An Overview of the Geological Control on Groundwater Potential in Lacamutu River, Alaua Kraik Area, Baucau Municipality, Timor-Leste ()
1. Introduction
Groundwater is among the most valuable natural resources supporting human health, economic development, and ecological diversity [1] [2]. The importance of groundwater is growing in both urban and rural areas of many countries [3] due to the increasing demand and pressure from population growth, irrigated agricultural activities and climate change [4] [5]. Most of Timor island has a tropical climate but with small rainfall compared to the other tropical countries due to its geographical location near the equator [6]. As a result, surface water is not obtainable throughout the year and groundwater is required to meet daily domestic industrial and agricultural [7] [8] [9] [10] [11]. Groundwater occurrence, source and movement are dependent on the geological characteristics such as rock type, thickness, structures, and permeability of aquifers [1]. Hence, geological reconnaissance mapping, hydrogeological investigations, and resistivity surveys are applied in this study to systematically assess groundwater conditions and their potential to supply water for human consumption. The domestic water supply for the local community in the study area is still depending on dug wells, springs, and main rivers. However, these are limited in both quantity and quality and cannot fulfill the demand for clean water, especially during the long dry season [12].
The 0.95 km2 study area is located in Alaua Kraik village near the Lacamutu River in Baguia Administrative Post (Baucau Municipality) ~150 km from the Capital City of Dili (Figure 1). It is low terrain topography with a contoured index ranging from 110 - 240 m.
The chaotic geology of Timor island is part of the non-volcanic Outer Banda Arc formed through the collision between Sundaland (Southeast Eurasia) and the Australian continent in the late Neogene [13] - [18]. The regional lithologic formation within the study area are Pre-Permian low-grade to moderate metamorphic rocks associated with ultrabasic gabbro to dolerite rock units of the Lolotoe Complex; a Cretaceous chert interbedded with calcareous shale of Borolalo Limestone; Holocene unconsolidated sediment of Alluvial deposits; Permian biocalcarenite and crinoid limestone of the Maubisse Formation; and Upper
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Figure 1. The study area of Lacamutu River, Alaua Kraik, Baguia Administrative Post, Baucau Municipality, Timor-Leste.
Miocene chaotic rocks of boulder-sized fragments within scaly clay matrix of Bobonaro Scaly Clay [19] [20]. All these lithologic units are classified as impermeable and are not likely to work as aquifers based on their thickness, porosity, and permeability.
In August 2021, only 65% (265) out of 408 households in Alaua Kraik area had been access to clean water, while 35% (143) reminds unobtainable [12]. In addition, surface water distribution including river and watersheds in the study area are mainly derived from rainfall and springs and are all categorized as intermittent types. Therefore, most of the local population lack clean water supply for their daily domestic needs during the long dry season [12]. To understand potential ways of mitigating this, the objectives of the study were as follows: 1) identify the available groundwater resources in the study area and estimate its potential for human consumption, 2) to define the aquifer system, the hydrostratigraphic units, and the groundwater flow direction modeling concept via geological reconnaissance mapping, resistivity study and well-log data.
2. Literature Review
The main origin of groundwater is rainwater. Rainwater that falls on the surface will flow as runoff and some will infiltrate into the ground through the infiltration process [21]. Infiltration is the flow of water (rain) into the soil as a result of capillary forces and gravity. After the topsoil layer is saturated, the water surplus flows into the deeper ground because of the earth’s gravity, commonly called percolation [22]. The amount of water that flows downward depends on the type of soil or rock itself [23], while the capacity to infiltrate rainwater is expressed as the infiltration capacity, which is mostly affected by several factors including soil surface conditions, soil structure, vegetation, soil temperature, etc. Infiltration capacity can be approached by knowing the porosity of a rock or soil (Table 1).
Groundwater is naturally limited by hydraulic boundaries which are effectively controlled by the geological and hydrogeological conditions of the local area [24]. The important factor that influences the process of groundwater formation is geological rock units, which are lithologic formations or other geological materials that store large amounts of groundwater, naturally known as aquifers [25].
The ability of a rock to store and pass water is largely specified by the porosity and permeability of the rock itself. Porosity defines the amount of water that can be accumulated in geological substrate whereas permeability describes how those pores are interconnected and determines water flow from one pore to the next [26]. The porosity and permeability help to define the hydrogeologic characteristics of rock units. The presence of fractures (secondary porosity) and the thickness of the material overlying the rock are factors that control groundwater storability and potential [21] [27] [28]. The type of porosity varies greatly depending on its genesis, cavity size, and the relation between pores which are affected by compaction, cementation, grain size, grain form, grain structure, and grain sorting, and it can be classified as large porosity, if it is greater than 20%, medium porosity ranges from 5% to 20%, and small porosity if it has a value of less than 5% (Table 2). High porosity is not a definitive indication that an aquifer will produce a large volume of water for a well. Meanwhile, factors that affect permeability are grain size, grain form, sorting, degree of compactness, and cementation thereof it appears the different values of permeability for several rock types [21] (Table 3).
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Table 1. The infiltration texture ranges based on the rock porosity [21].
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Table 2. The magnitude of rock porosity [21].
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Table 3. The permeability in several types of rocks [21].
Naturally, rock units that act as a medium for groundwater flow have favorable permeability, specific capacity, transmissivity, and storage where only lithologic permeability is able to consider in this study. According to Darcy [21], there is a relationship between permeability (k), laboratory coefficient of permeability (Ks), rock types, and their characteristics to groundwater flow (Table 4).
The existence of groundwater is very specific, and its distribution is also uneven depending on the geological subsurface conditions such as the distribution of permeable and impermeable layers. Accordingly, in determining the presence of groundwater it is necessary to do a geological subsurface study through resistivity surveys. The various resistivity values of rock units rely on the lithologic parameters themselves. These parameters are porosity and fluid contents (oil, gas, and groundwater). Typically, igneous and metamorphic rocks have high resistivity values. The resistivity of these rocks depends on their structure, texture, and the percentage of fractures that have been filled by groundwater [29]. The high porosity of sedimentary rocks which have been filled up by the fluids has lower resistivity than the consolidated sedimentary rocks with resistivity values less than 1000 Ω∙m whereas the unconsolidated material of sedimentary has lower resistivity values than sedimentary rocks. Groundwater has numerous resistivity relying on the concentration of salt dissolution with resistivity values ranging from 10 - 100 Ω·m. The resistivity values that are less than <0.2 Ω∙m are usually found in seawater due to a high salt composition (Figure 2). Furthermore, the resistivity of each material in the earth has a typical interval range, Table 5 below displays the variation of the resistivity values of rock materials, minerals, and various fluids.
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Table 4. The relationship between k, Ks, rock types, and their properties towards groundwater flow [21].
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Table 5. The resistivity value of rocks, minerals, and various fluids [29] [31] [32] [33].
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Figure 2. The electrical resistivity and conductivity values for selected materials [30].
3. Methodology and Materials
The study applies two (2) main methodological approaches, including geological reconnaissance mapping and resistivity surveys to identify the groundwater resources in the study area. The geological reconnaissance mapping method was applied thru field observations of geological structures and lineament interpretation from DEM (Digital Elevation Model), observations of rock units, descriptions of rock samples from well-log, and also identification of springs in the Lacamutu River area of Alaua Kraik. The resistivity study method is a geoelectric survey to detect subsurface lithologic conditions in a certain depth that will appear to indicate the hydrostratigraphic units of the subsurface and the presence of groundwater.
Geological structure data were analyzed using Dips 5.1 stereograph software to identify and characterize the structures. The rock units were described petrographically by using a polarized microscope to determine the types and the characteristics of the rock units themselves. Springs mapping was undertaken together with the measurement of discharge through volumetric measurement method. The 2-D resistivity survey used the Resistivity Meter McOHM Profiler 4 Instrument of Pole-Pole configuration with current electrodes spread (AB) of 160 m that can reach a maximum depth of 130 m. The rainfall precipitation data in the Baguia Administrative Post, from the [34] rainfall station during the period time of 2010 to 2018 was also calculated to determine the rainfall intensity, maximum rate of runoff [35], and infiltration [22] [23] based on the following equation:
where:
I = rainfall intensity (mm/hour);
t = duration of rain (hour);
R24 = the maximum rainfall for 24 hours (mm).
where:
Q = maximum rate of runoff (m3/s);
I = rainfall intensity (mm/hour);
A = catchment area (Km2);
c = values of the runoff coefficient [36].
where:
I = infiltration (mm/s),
k = infiltration capacity (%);
Q = maximum rate of runoff (m3/s).
All geological observation data were integrated into a shapefile and then displayed in ArcGIS 10.6 along with the other related data to create a data-based geological reconnaissance map. The resistivity data were analyzed and interpreted using Res2DInv Software and Golden Software’s Surfer 16, numbers in Figure 2 and Table 5, and the geological reconnaissance map. The geological cross-section lines are used to define the hydrogeological units for groundwater potential in the study area. Resistivity profile lines and well-log data were utilized to interpret the thickness of lithologic layers of subsurface, groundwater distribution, and the hydrostratigraphic units of the subsurface. Moreover, the aquifer system, the hydrostratigraphic units, and the groundwater flow direction concept are illustrated and modeled based on the geological reconnaissance map, resistivity interpretation, and well-log data in Global Mapper 13 and ArcGIS 10.6. Lastly, the data on rainfall precipitation, rainfall intensity, maximum rate of runoff, and infiltration are also used to constrain the groundwater potential in the study area.
4. Result and Discussion
Geology of the Study Area
Observation in the study area indicates the presence of both unconsolidated permeable and consolidated impermeable rock units. The fluvial deposits of the unconsolidated permeable sediment comprise various types of rock fragments including carbonate breccia chert, calcareous shale, breccia pumice, calcareous massive, chert with calcite intercalation, green shale, igneous mafic, crinoid limestone, red marl, and calcarenite. In the outcrop, the well-exposed light gray of fluvial deposits display normal bedding structure with poorly sorted grains ranging from angular boulder to clay size with a mud-clay matrix and opened fabrics along the river terraces where it was deposited with strong energy. The consolidated impermeable rock units are also displaying normal bedding structure and comprise interbedded red marl-chert and calcareous shale. The fluvial deposit has a thickness of about 3 meters and lies unconformably on the underlying consolidated impermeable units moreover, petrographic analysis of the sedimentary rocks intercalation hint at two (2) rock units, i.e., calcareous shale and marl (Figure 3). Well logs show that unconsolidated fluvial deposits are present
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Figure 3. The petrographic analysis of consolidated impermeable materials of calcareous shale and marl.
from 0 to 3 m depth; from 3 to 5 m is an impermeable layer of calcareous shale; from 5 to 18 m are impermeable red marl-chert; from 18 to 19 m is calcareous shale, and from 19 to 25 m there is red marl- chert.
DEM (digital elevation model) interpretation shows a general ridge and valley direction of Northeast-Southwest (N 334˚ E) with principal stress of σ1 = N 10˚ E, σ2 = N 0˚ E and σ3 = N 276˚ E (Figure 4).
Field mapping in the study area indicates there are four (4) types of geological structures present; 1) normal right slip fault [37], indicated by gash fractures found in the calcareous shale unit with fault plane of N 152˚ E/88˚, direction N 262˚ E/78˚ and stress direction σ1 = 4˚, N 81˚ E, σ2 = 79˚, N 330˚ E, σ3 = 10˚, N 170˚ E, rake 12, Net Slip 81˚, N 326˚ E. The structure of normal right slip fault has been formed by left slip fault with the direction of N 5˚ E/78˚; 2) left slip fault, observed in the impermeable red marl-chert and calcareous shale unit with direction of N 5˚E/78˚ which crosses over the thrust fault with West-East direction; 3) anticline upright horizontal fold [38] by Northwest-Southeast direction, direction limb 1= N 315˚ E/35˚, limb 2 = N 144˚ E/32˚, stress direction σ1 = 4˚, N 229˚ E, σ2 = 4˚, N 319˚ E, σ3 = 85˚, N 90˚ E, axial plane (hinge surface) = N 319˚ E/86˚, hinge line = N 319˚ E/4˚, rake (pitch) = 3˚; 4) thrust fault, found in the red marl-chert layer and calcareous shale unit with direction of N 145˚
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Figure 4. The DEM (Digital Elevation Model) lineament interpretation of Baguia administrative post.
E/40˚. The thrust fault is crossed by the left slip fault from the North to South direction (Figure 5).
Resistivity Interpretation
Using the geological reconnaissance map of the Lacamutu River area of Alaua Kraik, four (4) resistivity lines were selected to interpret the subsurface lithology to 130 m depth (Figure 5 and Figure 6). All these resistivity points are interpreted based on the numbers in Figure 2, Table 5, and the geological reconnaissance map of the study area. The geological cross-section lines of the study area depict the three (3) types of rock units and the documented structural features (Figure 5).
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Figure 5. The geological reconnaissance map with profile line of selected resistivity points in Lacamutu River area of Alaua Kraik, Baguia Administrative Post, Baucau Municipality, Timor-Leste.
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Figure 6. The interpretation of resistivity lines based on the geological reconnaissance map of Lacamutu River Alaua Kraik area, Baguia Administrative Post, Baucau Municipality.
The interpretation of resistivity lines demonstrates that the three (3) rock units are present in the subsurface. The alluvial deposit has a resistivity value ranging from 16 - 50 Ω∙m; the intercalated red marl-chert has resistivities ranging from 6 - 15 Ω∙m; and the calcareous shale resistivity ranges from 1 - 5 Ω∙m and wet calcareous shale from 0.05 - 0.5 Ω∙m. These impermeable materials are fractured, with these fractures conduits for surface water resulting in the wet calcareous shale (Figure 6).
Groundwater Potential of the Study Area
To define the groundwater potential in the study area, it is necessary to know the aquifer system, its hydrostratigraphic units, groundwater flow direction, and rainfall. The key to this is understanding the porosity and permeability of the rock units defined previously. All three rock units documented have good porosity but low permeability and represent poor aquifers (Table 2 and Table 3). Whilst the fluvial rock unit is permeable the thinness of the unit and the low permeabilities, driven by the mud matrix, render it a poor aquifer that cannot store nor pass significant volume of water. This also applies to the impermeable calcareous shale unit and the impermeable red marl-chert unit. Because of this, the Lacamutu River has a small discharge and is often dry especially in the dry season.
Aquifer system, hydrostratigraphic units, and groundwater flow conceptualization
The character and distribution of aquifer systems are controlled by lithological, stratigraphical, and structural factors. Due to these geological factors the rock units in the study area have different hydrogeological features. This is largely due to deformation generating fractures and turning lithologic confining units into localized aquifers and permeable-impermeable characters of unconsolidated rock units into intergranular alluvial aquifer. The geological reconnaissance map, resistivity study and well-log data in the study area show that the unconsolidated permeable material of alluvial deposit represents an intergranular low productivity aquifer system hydrostratigraphic unit with a thickness of about 3 m. The consolidated impermeable material of red marl-chert layer intercalated with calcareous shale represents a localized (confining unit – fractured) low productivity aquifer system hydrostratigraphic unit with a thickness of more than tens meters. As such, none of these rock units have limited potential to accumulate and pass significant volumes of groundwater [39] (Figure 7).
Fractures may both increase or decrease porosity and permeability. Fractured rocks will have good permeability when fractures are open, interconnected, not cemented and can provide conduits groundwater flow [40] . Groundwater flows in an aquifer generally bounded by hydrogeological boundaries such as rocks, faults, folds, or surface water bodies where the main flow directions in the
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Figure 7. The aquifer system, the hydrostratigraphic units, and the groundwater flow direction modeling concept of the study area.
fractures rocks of confining units are along the joint zone, fault zone, and discontinuity plane zone [24]. In the study area, the groundwater flow naturally controlled by the structural features with flow via the existing fractures of joint largely towards the left slip fault plane zone in the North to South direction. The groundwater on the west side flows through lateral fractures while the groundwater on the east side flows thru vertical fractures all from shear joints to the left slip fault where the controlling factors are pressure, temperature, coarseness, gravity, and geometry (Figure 7). The rock units in the study area generally have good intergranular porosity (Table 2) and interconnected fractures (Figure 7), however they have lower permeability values (Table 3) and represent poor aquifers (Table 4) due to the increased temperature by depth, compactness, cementation, age, weathering level, and rocks types.
Rainfall
One of the factors to characterise the groundwater potential in the study area are rainfall intensity, maximum rate of runoff, and infiltration. The average monthly rainfall in the study area from 2010 to 2018 is 210.70 mm with the median daily rainfall intensity being 23.64 mm/hour (Figure 8). Referring to the classification of rainfall from [41], the monthly rainfall and daily rainfall intensity in the study area are classified as moderate rainfall. With a rain catchment area of 0.95 km2, a runoff coefficient of 0.7 for sandy soil lawn – 2% slope [36] and an average daily rainfall intensity of 23.64 mm/hour, the maximum rate of runoff (Q) for the study area is 0.044 m3/s or 158.4 m3/hour. With an infiltration capacity (k) 40% for alluvial deposit and red marl-chert intercalated with calcareous shale (Table 1) and 158.4 m3/hour maximum rate of runoff (Q), the infiltration which occurred in the study area is 63.36 mm/s or 228,096 mm/hour.
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Figure 8. The graph of average monthly rainfall (top) and daily rainfall intensity (bottom) in the study area during the period time of 2010 to 2018 [34].
According to the [41], rain with 100 mm precipitation in a catchment area of 1 km2 is equal to 100,000 m3 of water. It means that with a median monthly rainfall of 210.70 mm and a catchment area of 0.95 km2 there will be 200,165 m3 of rain water, 158.4 m3/hour surface runoff and infiltration of 228,096 mm/hour or 228.096 m3/hour. We suspect much of the rainfall in the study area from 2010 to 2018 did not run-off and infiltrate into the ground but was lost to interception, evaporation, and transpiration.
5. Conclusion and Recommendations
Household data for the study area indicate that 35.05% of households are not able to access clean water. This suggests there are no groundwater resources available to the local community. Geological reconnaissance mapping, hydrogeological study, resistivity survey, and well-log interpretation have identified two main rock units in the study area. These are the unconsolidated permeable alluvial deposit unit and the highly fractured consolidated impermeable red marl chert layer interbedded with the calcareous shale unit. These units have good porosity but low permeability and represent poor aquifers. The key geological structures in the study area are Lacamutu anticline, a thrust fault that is crossed by the left slip fault which then forms a normal right slip fault. These materials aquifers are categorized as a hydrostratigraphic unit of intergranular aquifer system with low productivity and thickness of about 3 m and a localized aquifer system (confining unit—fractured) with low productivity and thickness of more than tens meters. As a result, these hydrostratigraphic units do not have sufficient aquifers to store and pass significant volumes of groundwater, even though the rainfall in the study area is categorized as moderate rainfall; also, the springs and Lacamutu River have low discharge and mostly dry in the dry season. To overcome this condition, it is highly recommended to install a series of canals for clean water supply to the local communities from Mosa-Ira Spring around 8 km from the study area. This spring has a 12 L/sec discharge which meets WHO (World Health Organization) water quality standards [12]. Relevant government stakeholders such as BEE.TL (Water and Sanitation Public Enterprise) and PNDS (National Program of Village Development) should also undertake a hydrogeological evaluation of sites before drilling for water to enhance clean water supply to the communities.
Acknowledgements
Expressing our utmost gratitude to the IPG-TL technical staff who were directly involved in this study, starting from the field activities, data acquisition, data processing, data analyzing and data interpretation as well as the laboratory’s petrographic analysis as follows; Geophysics & Regional Petroleum Study Team from Task Force with UNTL’s internship students and Laboratory & Lithoteca Team from Division of Geological Investigation. Also, our deepest thankfulness to UNTL (National University of Timor-Leste) for providing us with the Resistivity Meter McOHM Profiler 4 Instrument and Dr. Eujay McCartain as a proofreader of this paper. We thank the management board at IPG for facilitating the field and laboratory costs, the local people for the collaboration during the study, and the review and editorial board for the efforts.