“Brine Intrusion” – A catastrophe from the Hydrosphere

“Intoxication and kidney diseases” a life-threatening water-borne disease that has plagued Nigerians since time immemorial.

Usually, when one gets affected, pills or treatment is prescribed or in more severe cases, patients are administered in the hospitals.

While those methods have proven effective, not minding the root cause of this disease to tackle, the disease now is on the rise and increase spreading throughout communities due to ignorance, yet a question mark to be answered for this!
With time now! I have been haunted by the question in quote

“To what extent will this be?

And this… was a driving motive that prompted me into carrying out a research work on the root cause of this disease and to evaluate it.

The word “brine” - salty water has been a major cause of the aforementioned life-threatening disease; a diverse problem, affecting and damaging the groundwater system.

This research as it may was carried out in the coastal plains of Ibeno, Akwa Ibom state, Nigeria bordered in the south by the Atlantic ocean, north by Onna, Esit-Eket, and Eket and west by Eastern-Obolo Local Government Area.

In the course of this study, two methods where used in evaluating the brine intrusion; Geophysical and Geochemical methods

Materials used for this study were as follows: Steel electrodes, Resistivity meter, Measuring tapes, Labeled plastic sample bottles, Distilled water, Cooler box, Ice blocks, Spanners, Hammers were used in driving electrodes in to ground, 2 pairs current and potential of cable reels, 12V power source, etc., the global positioning system (GPS) was used to measure geographical coordinates of the point of interest.

With the geochemical method, water samples were collected from hand dug wells, boreholes and from the beach as a reference point with the use of 120ml plastic bottles pretreated and rinsed with distilled water of each labeled as A, for cation, B, for anion and C, for physical parameter.

These samples were preserved and subjected for the laboratory analysis which included analyses for the determination for the temperature of the water, total dissolved solids (TDS), and electrical conductivity (EC), sodium, potassium, magnesium, barium, calcium, chloride, fluoride, carbonates, bicarbonates, sulphate, nitrate, nitrite, iron, and manganese.

With the geophysical method, the electrical resistivity survey technique involving Vertical Electrical Sounding (VES) and Electrical Resistivity Tomography (ERT) methods were used.

In the field, For the VES technique, the data were acquired along the traverses. The Schlumberger electrode configuration was used in acquiring the VES data at station spread across at the survey sites. The station electrode was planted and the two measuring tapes were hooked to it and spread out in opposite direction. Current and potential electrodes were planted at their required positions from the center. The procedure was carried out by connecting both the potential and current cables to the electrodes and the equipment.

The power unit was connected and the equipment was switched on by turning the power button to the ‘ON’ mode. The resistant values displayed by the instrument for a particular electrode separation were copied.

For the electrical resistivity tomography which deploys the combination of Vertical Electrical Sounding (VES) and the Horizontal Resistivity Profiling (HRP). The Wenner electrode array was adopted which consists of four collinear equally spaced electrodes.
The outer two are usually the current electrodes while the inner two are potential electrodes in which the electrodes were regularly and equally spaced with a minimum separation of 5m and maximum spread of 150m.

The survey line was measured for the survey down perpendicular to the coastal line; hammers were used to drive the electrodes into the ground at intervals of 5m along the profile.

The power source was connected to the resistivity meter and the meter was put ON and prepared to record while the cables from the meter were also connected to the first four electrodes along the survey line for profiling, profiling done involved moving the whole array with constant spacing of 5m that is from 5m spacing and then expanded to 10m spacing, 15m, 20m, 25m, and 30m along the line of survey till the 150m spread was exhausted.

Readings on the resistivity meter were limited to shallow depth as there was no further increase of the electrodes. This was basically because of the nature of the beach. Resistance readings gotten from the resistivity meter were written down at every measuring point.

Problems encountered were the difficulty in moving cables from one point to another, and that made it a hectic experience though it was fun too.

After the field survey, the measured resistances were then converted to apparent resistivity values with the help of the geometrical factor of Wenner electrode array (2πaR) where R is the (resistance measured in the field otherwise known as the ground resistance) and “a” ( is the spacing between electrodes).

The data acquired were used to create a model layer curve for the VES using the WINRESIST software and also to create 2D Electrical Resistivity tomography for the ERT using the RES2DINV software program. The pseudo-section reflects the true resistivity distribution along the profile and therefore was used as a base for qualitative interpretation.

From the result acquired below, the 2D electrical resistivity tomography profiles measured with lateral extent of 200m, 200m, 135m, and 150m for the configuration and vertical extent ranging from 1.25 to 19.8m, 1.25 to 19.8m, 1.25 to 15.9m, 1.25 to 12.4m respectively showed that there was an intrusion of brine in this area.

From the three (3) modeled VES data below, it was also observed that two (2) VES point have five (5) layers, and (1) has four layers with, KH, HKH, and KHK curve types ie VES 1-3 which also showed an intrusion of brine in the area.

But the result from the geochemical method showed that there was no intrusion of brine in the area with the results of the physical and chemical analysis falling within the standards for drinking water as compared with the World Health Organizations standards.

In conclusion, Combined geophysical and geochemical investigations have been used to study the subsurface geology and to delineate the extent of the incursion of brine into coastal alluvial aquifer units within the study area.

The inferred lithologies from the VES and 2D ERT imaging shows an high level of incursion of brine in the area, perhaps due to increase in groundwater extraction; hydraulic connection between freshwater and seawater thereby allowing saltwater to push further inland beneath the freshwater.

Comparing the two methods (geophysical and geochemical methods) used in the research work, there is a variance in the result obtained were one delineate brine intrusion in the study area and the other does not.

The variances in the result obtained maybe as a result of the depth in penetration and lateral extent of the methods used. For the hand dug wells and borehole water samples gotten, the water was extracted at a shallow depth which is not compared to that of the geophysical method. With this result, it can be concluded that the two methods, are effective for brine evaluation as one will serve a check for another.

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REFERENCES

Adeoti L., Alile O.M. and Uchegbulam, 2010. Geophysical investigation of saline water intrusion into freshwater aquifers: A case study of Oniru, Lagos State, Sci. Res. and Essays. 5 (3): 248-259.

Adepelumi, A.A., Ako, B.D., Ajayi, T.R., Afolabi, O., Omotoso, E.J. 2008.Delineation of saltwater intrusion into the freshwater aquifer of Lekki Peninsula, Lagos, Niger. J. Environ. Geol. 56(5): 927 – 933.

Adewuyi, G. O., Oputu, O. U., Opasina, M. A., 2010. Assessment of Groundwater Quality and Saline Intrusions in coastal Aquifers of Lagos metropolis, Nigeria. J. Water Resources and Protection (Jwarp), 2: 849 – 853.

Barlow, P M. and Reichard, E. G. 2010. Saltwater intrusion in coastal regions of North America. USGS.

Frohlich, R.K., Urish, D.W., Fuller, J., Reilly, M.O. 1994.Use of Geoelectrical Method in Groundwater Pollution Surveys in a Coastal Environment. Journal of Applied Geophysics, 32 (2-3): 139-154.

Oteri AU (1977). Application of surface geophysics in hydrogeology. University of London and Diploma Imperial College, London, p. 94.