A preliminary groundwater investigation using geoelectrical resistivity survey in the Bengkalis Island, Indonesia

Groundwater resources are one of the natural resources that need to be preserved. The main problem of groundwater in coastal areas is the intrusion of seawater into the groundwater aquifer. The problem will be even more complex when the land is covered by peat soil. The purpose of this study was to investigate the potential of groundwater in the western part of Bengkalis Island. Geoelectrical resistivity data was taken with the Schumberger configuration with the increase in the electrode distance being varied to obtain accurate measurements. Groundwater samples were measured to obtain the physical character of the groundwater. The results show that groundwater close to the shoreline has been contaminated by the presence of seawater intrusion. However, in areas that are a bit far from the shoreline, the shallow aquifer groundwater conditions are still not contaminated by seawater. It can be concluded that the presence of seawater in the aquifer is caused by the trapping of seawater in the aquifer during sediment deposition in the past.


Introduction
Fresh water sources obtained from the ground are highly sought after for the needs of daily life, especially in rural areas [1].Groundwater in coastal areas, namely areas adjacent to the coastline, has generally been contaminated by the presence of seawater into the groundwater aquifer [2,3,4,5].The presence of seawater can be caused by an intrusion caused by the hydrostatic pressure of groundwater in the aquifer is already smaller than the pressure exerted by seawater intrusion, so the decrease in hydrostatic pressure causes the entry of seawater into the groundwater aquifer [6,7].Apart from this decrease in hydrostatic pressure, the presence of seawater can also be caused because the seawater has been trapped since the formation of sediments in the past in the geological time scale.
In addition to seawater intrusion into groundwater aquifers, a problem that often arises in coastal areas, especially on the eastern part of Sumatra island, is the presence of peat in the coastal lowlands [8,9,10].This peat extends widely covering almost all parts of the basin in the eastern part of Sumatra [8,11].The presence of peat in coastal areas will have a more complex impact on groundwater sources [12].Apart from the presence of seawater intrusion, the presence of this peat makes groundwater sources in shallow aquifers brown due to the influence of the peat [13].
Geophysical methods can be used to explore groundwater such as the ground penetrating radar [14,15] method, the very low frequency [16,17] method and the geoelectrical resistivity method [18,19,20,21].The geoelectrical resistivity method is very popularly used in areas dominated by sedimentary deposits [22], while the ground penetrating radar method and the very low frequency method are very dominantly used in areas with hard bedrock [23] and in areas with fractured igneous rocks.
The geoelectrical resistivity method has been reported successfully in its use to explore groundwater.In addition to exploring groundwater potential, the geoelectrical resistivity method has also been successful in determining the presence of nitrate in groundwater around oil palm plantations [24,25,26].Geoelectrical resistivity method is also used to detect the presence of lichate in landfill areas, and also to map the presence of heavy metal contamination in shallow aquifers [20].
In this study, we will discuss the use of 1D geoelectrical resistivity for groundwater potential in the western part of Bengkalis Island.This geoelectrical resistivity method is integrated with the method of measuring the physical character of groundwater.Thus the interpretation of geoelectrical resistivity data can be carried out properly referring to the data support for the physical character of groundwater.

Methodology
This research was conducted in the western part of Bengkalis Island, Indonesia.Figure 1 is a map of the research location taken from Google Earth. Figure 1 shows a yellow line and a blue circle.The yellow line is the location where the geoelectrical resistivity survey was carried out.And the blue circle is the location for groundwater extraction.The research area is mostly covered by the presence of peat soil on the soil surface.However, in some places, the ground surface is covered by mud soil and other marine sediment deposits.
Geoelectrical resistivity data were taken using the Schlumberger arrangement.Where the electrode distance is varied from 1 meter to an increment of 5 meters.The resistivity data is then formatted to be entered in the resistivity data processing software.In this research, Progress software is used which is based on inversion of block definite value.Groundwater is taken from the location of existing community wells, then measured for physical characteristics such as temperature, pH, conductivity, salinity and total dissolved solids.

Results and Discussion
Table 1 shows the results of measuring the physical characteristics of groundwater in the study area.The location of data collection can be seen in Figure 1.In the Table 1, it can be seen that the pH content of groundwater is from 4.9 to 6.39.The highest pH content is at the sample location B-06.However, it can be seen that the pH of the water in the study area is below the normal pH, which is pH 7. The cause of this low pH is the carbon dioxide or CO2 dissolved in the water which often occurs recently is the result of high levels of CO2 in the atmosphere, resulting in an increase in the concentration of hydrogen ions.(H+) which can lower or lower the pH of the water.The process of decomposition of organic matter or better known as the decomposition of organic matter in water can affect the decrease in the pH of the water.Organic matter in the form of a set of organic compounds from living things contains the element carbon (C).Organic compounds are usually unstable and easily oxidized to carbon dioxide (CO2) and water (H2O).The research area is a peat area, so the high organic matter in the soil causes the main cause of the low pH of the groundwater.The results of measurements of salinity, conductivity and total dissolved solids show that this groundwater contains seawater intrusion, where high conductivity indicates a high content of negative ions originating from the chloride content of seawater.This is supported by the presence of salinity in the water sample.This is also supported by total dissolved solid data which shows a high mineral content in the water Figure 2 shows the selected geoelectrical resistivity data from the measurement of geoelectrical resistivity in the study area.Figure 2 also shows the peat condition at the sites where the data were surveyed.The location of geoelectrical resistivity data collection can be seen in Figure 1.From Figure 2 it can be seen that the resistivity data shows that there is four layer of the subsurface detected.The first is the peat zone where the resistivity is around 10 ohm.m.This zone is expected contaminated by the seawater.This zone is the zone of the presence of seawater intrusion into the shallow aquifer.Thus, it can be seen that all geoelectrical resistivity data indicate the presence of seawater, but at the location L-03, the indication of seawater content is less than the other line survey.There is an increase in resistivity which indicates that the seawater content in the aquifer is slightly less than the seawater content at the other locations that is near to the coast line.

Conclusion
This research on groundwater potential investigation has been carried out in Bengkalis Island successfully.Groundwater potential is very adequate in the research area.This groundwater potential is indicated by the presence of an aquifer zone in the geoelectrical resistivity data.However, it is very unfortunate that the groundwater potential on Bengkalis Island has generally been intruded by seawater content.In some locations, the seawater content shows a fairly large number, but in the middle part of the research area it can be seen that the seawater content has decreased slightly.This seawater content in the aquifer is most likely the presence of seawater since the sedimentation process took place on a geological time scale.

Figure 1 .
Figure 1.The map showing the location of study area (source: Google Earth).

Figure 2 .
Figure 2. The geoelectrical resistivity data and the site survey showing the peat land condition.

Table 1 .
Physical characteristic of groundwater in the research area