By Willie D. Joshua –
Danger of soils becoming saline from bore-wells water in the Jaffna Peninsula
Fresh water supplies and arable land are at risk due to indiscriminate digging of bore wells resulting in saline water extraction. Simple methods for mapping the depth of usable water are proposed that can be used to guide well-depth and prevent significant environment degradation. This information can also be used to estimate the available groundwater for human use.
Recently, there has been a trend in drilling bore wells in the Jaffna Peninsula to extract groundwater for domestic use. This practice is to be implemented for houses being built with foreign aid to settle displaced people in the war torn area. These wells are constructed by drilling about 15cm (6 ins) circular holes right down to the groundwater. The water from these bore wells is used directly by hand pumps or is pumped to an overhead tank and distributed by pipes within the house for domestic use. The groundwater in the peninsula consists of a fresh water layer underlain by salt water. There is a danger that if the bore well goes down to the depth of the saline layer, the pumped out water will be brackish. Thus, there is a possibility of good land becoming saline if this water is used. There have been some instances in Jaffna, where the waters from the bore wells are brackish and not usable even for domestic purposes. Therefore, the depths of the bore wells have to be above the saline groundwater. The Northern Provincial Council has already been informed of this potential problem and remedial measures also have been suggested.
There are three types of wells in use in Sri Lanka namely agro-wells, tube wells and bore wells. Agro-wells, also referred to as dug-wells, are the ones commonly in use in the Jaffna Peninsula and elsewhere in Sri Lanka. Water from these wells are used both for domestic and irrigation purposes. These are large diameter (2-3m) wells whose depths do not go beyond 1m below the dry season water table.
Tube wells are similar to bore wells. They have a 15-20 cm diameter, but go to a depth of more than 70m (200 ft) to tap the deep aquifers of limestone and sandstone. These tube wells are found in the western areas along the Mannar-Pooneryn road.
Bore wells, as described earlier, are the shallower version of the tube wells. They tap only the groundwater formed by the percolated NE Monsoon rains. The need for bore wells arose because the cost of constructing the usual agro-wells has risen sharply. Another reason is houses in Jaffna are being built in small compounds, and there is lack of space to accommodate a large diameter well. In the 1970s and 1980s, people in Jaffna drilled bores of very small diameter of about 5cm (2 in) at the bottom of their agro-wells in the hope of intercepting some dissolution channels or cavities in the limestone aquifer to increase water supply in the well. Fortunately the waters were not brackish. Presumably, these drill holes were not deep enough to make the water brackish.
The large diameter agro-wells have been in use for a long time, and are preferred because a considerable volume of water is recharged and stored in the wells between the irrigation intervals of few days. This stored water itself is was often sufficient for irrigation. As the wells are shallow, most of the recharge of water would have been from the non-saline surface layer of the groundwater.
The officers who are responsible for authorising the construction of bore wells appear to be unaware of the saline water always present below the fresh water. If they knew the depth at which, for any particular location, the ground water becomes saline and unfit for human use, they could specify the maximum depth to which bore wells should be constructed in order to avoid salinity. Unfortunately, this information is still not available for the Jaffna groundwater.
Formation and quality of groundwater in Jaffna Peninsula
The limestone bedrock of Jaffna, with its large pores and cavities, extends below sea level and functions as an excellent aquifer (water bearing porous material). Even sands in the Vadamaradchi area act as an aquifer. These aquifers are charged annually by the North-East monsoonal rains between October and January. The rainwater percolates downwards through the soil and moves into the pores and cavities of the limestone/sands. The percolated fresh rainwater forms a layer within the aquifer and floats on top of the denser sea water that is already present in the aquifer at sea level.
According to the Ghyben-Herzberg Principle, the weight of the fresh rain water pushes down the denser sea water so that the overlying fresh water is in the shape of a convex lens within the porous limestone. The fresh water level (water table) has to be therefore above mean sea level. In reality however, the lower part of the fresh water lens is actually a transitional zone where the salinity level of the fresh water layer increases gradually with depth from fresh water at the surface to that of sea water at a lower depth. Therefore, the usable fresh water actually is the upper part of the lens that is formed by the rainwater.
The usable upper part is the depth up to which the brackishness (salinity) of the groundwater is low enough and fit for human use. Generally, the usable fresh water is considered to have salt (chloride) content lower than 500 parts per million. The thickness of this layer of good water is variable; depending on the location, season and the extraction rate. Usually, this usable water is thicker in the centre of the peninsula and becomes thinner towards the coast. In order to maintain the freshwater lens, there has to be continuous outflow of water from the lens to the sea, even though becoming slower with time. Consequently, the fresh water lens becomes thinner with the progress of the dry season and usage. The fresh water lens, although becoming thinner, has always been present till the end of the dry season.
As described above, all aquifers in the peninsula will always have fresh water in the upper layer followed by the transitional zone of increasing brackish water. In all probability, individual aquifers are present in the three landmasses (Valikamam, Thenmaradchi, Vadamaradchi) separated by the lagoons and also in the Islands. The quality of water in the wells will depend on the thickness of the fresh water layer and whether the bottoms of the wells are in the fresh water part or in the brackish part. Since the thickness of the fresh water layer becomes smaller towards the coast, the depth of wells have to be finely judged to intercept only the fresh water layer to the middle of the lens to keep the water always fresh. As a rule, coastal wells, whether they are in the peninsula or in the islands, have to be shallow in order to avoid salinity. A good example of this situation is the well located on the island close to the Jaffna Lagoon, in the compound of the Catholic Church in Charti. The well is shallow and the water is always fresh.
Very often, people dig or drill deeper wells to get more water but end up only getting brackish groundwater. Even pumping tests in shallow fresh water wells are meaningless because even little excess pumping may bring up the underlying salt water like a cone. There is an urgent need to determine the thickness of usable freshwater (the upper layer of the lens with chloride content less than 500 parts per million) and its depth variations with the progress of the dry season.
Remedial measures to avoid salinity
It would be greatly desirable to estimate the depth and its variation of the usable fresh water at different locations in the Peninsula. This information not only helps to determine the permissible depth of bore wells, but also helps to reliably estimate the total amount of water available for human use. There are many methods available, both physical and electronic, to determine the depths of usable fresh water in the lens.
1. Physical measurements: In principle, it is possible to establish a network of deep boreholes of small diameter, in a grid at regular intervals in the peninsula. The groundwater in the bore holes are measured periodically for water table depth and salinity. This will yield information on variation and depth of the usable water in the lenses for the entire peninsula over the seasons. The details of the actual procedures have to be worked out. There are already many boreholes drilled in the peninsula for previous studies, locations of which may be available with the Water Resources Board.
2. Electronic measurements: Radioactive and electronic techniques exist that may be applicable, but the author does not have the relevant expertise to explain the methods in detail. However, any instruments used for either electrical sounding, radar or gamma ray methods need local calibration, as manufacturer’s calibration has been found to be unsatisfactory. Several identified bore holes that are in the peninsula from earlier studies can be used for this purpose. In 2006, a detailed study was undertaken by SMEC International Pty Ltd on groundwater of some locations in Jaffna. The boreholes in this study have all the information regarding relief, coordinates and borehole drilling log etc. SMEC has identified electronic equipment that could be used for identifying salinity levels at different depths of groundwater without actually drilling boreholes. The reports are available at the office of National Water Supply and Drainage Board, Jaffna. This information has been passed on to the Northern Provincial Council.
It is the author’s urgent recommendation that those responsible for management of water supply in the Jaffna peninsula, namely the National Water Supply and Drainage Board or the Water Resources Board, take immediate action to map the depth of usable freshwater in the Peninsula, and use this information to guide the regulation of new wells. The persistent digging of too-deep wells caused by ignorance will result in an inadequate supply of fresh water and irreversible salinisation damage to good land, jeopardising the future liveability of the Jaffna peninsula. Prompt, simple action could prevent a major environmental disaster.