Geo Factos

Groundwater

The hidden resource

The importance of water is immense, and no one needs to exaggerate (make something seem bigger) on this since we all know how it sustains the existence of Earth and its living beings.

Water becomes extremely significant because although 98 per cent of the globe is covered in water in the form of oceans, only about two per cent of the planet's water is fresh. Out of this ,1.6 per cent is locked up in the polar ice caps and glaciers.

Another 0.36 per cent is found underground. Only about 0.036 per cent of the planet's total water supply is found in lakes and rivers. Today let's look at the 0.36 per cent which exist as groundwater.

Groundwater, aquifers and the water table

Simply put, groundwater is the water that is trapped below the Earth's surface. That being the general description, three other terms can be introduced which resemble the amount of water below the Earth's surface.

The first is 'aquifer'. An aquifer is a water deposit made of permeable (ability to flow into all parts) rock or other loose material which contains a sufficient amount of water. The wells in your homes generally tap water from aquifers. Aquifers can be spread over an extent ranging from a few acres to thousands of square kilometres, and they can also have a thickness ranging from a few metres to few hundreds of metres.

The next term associated with groundwater is 'water table'. This involves a higher quantity of water compared to an aquifer.

When the soil pores or other loose material below the surface get completely saturated (full) with water, such a region is called a water table.

The region above the water table is known as the 'unsaturated zone' which has some water as in aquifers, but not saturated.

Groundwater has a cycle. The water seeps throughout the surface in to the underground. This is known as 'recharge'.

However, eventually, groundwater will 'discharge' at springs, wetlands and oases. This process is known as the 'water cycle' associated with groundwater.

The duration of this cycle can vary from a few days to thousands of years.

 Sri Lanka’s groundwater

Groundwater has been utilised for a range of purposes in Sri Lanka ever since the time of our ancestors. Until the recent past, more than 95 per cent of the groundwater utilisation of the country was for agricultural purposes. The rest was shared between industrial applications and other activities.

However, like many other resources, there is a necessity for conserving this resource, and many management strategies are under way with this objective in mind.

Current problems

Several problems that have come to notice globally, which require effective strategies for groundwater management, are seen in Sri Lanka as well.

The intense pumping of groundwater for agricultural activities has started to lower the water table and consequently, a requirement has arisen to increase the depth of wells.

In addition, farmers in the dry zone complain about the lack of groundwater, mainly because over-pumping has dried off aquifers. With more agro-wells tapping into the same limited resource, and more intensive agriculture taking place, it is likely to mean that shallow groundwater resources will become further depleted and more severely polluted.

In some areas, like the Kalpitiya peninsula, high concentrations of nitrates and agrochemicals are already being found in groundwater. Several deep tube-wells constructed recently to provide drinking water in the dry zone have been abandoned because of high iron and fluoride concentrations.

The coastal sand aquifer which feeds the Kalpitiya peninsula, which is popular for its intensive groundwater-based vegetable production, is experiencing a gradual build-up of salt and agrochemicals.

Similarly, in Batticaloa, salinity (salt content) and nitrate pollution is affecting agro-wells and the nitrate levels found were three to five times higher than the recommended limit set by the World Health Organisation.

This becomes prominent since the aquifers in Sri Lanka are situated in a very shallow level. This directly affects rural drinking water since 80 per cent of the requirement is satisfied by open dug wells.

The tsunami and groundwater

The tsunami which struck Sri Lanka in 2004 had a direct impact on the coastal groundwater, mainly on the aquifers. The flooding waves of the tsunami caused millions of gallons of sea water to enter the underground reservoirs through open wells.

The salty waters had infiltrated (sunk) through the permeable sands that are typical of coastal aquifers. It was reported that over 40,000 drinking water wells were either destroyed or contaminated.

This infiltration of seawater had caused the salinity levels of groundwater to increase. Extensive pumping has been done in order to remove the saline water from the wells, but the salinity levels were expected to drop only after a few monsoons which would result in recharge of groundwater.