Groundwater under the Deccan Basalts in Nagpur District

A large part of Nagpur district is covered by the Deccan traps, remnants of volcanism that spewed lava, more than 65 million years ago. The lava spread horizontally and hardened into basaltic rocks and underwent weathering and sedimentation before the next bubbling up of lava that put another thick coat of basalt on top, creating the huge staircase-like appearance (or trapps, as they are called in Swedish).

To get to the groundwater under the hard unfriendly layers of basalt is, by no means, an easy task – trial and error is too costly. But not to be able to delineate groundwater aquifers is even more disastrous for the burgeoning populations in the region. 

That is why scientists at the CSIR-NGRI started their groundwater explorations there.

Spatial pattern of lineaments, drainage pattern and vegetation derived from satellite imageries, surface topography and land use land cover provide initial indication of groundwater prospecting zone. The researchers took these into account as control points in the hydrogeology of the area covering 372 square kilometres of their study.

There are several villages in the area. And farmers there dig large diameter wells in the hope of striking groundwater. If the basalt is massive with no vesicles or have vesicles that that are filled by minerals it cannot hold water. Those with interconnected vesicles can contain water. So in some villages, people drill holes horizontally, deep inside the wells, hoping to tap the groundwater from water-filled vesicles in the basalt.

 

Rock samples of basalts collected during field work (a) basalt with isolated vesicles, (b) basalt with interconnected vesicles, (c) basalt with secondary intrusions, (d) massive basalt

The water yield data from these existing wells as well as the data of electrical logging in the exploratory bore wells drilled in the region by the Central Groundwater Board, will provide opportunities to test the theoretical and experimental extrapolations on aquifers under the basalts.

Technologies to locate groundwater

To locate groundwater, there are many different geophysical techniques, each with its own strengths and weaknesses. The researchers decided to combine all of them to get a clear and comprehensive idea about groundwater locations and potential yield from the aquifers.

Vertical electrical sounding using current electrodes separated by about 500 metres and measuring resistivity provide clues on the nature of the underlying rocks. The resistivities of vesicular basalt, of the massive basalts, and of the underlying Gondwana formation are all different and the signals received can distinguish aquifers with high transmissivity. The researchers collected data from more than 100 sites in the study area.

Working principle of the vertical electrical sounding method

The ground transient electromagnetic method measures the penetration of transient electromagnetic pulses and their diffusion in time, to provide clues about the conductivity of the structures that lie underneath. When the electric current in the transmitting loop is switched off abruptly, it generates eddy currents in the underlying conducting structures. The eddy current induces a secondary magnetic field. The propagation of this field depends on the conductivity distribution in the subsurface structures. The conductivity of water is much higher than that of the basalt formation. The depth to which measurements can be made depends on the magnetic loops used as antennae for transmission and reception. The researchers used 40 metre loops, to peep into the properties of the ground to depths of more than a hundred metres, and generated data from more than 40 locations.

Electrical resistivity tomography is yet another advanced method of two-dimension mapping. It’s a multi-electrode resistivity imaging method where, at a time, it measures multiple electrodes in the ground in a line, apply a measured DC current to the outer two electrodes, and measure the potential difference between the inner two electrodes. The resistivity of the subsurface structures can be calculated using Ohm’s Law and geometric factor. The researchers collected 17 tomographic profiles along a linear distance of more than 14 kilometres. The data enabled the creation of seventeen two-dimensional images of the resistivity under the ground so that scientists can extrapolate interconnections of aquifers within the region.

Electrical resistivity tomography image near Gumthala village

The helicopter‑borne transient electromagnetic method uses a rigid boom that carries a large transmitter loop and a small receiver. High-frequency signals captured early provide clues about the shallower parts of the conducting subsurface and the low-frequency signals captured later provide clues about the deeper parts. The researchers captured the data for about 6 km over the same 14 kilometre line measured with electrical resistivity tomography, for comparison and as evidence to support their interpolations between the 17 two dimensional tomographic images.

All these data gathered need some empirical anchoring. The data collected from sixteen exploratory bore wells which penetrated the layers of basalt to reach the sandstones and shale of the Gondwana underneath provided the anchoring. The data contained measurements from three different instruments. The tool that is lowered into the borehole contained an instrument for electrical logging. The spontaneous potential between the electrode in the borewell and an electrode at the surface is continuously recorded as the electrode in the borewell is brought up. Layers of clay can be easily detected as low potential regions.

The same tool can also carry an instrument for measuring resistivity. The researchers used the electrical logging data. In electrical logging, the electrodes with a 16-inch separation for resistivity changes at shorter lengths and electrodes with a 64-inch separation for resistivity at longer lengths are used. Besides giving clues about the surrounding lithospheric structures at various depths, these measurements can give clues about the water quality since the resistivity of freshwater is much higher than that of saline water.

A third type of instrument used in the borehole was a gamma counter. When traversing the Gondwana shale, the instrument will record higher gamma radiation, making it easy to distinguish.

ERT field survey in hot environmental conditions (~48°-50°C May-2013) in Nagpur district

Synthesis of data to map aquifers

From these borewell logs, it was clear that the western parts of the study area had thicker basalt. By integrating the data from different sources, the thickness of the basalt flows in the south eastern part was found to be only about 20 metres. A thickness of more than 400 metres towards the north western part indicated that the basalt was more massive, with low amounts of vesicles. And this reduced the aquifer potential there, confining water to only the weathered and fractured parts of the massive basalt.

Aquifer potential was higher in the northeast due to the further thinning and fracturing of the basalt. The porous and permeable Gondwana sandstone there increased the potential for aquifers. 

In the middle part, the vesicles in the basalt are mostly filled with minerals, but the fractures hold some water. The southern part had weathered and fractured basalt that can hold water.

Map showing the spatial variation of potential aquifer systems, based on lithology

All the data from borewell logs were useful and necessary to correlate with the vertical electrical sounding and electrical resistivity tomography. There was wide variation in resistivity measured by different geophysical methods for the lithological units. The scientists started comparing and contrasting the results from each. For example, below 100 metres, the vertical electrical sounding failed to detect clay deposits if they were less than 5 metres thick. The measurements from the borewell logs were clearly able to demarcate these layers. The electrical log could also distinguish separate layers of basalt. The ground transient electromagnetic method was unable to characterise the top massive basalt, but could demarcate thick layers of red and green clay far below.

Validation of results from vertical electrical sounding with bore hole lithologs at Raulgaon village

In general, the lineaments and the zones between the layers of traps (intertrappean beds) were found more potential for aquifers. The two dimensional profiles generated from the electrical resistivity tomography data treated with the inverse model highlighted the pattern of aquifers along the 14 kilometre stretch. The water level was at shallow depths of 4–5 metres below ground level in the south. It deepened to 18 metres below ground towards the north. This suggested groundwater flow from south to north.

The helicopter‑borne transient electromagnetic method showed that, in most locations, there were at least two and, in one, even three aquifers, at different depths. The first level aquifers were mostly in basalts with vesicles. And the aquifer at the lowest level, at the contact of the basalts and the Gondwana, would yield more water, say the researchers. 

The Gondwana base below seemed to have a bowl shape. 

Hydrogeological model of the basaltic aquifer system underlain by Gondwana formations

The spatial variability of potential aquifer indicates that the north-eastern part has been confined to the Deccan traps covered Gondwana sandstone, the north-western part with weathered and fractured massive basalt, the middle part with fractured amygdular basalt, and the southern part with weathered and fractured basalt.

Cross plot showing the correlation of depth in metres with groundwater yield in litres per second.

At Jhunki village, the basalt meets Gondwana at a depth of 27 metres and the groundwater yield is barely one and a half litres per second. But, at Ramgiri village, the contact is at 172 metres and the yield is more than 6 litres per second. Overall the probability of increased groundwater yield with depth to B-G contact is the novel observation in this study, says Subash Chandra, CSIR-NGRI.

The contour map of potential aquifers created so painstakingly will now assist in the management and planning of groundwater resources including the enhancing of recharge, says Paras R. Pujari, CSIR-National Environmental Engineering Research Institute.

The successful mapping of the aquifers under Nagpur district has provided the experience and insights needed to tackle explorations for aquifers under the 500,000 square kilometres covered by the Deccan traps, says Sahebrao Sonkamble, CSIR-NGRI.

Reference:

Sahebrao Sonkamble, Subash Chandra and Paras R. Pujari, Application of airborne and ground geophysics to unravel the hydrogeological complexity of the Deccan basalts in central India

Hydrogeology Journal, June 2022; DOI: https://dx.doi.org/10.1007/s10040-022-02503-7 

Acknowledgements:

The research work was carried out jointly by CSIR-NGRI and CGWB under the Aquifer Mapping project (AQUIM) funded by the Ministry of Jal Shakti (formerly Ministry of Water Resources, River Development and Ganga Rejuvenation), Govt. of India. CGWB provided the exploratory drilling lithologs and electric logs.