Like many of the tropical islands, the population of Andaman and Nicobar Islands, though not directly, relies predominantly upon rain water harvesting to quench their need and also depends on the groundwater sources. In the background of climate change, severity of hydrological cycle is much anticipated which may cause more extreme and unusual precipitation. It is quite essential to have other alternatives. Accordingly, groundwater could be exploited as a potential alternative. The present study intends to find out the potential groundwater source and estimate aquifer parameters in Kodiyaghat (KD) and Burmanallah (BN). As these areas are composed of very hard rock, Wenner-Schlumberger array has been used to carry out a 2D Electrical Resistivity Tomography survey to find out the fracture zone as well as to delineate the aquifer. KD and BN show maximum resistivity of 25,416 Ωm and 5985 Ωm indicate very hard rock terrain. Similarly, the minimum values of resistivity (21.6 Ωm and 30.4 Ωm) were observed at KD and BN define the presence of freshwater aquifers respectively. The aquifer identified was found to be at a depth of 5 m to 19.9 m at KD and 2.5 m to 20 m at BN. The calculated Hydraulic conductivity (14.85 m/day and 30.14 m/day), transmissivity (86.25 m2/day and 271.27 m 2/day) and porosity (28.7% and 31.24%) values at KD and BN confirmed that, the located aquifer was of fresh ground water quality and can be utilized for drinking and house hold purposes. According to the results, almost 70% of the study area is hard rock terrain and 30% comes under potential aquifer zone. The results also show that, both the areas were characterized by Horst and Graben topography and suggest possible groundwater sources for future exploration.
Aquifers in fractured rocks are generally considered of minor importance compared to those in primary porous media, on which the attention has mainly been focused up until now. In fact, the amount of ground water available in fractures is generally limited, at least in arid and semi-arid regions. Nevertheless, this type of aquifer is a primary source of water for man in vast area throughout the world, where aquifers with primary porosity are practically non-existent and surface waters are ephemeral [
The data presented here were collected at Burmanallah (N.11˚34’-10'' lat.: E.92˚43'- 42.5'' long) and Kodiaghat (N.11˚31'-53.6'' lat.: E.92˚43'-22.2'' long) respectively (
Profile Orientation:
The profile orientations were done with respect to the fault trending and the weak zone indicators like streams, fractures etc. In Kodiyaghat the stream orientation was East-West, so the 1st profile was made perpendicular to the stream, followed by second profile trending North-South. Whereas, in Burmanallah the stream oriented North-South direction and the profile was made perpendicular to it, South-East to North-West. Some of the common features of these regions like presence of perennial stream lines, subsided landmass and some open water sources have contributed to the ground trothing and profile orientations.
Geological Settings:
Ophiolite occurrences are reported from almost all the major Andaman Islands, but are well-exposed in the south Andaman Island [
Among the various geophysical methods of groundwater investigation, the Electrical Resistivity Method has the widely applied for groundwater exploration [
Three 2-D electrical resistivity tomography (ERT) surveys were conducted in Kodiaghat and Burmanallah areas. By using CRM500 system, the surveys used Wenner-Schlumberger array with 10 m and 5 m electrode spacing. Depth wise resistivity values are measured and calculated for Apparent Resistivity and interpreted using RES2DINV software on an inexpensive microcomputer.
Wenner-Schlumberger array is a new hybrid method used on a system with the electrodes arranged with constant spacing (
Following steps were followed to estimate Aquifer Parameters [
Water sample were collected from the respective study area and analysed.
TDS (Total Dissolved Solvents)
TDS (ppm) = (Alkalinity + hardness + chloride) × 1.2
Conductivity of Water
Conductivity of water (μS/cm) = TDS (ppm)/0.64
Resistivity of Water
Resistivity of water (Ωm) = 104/conductivity of water (μS/cm)
Formation Factor FF
Formation factor = aquifer resistivity (Ωm)/resistivity of water (Ωm)
Porosity (%) (Fracture or Secondary Porosity)
Porosity = (1/formation factor)1/2
Aquifer Conductivity
Aquifer conductivity (μS/cm) = 104/aquifer resistivity (Ωm)
Hydraulic Conductivity (m/dy)
Hydraulic conductivity (K) = cross sectional area of aquifer (m2)/aquifer conductivity (μS/cm)
Transmissivity (m2/dy)
Transmissivity = hydraulic conductivity (K) × aquifer thickness (h)
Profile 1―Kodiyaghat (KD)
Array Used: Wenner-Schlumberger, Electrode Spacing: 10 m, Depth Covered: 34.6 m.
The profile 1st Wenner-Schlumberger trends East to West direction to a length of 320 m (
The profile is characterized by:
1) Horst and Graben topography holding basin at both sides (
2) Deep vertical fracture zones with water bearing property along with hard rock intrusion towards the right side about 10 m width and 10 m length.
3) Two aquifers are marked on the extreme left side (60 m to 70 m electrode spacing) and middle (170 m to 180 m electrode spacing) of the profile.
Content | Resistivity Ranges (Ωm) | Depth Range (m) |
---|---|---|
Aquifer | 29.5 - 204 | 7.75 -19.9 |
Weathered Rocks | 535 - 1404 | 5 - 21 |
Hard Rocks | 3686 - 25,416 | 7.75 - 34.6 |
Profile 2―Kodiaghat (KD)
Array used: Wenner-Schlumberger, Electrode Spacing: 5 m, Depth Covered: 30 m.
The profile 2nd Wenner-Schlumberger trends North to South direction to a length of 160 m. Resistivity values from water layers show the variation from 21.6 to 14,262 Ωm (
The profile is characterized by:
1) Successive Anticline and Syncline folds respectively cover the bedrock profile of pseudo section (
Content | Resistivity Ranges (Ωm) | Depth Range (m) |
---|---|---|
Aquifer | 21.6 - 138 | 5 - 10 |
Weathered Rocks | 349 - 883 | 10 - 14 |
Hard Rocks | 2232 - 14,262 | 15 -30 |
2) Horst and Graben topography holding basin on both sides at 60 m to 100 m length.
3) Deep vertical fracture marked by hard rock intrusion towards the left side, almost above the anticline fold about 10m width and 5m length.
4) Small vertical fracture zones with water bearing property along with hard rock intrusion towards the middle side.
Profile 3―Burmanallah (BN)
Array Used: Wenner-Schlumberger, Electrode Spacing: 10 m, Depth Covered: 35 m.
The profile 3rd Wenner-Schlumberger trends North-West to South-East direction to a length of 320 m with 32 electrodes spacing 10 m (
The profile is characterized by:
1) Horst and Graben topography characterized by tensional cracks resulting basins and rifting.
2) Open fresh water sources abundance almost from 15 m - 35 m, 70 m - 100 m, 130 m - 200 m, 210 m - 240 m, 260 m - 290 m electrode spacing (
3) Small vertical fractures and dipping in the bed rock strata.
The 2D ERT of study area shows maximum resistivity of 25,416 Ωm and 5985 Ωm indicate very hard rock terrain. Similarly, the minimum values of aquifer resistivity (21.6 Ωm and 30.4 Ωm) were observed at KD and BN define the presence of freshwater respectively. As the resistivity of fresh Groundwater water varies between 10 Ωm to 100 Ωm [
In accordance to parametrical analysis of identified aquifer, porosity value varies between 28.7% to 31.24%, which signifies the fresh water porosity zone for hard rock environments. According to Davis [
Content | Resistivity Ranges (Ωm) | Depth Range (m) |
---|---|---|
Aquifer | 30.4 - 137 | 2.5 - 19.9 |
Weathered Rocks | 292 - 1323 | 5 - 20 |
Hard Rocks | 2813 - 5984 | 13.5 - 34.6 |
Profile | Total Dissolved Solvents (ppm) | Conductivity of Water (μS/cm) | Resistivity of Water (Ωm) | Aquifer Thickness (m) | Formation Factor (FF) | Secondary Porosity (%) | Hydraulic Conductivity K (m/dy) | Transmissivity (m2/dy) |
---|---|---|---|---|---|---|---|---|
Prof-1 | 542.12 ± 3.14 | 858 ± 4.98 | 11.655 ± 0.06 | 10.5 ± 0.69 | 12.07 ± 3.43 | 28.7 ± 4.57 | 14.84 ± 3.59 | 155.82 ± 42.53 |
Prof-2 | 542.12 ± 3.63 | 858 ± 5.76 | 11.655 ± 0.07 | 5 ± 1.65 | 11.84 ± 1.43 | 29.06 ± 2.50 | 17.25 ± 3.02 | 86.25 ± 11.89 |
Prof-3 | 624 ± 13.72 | 975 ± 21.42 | 10.25 ± 0.22 | 9 ± 1.31 | 10.24 ± 3.47 | 31.24 ± 7.59 | 30.14 ± 3.35 | 271.27 ± 30.62 |
Further analysis, including permeability, production yield, water quality etc. has to be done to dig a bore well and extract the potential fresh water for drinking purpose.
Potential groundwater sources from secondary porosities in hard rocks were identified and located in the respective study areas using 2D Electrical resistivity technique. Some of the important conclusions are:
➢ The analysis of the pseudo section along the profile one to three clearly shows 3 broader divisions viz, aquifers (fresh water zone), hard rock and very hard rock region.
➢ Structural and tectonic factors (faults, fractures) play a vital role in locating potential groundwater sources and its movement. Overburden soil thickness is comparatively less so that ground water in this terrain is localized to fracture zone & rifted basins with the presence of Horst & Graben topography. The horst and graben topography that holds the basins is found to be a very good source for ground water. These features consolidate the fact that the study area belongs to a tectonically active region.
➢ The estimation of aquifer parameters has strengthened the evidences for locating fresh groundwater sources in the study areas. Favourable porosity values along with hydraulic conductivity and transmissivity values give appropriate groundwater source location and thereby, techniques for its extraction.
➢ The Profile 1 is very suitable for digging open wells and drilling Bore wells because the area comprises confined aquifer in addition to unconfined aquifers and streams. Profile 3 and 2 are appropriate location for open wells and reservoirs of fresh groundwater as the aquifer thickness is less compared to profile 1 and very few confined aquifers.
➢ In the hard rock area, it is difficult to locate the availability of ground water resources. Rock characteristics in the area show distribution of Ophiolite suite, mainly Basalt. Fractures and overflowing streamlines are the main indicators of groundwater in the hard rock environments. According to the results, almost 70% of the study area is hard rock terrain and 30% comes under potential aquifer. The application of electrical resistivity technique has helped to delineate the fractures and aquifer zones in the study area without making bore hole and conducting pumping tests, which are meant to be very expensive.
➢ The extracting groundwater can be supply for the house hold as well as agriculture purpose at the time of The Time of Seasonal Rainfall Failure.
➢ The 2D Electrical Resistivity Imaging technique, thus is a powerful tool for shallow subsurface groundwater investigation study in the hard rock environments.
The following recommendation can be inferred from the results and conclusions of the study:
➢ The 2D Electrical Resistivity technique can be recommended for groundwater exploration and study of aquifers characteristic in hard rock environments because it is easy to operate, low survey cost and the capability to distinguish between the saline and fresh water zones.
➢ Open wells, bore wells and reservoirs can be constructed in the suitable aquifer zones with reference to depth variations and type of aquifers present.
➢ Augmentation of groundwater resources is necessary where surface storage structures are inadequate to meet the demand or when annual exploitation of resources exceeds its replenishment in a given area.
➢ To maintain the sustainability of the fresh water bearing aquifer zones, large scale rain water harvesting can be advised to manage and control the groundwater resources for future.
➢ Hydro chemical study can be carried out in the area to determine the water quality for both municipal and industrial uses.
➢ Other advanced imaging techniques like Vertical Electrical Sounding (VES), Azimuthal Square array, Cross hole electrical tomography can be used in the area to validate this result.
The author would like to thanks Pondicherry University for providing the instrument and tools to this research work. Not forgotten, to Maulana Azad National Fellowship of Maniruzzaman SK for financial support. We would like to express our heartfelt thanks to Mr. Ravi Ranjan Kumar and Raj Kiran Lakra for helping us to improve the quality of manuscript. Mr. Gulam Rasool Bhatt especially recommended several modifications that tightened and strengthened the interpretations presented in this paper.
Sk, M., Ramanujam, N., Champoil, V., Biswas, S.K., Rasool, Q.A. and Ojha, C. (2018) Identification of Groundwater in Hard Rock Terrain Using 2D Electrical Resistivity Tomography Imaging Technique: Securing Water Scarcity at the Time of Seasonal Rainfall Failure, South Andaman. International Journal of Geosciences, 9, 59-70. https://doi.org/10.4236/ijg.2018.91004