Hydrogeophysical modeling of the groundwater aquifer units under climate variability in parts of Peninsular Malaysia: A case study of the climate-water nexus approach to sustainability

Understanding of the climate-water nexus for sustainability, required good knowledge of the climate effects on groundwater aquifer units, particularly in rural communities. The studies were achieved using RES2-D modelling of the subsurface structures at the study site. Geophysical exploration with the application of 2-D Electrical Resistivity Imaging (ERI), combined with Induced Polarization (IP) method, were carried out to identify groundwater aquifers during extreme weather at Kampung Kuala Pajam, Beranang, Selangor, Peninsular Malaysia. The signatures obtained from geophysical explorations were used to better understand the phenomena that are responsible for groundwater depletion in the area. In recent times, there had been seasonal fluctuations in the water supply from boreholes serving the community. During the drought season, subsurface underlain this area experienced perennial acute shortages of groundwater supplies due to annual climatic variations that call for immediate solution by meeting the agricultural, domestic, and industrial water usage of the State of Selangor. A Pole-dipole techniques, using seven parallel lines of 400 m each at 5 m inter electrode spacing deployed to study the groundwater accumulation/aquifers within the area. Saturated groundwater occurrences zones were delineated as areas with average resistivity values of about 125 Ω-m, with corresponding chargeability of 30 ms. The methods used identified major faults along the northeast-southwest (NE-SW) directions, suitable for groundwater occurrences with approximate volume of about 2.86 Mega cubic metre (CBM), to proffer lasting solutions to the challenges being experienced by the community using a climate-water nexus sustainability.


Introduction
In recent years, there had been rapid unpredictable changes in the Peninsular Malaysia's climatic conditions. The changes have been shown to adversely affects water and agricultural sustainability of most urban and rural communities in the Peninsular. Although, in the efficient water resource management. With availability of sustainable fresh and portable water will perhaps leads to food security and economic growth of these communities.
Ambiguities arises from surface and subsurface geophysical mappings due to data acquisition, processing and validation of results, humans and errors from geophysical instruments used, together with the subsurface stratum conditions, necessitates integration of the surface mapping techniques with in-situ borehole well logs to study the lithological units, e.g. Ref. [37]. Since observed groundwater levels within an aquifer unit are non-continuous in a given time and space, the results obtained from RES2-D and IP alone are inadequate to model the groundwater estimated volume, e.g. Refs. [33,36]. The aquifer parameters are none distributive which give rise to qualms in the data interpretations if proper care is not taken into consideration. The in-situ borehole well logs helped to overcome the uncertainties that would have arise from the estimated volume of groundwater occurrences in the study area if surface geophysical methods are applied only.
To connect effectively with the people in these communities, and facilitate water policies for adequate resource allocation, the hydrogeophysical models of groundwater quality index obtained from the results of geophysical mappings for the study area, and the boreholes well logs, serves as guides for the detail needed information on the boreholes sitting, depths to the aquifers units, and other geophysical parameters that support exploitation of clean sustainable hygienic groundwater for the community's food security and social economic stability as population of the area soars. Sustainable climate-water nexus is for the policy makers in the natural resource management to adequately plan for sustainable adaptation strategies through corrective measures that will minimize the impact of weather and climate on groundwater resources.  Fig. 1a, showed the Topographical map that was published by Jabatan Ukur dan Pemetaan Malaysia (JUPEM) in 1994, and extracted from the topographic sheet number 3856, e.g., (Seremban Topographical map). The present study area is located at the hilly area in the Rubber Estate with highest elevation contour of about 55 m and could be accessed through Jalan Enam Kaki route.

The study area
The area is drained by rivers Pajam and Beranang. River Pajam is located approximately 860 m from the northwest (NW) to the study site. River Beranang is located approximately 250 m from the eastern side of the study area. The geophysical mapping was carried out behind the Masjid Kampung Kuala Pajam as shown in Fig. 1b. The selection of the study area was based on mapping groundwater potential zones in hard rock's geological terrains according to the models reported by Ref. [2].
Average annual precipitation recorded in the area was about 2500 mm, with the northeast (NE), Monsoon seasons producing the maximum peak values from months of October to January yearly which is characterized by significant seasonal variations in precipitations, temperature, evaporation, extremes wets and dryness periods. The meteorological station in Selangor recorded maximum temperature of about 39 ᵒC, and a minimum of about 23 ᵒC, with >96% mean relative humidity for this area. The research work being reported covered Kampung Kuala Pajam, Beranang, Selangor State communities. Geologically, i.e., Fig. 1c, the study area is underlain by Permian Formations that consist mainly of phyllites, slate, and shale with sandstone, siltstones, and schist as the subsidiary rock units, and a major faults trending along the NE-SW directions as delineated from the surface geophysical methods.
Geologically, Kampung Kuala Pajam is located at the Kenny Hill Formation as shown in Fig. 1a. The possible age of the formation was reported to be of late Paleazoic; Carboneferous by Ref. [38]. The formation consists of sequence layering of shale, siltstones, and sandstones. Shale and siltstone have metamorphosed into phyllite (e.g., Fig. 1c), and shows some presence of foliations and cleavages that are not parallel to the bedding planes. Generally, the sandstones are much richer in quartz, with the abundance of quartz veins observed at the study site (e.g., Fig. 1c). Some of the Outcrops observed at the Kenny Hill Formation were also found around Kuala Lumpur, and Petaling Jaya areas, e.g. Ref. [39]. The same formation units were also located at a bigger area within Kajang and Bangi environments. Some of the phyllite outcrops has been exposed as observed near the study site. The Kenny Hill Formation is part of the orogeny Formations that consists of various types of rock units with varieties of origins.

Method of data acquisition
Geophysical field data acquisition was carried out using ABEM SAB4000 Terrameter land imaging equipment in combination with auto selection ABEM ES 10-64 unit. The RES2-D ERT surveys were accomplished at 5 m electrode constant spacing, using 61 metal steel electrodes via two reels of multi-core cables [37,40]. A microcomputer unit was incorporated to the ABEM ES 10-64 selector as a switcher unit, which automatically pick-out four energized electrodes promptly, was used for each measurement. Each of the RES2D ERT surveyed profile lines, was made up of a single electrode spread of 400 m, with the application of pole-dipole electrodes array technique. The two dimessional resistivity inversion (RES2-DINV) software, by Ref. [41], was applied to model the recorded resistivity field datasets obtained from Beranang, Selangor community. The data acquired necessitates a qualitative approach to using a 12 V, 30 Amp-hr d.c battery as the source of current (A), injection to the subsurface strata, through the metal steel electrodes at points A & B, (e. g., current electrodes C.E). The consequential potential difference (V), dropped across these metal steel electrodes at points M & N, (e. g., potential electrodes P.E), are subsequently measured at other electrodes positions in the regions of the current flowing through them using parallel connections, e.g., Fig. 2.
Observations with subsurface structural layers are better mapped using the electrode cable geometry together with a switching cycle that defined the address and protocols files which allows user-defined arrays and survey approach. For this work, the Wenner- Schlumberger (W-S), configurations were selected. On the other hands, irregular heterogeneity in the subsurface structural layers is better delineated adequately with the application of the Pole-Dipole (PD) configurations. A combination of both W-S and PD will always produce better results. The PD array with the W-S configuration were selected for the data acquisition, because of their influence on the RES2-D inversion model applied. The combination has comparatively high-quality regular coverage, and higher signal to noise strengths' ratio. The model helped to eliminate ambiguities from the recorded field data, and much less sensitive to telluric noise than the pole-pole array, e.g. Refs. [1,21,22].
The pole-dipole array has four collinear electrodes. The second current electrodes (e.g., C2), was positioned at an "effective infinite" distance that must not be paralleled to the survey line, at approximately five times larger than the distance of C1 and P1. The other current electrode (C1) was positioned in the vicinity of the two potential electrodes (P1 and P2). This geometry was used because of its higher signal strengths compare to the dipole-dipole configurations. Thus, the geometry will generate higher depths of investigation. Besides, the less exposure to telluric noise datasets produces from these electrode arrays as both the potential electrodes are positioned according to the survey lines.

RES2-D ERT and IP data processing technique
The raw data were collected and transferred to the computer using the software for SAB4000 Utilities and *DAT format for data processing with the RES2-DINV software. SAB4000 software also has a presetting for the exchanges of data file formats, e.g., *.B4K to some other format similar to *REX, *.LAS, *.AMP, *.RPD, *.REX, and *DAT. Data files that have been transferred to the *DAT format is processed using RES2-DINV software to generate the RES2-D profiles. The step-by-step processing of the field data is presented in Fig. 3.
Data quality check was carried out as soon as the data files transfer is completed. The topography data of the area was checked for correctness before generation of the initial RES2-D model. Data from RES2-D profiles are then processed using RockWorks software to generate and produced a 3-D model. Prior to the production of 3-D model, the recorded field data in excel file format. i.e., (.xlsx) was imported into the software first. The field data files contain data for distances in metres of the survey points (X), spacing between the survey lines (Y), elevation (Z), and resistivity values in Ω-m, and the chargeability in ms. These datasets were therefore processed into Step by step RES2-D and IP data processing flow chart. Adopted from Ref. [37]. solid model of the 3-D generated for the study area.

RES2-D and IP profiles along survey line B1
RES2-D and IP profiles along survey line B1 is presented in Fig. 4, with approximate line orientation in the N 300 • E direction, and a total survey length of 400 m, at inter electrode spacing of 5 m. The subsurface strata resistivity values recorded varied between about 0 Ω-m and 4000 Ω-m. Three major lithologic units were clearly delineated in the interpreted RES2-D data as; (i) the groundwater saturated zone/aquifer zone, (ii) the unconsolidated soil, and (iii) the weathered bedrock comprises mainly of phyllites rock formation. The lowest resistivity, and chargeability values respectively varied from 0 to 100 Ω-m, and 0-30 ms. This unit was interpreted as the groundwater saturated zone, located at about 20-80 m along the survey line, at depth of about 20-80 m. The highest resistivity values of 500-2000 Ω-m, was interpreted as the phyllite bedrock which is located at an approximate depth of about 20-140 m.
The RES2-D and IP profiles results showed delineation of faults with NE-SW orientations at about 210 m, along the horizontal distance of the survey line. Two borehole positions were pinpoints at 80 m, and 330 m respectively, with a view to salvage the community sustainable water needs.

RES2-D and IP profiles along survey line B2
Resistivity and Induced Polarization profiles along survey line B2, is as shown in Fig. 5. The lowest RES2-D value varied from 0 to 100 Ω-m, that was delineated as water saturated zone/aquifer unit. Low chargeability value, i.e., 4-20 ms, was recorded for the zone, at depths of about 30-60 m. The highest resistivity and chargeability values respectively varied from about 100 to 1000 Ω-m, and 50-300 ms. The weathered interpreted phyllite bedrock unit was delineated at horizontal distance of between about 110 and 230 m, along the survey line.
From the RES2-D and IP profiles, the faults zone was delineated along Northeast-Southwest directions at horizontal distance of about 220 m.

RES2-D and IP profiles along survey line B3
The RES2-D and IP profiles along survey line B3 is presented in Fig. 6  Based on the two profiles, the NE-SW trending faults zone was delineated at about 210 m. From the results obtained, borehole drilling location was pinpoint at 260 m along the survey line for the groundwater exploration that would serve the community.

RES2-D and IP profiles along survey line B6
Line B6, i.e., Fig. 9, showed the delineated groundwater saturated/aquifer unit with RES2-D and IP geophysical methods given  The RES2-D and IP geoelectric parameters was able to delineate the NE-SW faults at about 220 m along the horizontal distance. Two proposed borehole locations for groundwater drilling were respectively pinpoint at 210 m, and 300 m along the survey line. The NE-SW trending faults zone was delineated at about 220 m. Based on the results obtained, borehole drilling location was pinpoint for groundwater exploration at 260 m along the survey line with probable depth of about 25-50 m that would alleviate the challenges facing the people in the community.

Hydrogeophysical modelling of the RES2-D and IP results with the borehole well logs
The primary focus of this work is to model subsurface zones saturated with groundwater accumulations for borehole sitting that would serve the community's sustainable hygienic water needs on the basis of the geoelectric parameters recorded in Table 1. Three dimensional (3-D), hydrogeophysical models of the results from RES2-D and IP geoelectric, was constructed to quantify volume of the groundwater occurrences at the study area as presented in Fig. 11 and 12. The models were generated through the combinations of the geoelectric results from all the survey lines at inter-profile distance of 5 m. However, Fig. 12, showed the groundwater potential/ aquifer zone interpreted based on the recorded resistivity values of <125 Ω-m, and chargeability values of between 0 and 30 ms. The units were delineated between 100 and 200 m, for the first segment, and from 300 to 400 m, in the second segment, at varied depths of between 30 and 100 m. Volume of the groundwater accumulations residing within the subsurface reservoir of the study area was then estimated to be about 2.86 Mega CBM.
Two validating boreholes wells were drilled along the horizontal distance of 210 m, on the survey line profile B1, and 260 m, on the survey line profile B7 respectively, i.e., Tables 2 and 3. The detailed borehole log results, (i.e., Appendices A1 and A2), showed good correlations between the resistivity profiles results and the borehole well logs data. Wet silty-gravely-sand zones from the depths of between about 10 and 39 m, and the weathered phyllite bedrock unit was delineated at about 39-97 m, as shown on the first validation borehole. Groundwater yields was extracted until depth to about 97 m. The consequences of groundwater quality deterioration occasion by deeper boreholes in regions closer to coastal areas that could rendered groundwater unsustainable were carefully observed. The geopotential height of the study area, the borehole pumping pressure and the study site distance from the coastal sores provided the needed advantages to overcome challenges of saltwater intrusions that could rendered the groundwater unsuitable for agricultural, domestic, and industrial consumptions.
The top/unconsolidated soil filled up the first lithologic unit from the ground surface down to about 9 m, Though, grains of weathered phyllite rock formation with other rocks such as silty sands were delineated from about 9 m downwards.
The second validating borehole well was drilled at point 260 m, along the survey line profile B7, with a maximum depth of about 55 m. The well log data from the borehole lithologic sequence showed correlated results with that of the resistivity and IP surveys. Depths from between about 10 m and 55 m, below the ground surface, produced wet aquifer zones. Depths of between 18 m and 49 m, was filled with gravel, quartz, and phylite pebbles with increase in sizes as the depth of the borehole increases but it has no effect on the groundwater yield, as it increases as the depth of the borehole increased.

Conclusion
In planning sustainable adaptation strategies for growing population of the state of Selangor, Peninsular Malaysia, the role of groundwater resources in ensuring food security, socio-economic development, and societal wellbeing, is critical in the face of unpredictable climate variables and droughts. Many research proposed measures to correct or minimized the impacts of climate variability and droughts on water resources, e.g. Ref. [25]. However, a forward-looking approach intended to considerably reduce the  Kayode et al. human vulnerability to the future risks of exposures that could threatened socio-economic development, food production and other societal sectors is crucial.
The results generated from the seven RES2-D and IP profiles geoelectric parameters together with the borehole logs data considered, clearly delineated water saturated aquifer units as presented in Table 1, and the models generated in Fig. 11 and 12 which helped in the estimation of groundwater reservoirs. Based on the borehole logs data, the subsurface geology of the study area was shown to be by and large, mostly shales, and siltstones that metamorphosed into phyllite rock units that sustained groundwater aquifers in the area which will help water resources policy management and authority to enable greater resilience and sustainability irrespective of the magnitude of future droughts.
The model generated from our results showed availability of groundwater resources that would meet the demands of the communities irrespective of the future climate and weather scenarios. The RES2-D and IP parameters recorded also showed great promising aquifers yields in the weathered phyllite hard rock formations to meet the diverse demands without fear of experiencing supplies deficits as the demands soars within and outside sustainable livelihoods. In all the seven survey lines executed, the major faults trending along NE-SW directions was delineated. The geoelectric results obtained from the survey lines B4, B6, and B7, produced abundance subsistence groundwater occurrences capable of serving the community's sustainable domestic and agricultural water needs irrespective of the pumping rates with no significant impact on the ecosystems and the environment for future planning and adaptation strategies. The modeled results produced potential bedrock deformations along these three profile lines with deep cracks/ fissures, and weathered bedrock with displaced subsurface hard rocks that formed groundwater aquifers units at depths >50 m.
Though, the interpreted validation borehole well logs data from the two validating monitoring wells provides same rock type at various depths, the differential in their resistivity and chargeability values may not be unconnected to the influence of fractured bedrocks, the degree of weathering, saturated formation water, and the mineral contents on the resistivity and IP of the subsurface rocks, e.g. Ref. [40]. Based on the results from the two-validation borehole well logs data; RES2-D and IP profiles, together with the modeled 3-D generated, it could be therefore concluded that.     -80  20-80  0-100  10-30  80  330-360  20-60  0-100  0-20  330  B2  30-90  30-60  0-100  4-20  70  330-360  30-60  0-100  5-20  345  B3 45  • The groundwater saturated/aquifer zones were clearly delineated as confirmed by the correlations of the results that could leads to food security and economic growth of these communities. • The phyllite outcrop sited at the vicinity of the study area, together with the two validating boreholes well logs, provided evidence of the delineated weathered phyllite bedrock units underlain the area to produce great quantity of subsistence groundwater capable of serving the communities. • The delineated NE-SW trending faults along all the survey lines, could be the conduit water ways to recharge the aquifer units even though hard rock aquifers present small buffer storage. This is significant to the aquifer recovery during prolong future drought episode as the impact will be minimal or no effect. • Saturated groundwater reservoir estimated volume of about 2.86 Mega CBM within the subsurface of the study area, would be a long-lasting solution to the perennial acute water shortages confronting the community during prolong drought seasons as it provides an acceptable guide and realistic approach to planning for sustainable hygienic water supplies by the decision makers.
The success from 3-D hydrogeophysical modeling of groundwater aquifer units with the applications of borehole well logs data, RES2-D and IP profiles, resides on the support of climate-water nexus sustainability, looking from the volume of groundwater produced through the geophysical results, and the two validated borehole well logs data to meet the Beranang, Selangor, Malaysia, daily portable hygienic domestic water usage, irrespective of the prevailing climatic conditions. Although, the consequences of groundwater quality deterioration occasion by deeper boreholes in regions nearer to coastal areas was taken into consideration while drilling the two validated borehole wells. The maximum depth of the first well at < 100 m is much shallower to creating source of concerns on aquifer degradation from saltwater intrusions that could lead to unsustainable groundwater extraction. Long-term sustainable clean and hygienic groundwater implementation across the Peninsular Malaysian states could put an end to acute water shortages that bring about disruptions to foods and industrial production. Our approach is devoid of any consequential impacts from groundwater depletion.
To transform groundwater resources towards achieving improved, and sustainable resilience irrespective of the magnitude of future climatic conditions involved commitment by all sectors and players in the management of water resources. Therefore, it is a call for action by the political office holders, decision makers, public and private sectors practitioners, scientific community, and the society in general to jointly build a sustainable environment for the wellbeing of all. Silty fine sand with some gravels.

Table 3
Validating borehole well log data at point 260 m distance along survey line B7.

Funding statement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement
Data will be made available on request.

Declaration of interest's statement
The authors declare no competing interests.