A Quantitative interpretation of salt water mixture in the shallow aquifer through the geoelectrical resistivity data

The main problem of groundwater in coastal areas is the intrusion of saltwater into freshwater aquifers. This study aims to predict the amount of saltwater content in aquifers polluted by intro-sea saltwater. In this study, an attempt is made to provide a direct correlation approach for resistivity measurements with the physical characteristics of groundwater. A 2D resistivity survey with the Wenner configuration have been carried out in the coastal area. The geoelectrical resistivity survey is about 3 km from the shoreline. The chemical measurement was done on freshwater, seawater and fresh-seawater mixture, which the mixture was consisted of 25%, 50% and 75% of seawater content. The results showed that the resistivity below 5 ohm.m indicated the presence of seawater with a concentration of more than 50% mixed with fresh water. In general, it can be seen that the decrease in resistivity appears to decrease exponentially with increasing seawater content in the aquifer.


Introduction
One of the groundwater problems faced in coastal areas are the intrusion of seawater into fresh aquifers [1].Fresh water sources in coastal areas in some places can no longer be used for people's daily consumption because the taste of fresh water has started to taste a bit brackish and even tastes salty in some places [2,3].In addition to the condition of groundwater that has been contaminated by seawater intrusion, in several places in this research area, the soil surface is also covered with peat where the thickness of this peat varies from a few centimeters to several meters.This condition adds to the severity of the problem of water sources in the study area because shallow water sources are also contaminated by the presence of peat [4].
Geophysics methods, especially geoelectrical resistivity methods have often been used in characterizing subsurface soils [5,6,7,8].Geoelectrical resistivity method is also used to detect the presence of heavy metal in shallow water aquifers [9].Another study also reported on the use of geoelectrical resistivity to detect the presence of leachate in landfill areas [10].Geoelectrical resistivity has also been successfully used to image the presence of geothermal sources [11], detecting the slope of the sliding zone of the landslide zone [12].
Several research results provide an explanation that the resistivity value of a material also depends on the fluid that fills the pores of the material.The fluid type content in the soil pores will give the overall resistivity reading depending on the matrix that makes up the soil.Sandy soil if the pores are filled with fresh water, the results will be very different if it is filled with a fluid with good electrical conductivity such as sea water [13].Thus the results of this resistivity measurement will provide information on the physical character of the combination of matrix and fluid in the soil pores.
In this study, it will be studied how to obtain quantitative data on the amount of seawater that has been mixed with fresh water in aquifers in coastal areas.Thus, from the geoelectrical resistivity data measured at the ground surface, it can be predicted how large the percentage of seawater content is mixed in the groundwater.

Methodology
The map of the study area is given in the Figure 1.In the figure, the line coloured yellow indicating the geoelectrical resistivity survey location, and the blue circle is indicating the groundwater sample location.In this research, the groundwater samples were taken from existing wells that used by community for their daily activity.The groundwater sample then was measured their physical characteristics such as temperature, pH, total dissolved solid, salinity, and conductivity.Besides, the depth of well was obtained from direct measurement.
Seawater samples were taken from the nearest beach, then the content of Chloride, Sulphate, Ca, K, Na and Mg of seawater was analyzed using ion chromatography (IC) and inductively coupled plasma (ICP).Furthermore, the seawater is mixed with fresh water obtained from groundwater samples taken from an aquifer about 10 km from the coastline, where this groundwater sample has no indication of the presence of seawater.Seawater and fresh water are mixed with a composition of 25%, 50% and 75%, and the resistivity of the water mixture is measured.
Geoelectrical resistivity data was taken using a resistivity meter with Wenner configuration.The data obtained were inverted using Res2Dinv [14].The resistivity value of the measurement of the mixture of seawater and fresh water is used as a guide for quantitative interpretation of the percentage of seawater mixed in the aquifer with fresh water.

Results and Discussion
The content of Cl, SO4, K, Ca, Na and Mg seawater for each percentage of the amount of seawater are given in Table 1.The standard anion and cation content obtained from the reference Hounslow [15] are shown in the bottom row of Table 1.From this cation data, it can be seen that the anion and cation content of seawater from the study area has a concentration that is almost the same as the cation and anion content obtained from the reference.In general, it can be concluded that an increase in the  The results of the resistivity measurements of saturated soil with 0% seawater content and mixed seawater content of 25%, 50 and 75% and 100% seawater resistivity can be seen in Table 2. Obviously, in Table 2 it can be shown that in general the peat soil sample has a slightly higher resistivity compared to the soil sample for the same fluid content at 0% seawater content.However, for the seawater content of 25%, it is seen that the resistivity decreases drastically.This is because the negative ion content of seawater makes it easy for electricity to flow. Figure 2 is the result of geoelectrical resistivity survey modeling at the research location which is about 3 km from the coastline.At the location of data collection, the surface soil condition is generally sandy soil.Meanwhile, at the end of the survey, the soil was a bit muddy in a slightly hardened state.It can be seen in the geoelectrical resistivity data that the low resistivity is at a depth of approximately 10 m to 22 meters above ground level.From this inversion data, it can be seen that the resistivity price ranges from 30 ohm.m and decreases towards the Northeast to around 10 ohm.m.It is very visible that there is an increase in seawater content, namely from conditions without seawater to an increase of about 25% of seawater content at the end of the survey track.This interpretation is based on what is shown in Table 2, where the resistivity decreases drastically from 0% to 25% of seawater content.

Conclusion
This research has shown a valuable approach to predict the percentage of seawater content mixed into fresh water in groundwater aquifers.The prediction is based on the correlation between seawater content and groundwater resistivity obtained from direct resistivity measurement on the surface.Increasing seawater content in water samples shows a drastic decrease in resistivity relationship.Through this research, in the Dumai area, it can be seen that the percentage of seawater content in groundwater generally increases with the closer the location to the coastline.
seawater results in an increase in the content of each cation and anion in the mixture.

Table 1 .
Concentration of seawater and seawater-fresh chemical content of water mixture.

Table 2 .
Resistivity value obtained from salt-fresh water mixture