in the eastern part of Oddar Meanchey province (Fig. 1), covers the districts of Along Veng and Trapeang Prasat where about 71,000 people were living in 2008, which was about 30 % of the total population of the province (National Institute of Statistics 2009). Ninety per cent of the rainfall occurs from April to October (1,754 mm/year on average, see section “Aquifer recharge” and Fig. 9) and the annual mean air temperature is 29 °C.
The area is a plain which gently slopes from the north-east to the south-west and is mainly drained by the Stoeng Sreng River, which discharges to Tonle Sap Lake. Most of the rivers dry up after the rainy season and there is no perennial runoff. To the north, the plain ends at the Dangrek Hills along the border with Thailand. The Dangrek Hills, formed from sandstones dating from Upper Triassic to Lower Cretaceous (Dottin 1972), are the edge of a sandstone escarpment which extends into Thailand. The beds are gently dipping to the north (5–8°) and they comprise conglomerates, fine-to-coarse-grained sandstone and variegated siltstone and mudstone. Tectonic faulting in the sandstones has not been observed (Dottin 1972). In the investigated area, the sandstones have been eroded (they clearly outcrop further south) and then have been covered by alluvium dating from the Pleistocene to the Holocene (known as Old Alluvium). The alluviums which are composed of clay, silt and sand are the most widespread geological unit in the investigated area. They are thin (i.e. few meters thick), thus making outcropping of the upper sandstones possible in different places (Fig. 1). The south of the investigated area is probably an anticline of east–north-east to the west–south-west direction, of the Devono-Carboniferous substratum, which locally outcrops as Phnom (i.e. small isolated hills) in the south-west of the area (Dottin 1972).
Materials and method
The study was implemented over a 4-year period (2011–2014) and combined several methods including statistical analysis of a database of 211 wells, geophysics and pumping tests at nine experimental sites, 79 measurements of MRSs to complement aquifer properties, estimation of recharge from the monitoring of 12 piezometers (including six coupled with rain gauges), two piezometric campaigns at 36 locations, and water isotopes analyses from 24 wells and rainfall.
Estimation and supplementation of aquifer productivity and storage
The only available data regarding groundwater in the study area were from the well inventory of the national database (Ministry of Rural Development of Cambodia 2010). Among the 332 records, only wells with coordinates and described basic properties were selected; these were then added to a set of 30 wells (drilled by the CRC/FRC within the framework of this study) out of which the total number of wells used for the study was 211. Based on this data set, basic statistics and spatial patterns of drilling success rate (i.e. ratio of the number of successful wells that have an instantaneous yield higher than 0.5 m3/h to the total number of drilled wells), well depth and productivity (i.e. instantaneous yield and specific capacity) were documented. Moreover, 61 drilling reports were available at the CRC/FRC office whose lithologs were used to complement the description of the geological context given by Dottin (1972).
For assessing aquifer transmissivity (T) and specific yield (Sy), nine experimental sites were setup all over the study area. At every experimental site, pumping tests were carried out using a pumping well and one or two observation wells located between 8 and 23 m away from the pumping well. Pumping durations lasted between 72 and 120 h and pumping yields ranged between 1.3 and 9.4 m3/h. Pumping tests were interpreted by Vouillamoz et al. (2012a) using analytical solutions and drawdown log-derivative analysis (Renard et al. 2009) implemented in AQTESOLV/Pro v4.5 software.
To complement the transmissivity (T) obtained at the nine experimental sites, a relationship between the transmissivity and the specific capacity (Q/s) was defined (e.g. Razack and Huntley 1991). Using this relationship, transmissivity values were calculated at 108 locations scattered over the investigated area where Q/s was documented in borehole reports.
The MRS measurements were carried out using the NumisPlus apparatus from Iris Instruments with a square-shaped loop of 50–100 m length per side, thus giving an investigation depth of 50–70 m below ground level (Bernard 2007). The measurements were interpreted with Samovar V11.43 software (Legchenko et al. 2008).
Estimation of aquifer recharge
Next to six piezometers, an auger rain gauge equipped with a datalogger was also installed to look for the link between recharge and rainfall. The auger rain gauges was checked using an appropriate procedure (i.e. adding a known volume of water at a controlled intensity) and the rain flowing out of the augers was collected into buried containers for cross-checking the recorded volume of rain. Rainfall events and groundwater level were monitored for 4 years at the six locations equipped with a piezometer and rain gauge. At the six other locations only equipped with a piezometer, the groundwater level was monitored for a shorter duration, i.e. 12–48 months.
To better understand the process of the recharge, 52 analyses of stable isotopes of oxygen and hydrogen (i.e. 18O and 2H) were done, including six analyses of rain samples. Groundwater samples were collected in September 2012 and then between April and December 2014 at 24 locations scattered over the investigated area. Rain samples were collected in May, June, August, September and October 2014, that is, between the early and the late rain events of the monsoon. Rainwater was collected in containers that were connected to rain gauges by a pipe. To prevent isotopic fractionation due to evaporation, the pipes reached the bottom of the containers; the containers were properly closed and they were buried to be protected from the sunlight. The collected rainwater was composed of several rain events that occurred within 1–2 months. The samples were sent to a laboratory in Thailand (Bangkok) to carry out the analyses. Two piezometric monitoring campaigns were also carried out by measuring the surface-water level (SWL) at 36 locations in November 2012 (after the monsoon) and May 2013 (before the next monsoon).
Of the 211 boreholes of the database, there were 37 unsuccessful holes (i.e. instantaneous yield Qi measured after well development is less than 0.5 m3/h), thus giving a success rate of 82 %. However, numerous unsuccessful boreholes are most probably not reported and thus are not recorded in the national database; indeed, when only considering the well-controlled database of CRC/FRC (69 wells, which are part of the 211), the success rate is 64 %. This latter figure is consistent with the anecdotal observations of drillers. Note that the use of geophysics (i.e. resistivity and MRS) for siting boreholes in Oddar Meanchey province has increased the success rate of the CRC/FRC from 64 % (39 wells) to 97 % (30 wells), as previously observed in the neighbouring province of Siem Reap (Vouillamoz et al. 2002). The success rate seems to be higher in the north-east of the area, but the major finding is that positive and negative boreholes can be as close as a few tens of meters, thus indicating a strong spatial heterogeneity of the aquifers.
Aquifer transmissivity and storage
Aquifer properties, recharge and rainfall
No. of values
Decile No. 1
Decile No. 9
1.6 · 10–5
2.9 · 10–4
5.1 · 10–3
1.2 · 10–5
3.4 · 10–4
1.1 · 10–2
Specific yield [%]
Annual recharge [mm]
Annual rainfall [mm]
Aquifer properties and recharge
The median recharge that occurred during the 4 years of the study is 28 mm (10–70 mm), which represents only a small percentage of the rainfall (from 0.5 to 4.3 %). To the authors’ knowledge, no previous study has estimated the recharge in the Upper Triassic to Lower Cretaceous sandstones in Cambodia. Estimations of recharge have been carried out in unconsolidated younger sediments using several approaches of groundwater modelling (IDE 2009), water balance (JICA and MRD 2002), interaction of small streams and shallow groundwater (Araki et al. 2008), or modelling of floods of the Mekong River (Kazama et al. 2007). The recharge estimated for this study is based on numerous field observations (i.e. a total of 31 annual data obtained from the monitoring of 12 sites) which have been interpreted with the water-table fluctuation method (WTFM). WTFM assumes that the groundwater moves away from the recharge location at a rate that is slower than the rate at which recharge water arrives from ground surface (Healy and Cook 2002). Assuming this is the case in this study, the application of the method is valid. The comparison of piezometry before and after the rainy season indicates that the horizontal component of the groundwater flow does not change seasonally, thus suggesting a diffuse recharge, i.e. a discharge that is distributed all over the area as a direct response to rainfall events. The isotopic analyses also confirm that rainwater infiltrates directly and rapidly (i.e. no prior evaporation).
Although the data set produced in this study by field characterization of the aquifer is unique in Cambodia (i.e. 108 values of transmissivity, 79 values of storage and 31 values of recharge over an area of 3,375 km2), there is no clear proof of any spatial pattern in the distribution of the properties. The aquifer in the east and north-east of the investigated area generally seems more productive, but transmissivity and storage can be highly different at the scale of a few tens of meters. The main explanation for the variation of the properties of the aquifer is probably the variation of clay content of the sandstones.
A few boreholes that reached the substratum (schist in Fig. 12b) below the sandstone are clearly more productive than boreholes drilled in the common sandstone. Unfortunately, this observation could not be confirmed with new boreholes since the two attempts to drill deep enough into the Devono-Carboniferous substratum were unsuccessful: the drilling rig was either not powerful enough or the hole too unstable to reach the targeted depth at Kok Sampor village. Note that the cost of deep drilling is several orders higher than the shallow drillings, making aid agencies focus on the latter.
The first question asked by the humanitarian and development actors to the researcher concerns the use of groundwater to supply people with drinking water. The national census of 2008 recorded the population of the investigated area at 71,651 (National Institute of Statistics 2009). The average annual growth rate between 2008 and 2013 in Oddar Meanchey province was 4.39 % (National Institute of Statistics 2009, 2013) and the national average rate is estimated to be 1.38 % from 2013 to 2030 (UNICEF 2015), thus giving a total population in the investigated area of about 112,000 people in 2030. Today, the daily water consumption is not known exactly but it might increase as people move out of poverty. Assuming that the daily consumption will increase to 100 L per capita per day, and assuming that the entire supply will come from groundwater abstraction, the required volume will be 11,200 m3/day or 1.2 mm/year of equivalent water thickness. As compared to the current groundwater recharge (R ≈ 28 mm/year), the abstraction for domestic supply is definitively low. Moreover, the groundwater storage (GWstorage = 173 mm) can clearly buffer any change in the recharge, thus increasing the resilience of the population to climate or anthropogenic changes.
Concerning groundwater quality, the investigated area is usually classified in the “low” to “very low” risk of arsenic pollution (e.g. Berg et al. 2007); however, a systematic control of the arsenic content is recommended when groundwater is used for drinking, cooking or irrigation (e.g. Lado et al. 2008) in areas where no or only few data have been collected; thus, arsenic concentration was checked by the FRC/CRC in 2010 before the beginning of the study. All the samples collected in 138 wells scattered in the investigated area indicated an arsenic concentration less than 2 μg/L (the WHO guideline for drinking water is 10 μg/L).
Replicability of the study
This study was carried out in the framework of a collaborative project between two parties, i.e. aid agencies (Cambodian and French Red Cross, CRC/FRC) and a French research institute (Institut de Recherche pour le Développement, IRD). The interest in such a collaboration is the complementarity of the skills. On the one hand, aid agencies have direct access to the human communities and have effective logistics to implement activities in the field, while, on the other hand, scientists can support aid agencies to address development challenges through specific inputs. Lessons learned from this 4-year study indicate that the compulsory conditions to be met to obtain useful results for both parties are (1) scientific activities and expected results clearly defined and integrated in the proposal of the development project, (2) dedicated staff and budget allocation to implement the scientific field work, and (3) a sufficiently long period of time for assessing the hydrological cycle, i.e. 4–5 years. Fulfilling these conditions may lead to a successful replication of such a study in other regions.
This work is the first ever study carried out on the groundwater resource in Oddar Meanchey province to support governmental and aid agencies in the implementation of drinking-water supply and food security projects. The study was implemented over a 4-year period from 2011 to 2014. Hydrogeological techniques were used together with the geophysical MRS method, resulting in an estimation of aquifer productivity, storage and recharge. It was found that the sandstone aquifer is generally clayey, thus exhibiting a low-to-medium transmissivity and a median storage of 173 mm. The recharge has been quantified to range between 10 and 70 mm or 0.5–4.3 % of the annual rainfall. Based on this unique data set, the capacity of the sandstone aquifer to supply 100 L of drinking water daily to the expected population in 2030 has been checked and it was found that the sandstone aquifer can easily supply drinking water to the Oddar Meanchey people. However, the shallow sandstone aquifer can generally not supply enough water for the large-scale supplementary irrigation of rice. Because the aquifer is heterogeneous, it can locally exhibit more advantageous properties that can be sufficient for irrigating small plots for a few families for about 2 months. Detailed studies will have to be carried out at the local scale for quantifying the potential for such irrigation.
Deeper aquifers have not been studied in this work, even if some results indicated that groundwater resources can be higher in the fractured schist of the Devono-Carbonifereous substratum as compared to the surveyed Upper Triassic-Lower Cretaceous clayey sandstone. The development of a new project focussing on the deeper groundwater resource can hopefully confirm this assumption and then propose new options for using groundwater for irrigation.
Finally, groundwater is a component of the water cycle and a more comprehensive assessment of the water resource can be achieved by an integrated water resource survey. This study is a first step in the quantification of the water resource of Oddar Meanchey province, and it aims at suggesting that appropriate quantitative studies must be promoted to support strategies to address current needs and to adapt to future demands.
The authors would like to thank the reviewers as well as the associate editor for their helpful comments and thorough review of the manuscript. This work has been carried out in the framework of the Institut de Recherche pour le Développement and the French Red Cross collaborative project 39842A1-1R012-RHYD, with financial support of the European Union (grant DIPECHO SEA ECHO/DIP/BUD/2010/01017 and grant DCI-FOOD/2011/278-175). We thank S. Sokheng, P. Sophoeun and O. Bruyère for their efficient assistance in field work. We also thank H. Thyberghien, A. Petibon, L. Anstett and A. Chappate for making this project possible.
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