Reports of water contamination in Raddolugama in the Gampaha District are trickling in. Residents of the Raddolugama Housing Scheme complain that the pipe-borne water is salty and undrinkable.
“No matter how much water you drink, it won’t quench your thirst. There has to be some problem in purification. Children simply refuse to drink it,” said Jagath Peramune (32) a resident of Raddolugama. It’s obviously a case of salt water intrusion, an issue that most Colombo residents who depend on pipe-borne water will have to deal with given the worsening drought.
When inquired, National Water Supply and Drainage Board, General Manager, Engineer GA Kumararathna informed that this is indeed a case of salt water intrusion, an issue that affects Raddolugama residents from time to time. “This arises due to salt water intrusion into the Aththanagalu Oya due to the low water level in the Oya at the moment. There are two anicuts in Opatha and Bolanda. We take water from a point in-between these two anicuts, 9.5 kilometres away from the sea. However water has a high salinity level of 1,470 per milligrams per litre,” explained Kumararathna. Normal salinity level is between 300 and 500 milligrams per litre. He mentioned that the water tanks in the area are currently being filled by water bowsers and a supply of clean water would be made available to the residents soon.
According to Colombo University, Department of Geography, Senior Lecturer Dr. Ranjana Piyadasa, salt water intrusion is a natural phenomenon that occurs due to the difference in densities between fresh water and saline water.
“Saline water density is higher than that of fresh water. As a result saline water flows inland easily. Normally, fresh water density is 1.0 gram per cubic centimetre, whereas saline water density is 1.06 grams per cubic centimetres. This is quite normal in the Kelani or any other river basin,” he says.
Current illegal human activities being carried out along the river basin have contributed to salt water intrusion. These include sand, gold and gem mining. Gold mining is carried out in the Malwana area especially in the western region along the Kelani River.
Due to sand mining, the river bed is beyond the main sea level. This increases the salt water intrusion where salt water flows several kilometres landward. “If sand mining is prohibited, with time sand would accumulate, bringing up the level of the river bed to the main sea level. However, sand is needed for development projects and it’s important to think of alternative sources for sand. It is also important to conduct scientific research studies on the river environment. This way the sites with an abundance of sand can be identified for mining,” Dr Piyadasa said.
The research done in the past along the Kelani River has discovered the river bed to be two or three metres below the main sea level. According to some of the studies, the salt intrusion takes place up to the Kaduwela area.
“Now it is minimized due to the sand barrier placed just before the Ambatale area where the main water inlet for the Colombo city is located. Without the sand barrier, the Ambatale water intake would be affected by the saline water,” Dr. Piyadasa disclosed.
A recent study conducted on the Bentara River revealed that salt water travels a distance of 25 kilometres up from the river mouth. Salt water intrusion affects agriculture, infrastructure and human health at different levels. The saline water flows along the tributaries of a particular river and mixes with the ground water.
Along the Bentara River, paddy fields are abandoned due to a freshwater variety of paddy grown not able to survive in the salt mixed water. Along the Bentara River, 1,000 hectares of paddy lands have been abandoned. The buildings near the river banks could experience rapid cement erosion.
Generally, salt water forms the bottom layer that is closest to the riverbed, due to its high density. When a sand barrier is enclosed, the less dense fresh water flows above it, minimizing the more dense salt water intruding inlands. Sand barriers are a temporary solution that is currently used in Sri Lanka. However, sand barriers are disrupted during the floods, while permanent solutions for the issue remain expensive.
“There are many permanent solutions available, including a barrier that resembles a balloon. It could be placed when there is salinity intrusion and be removed during the season when there is no salinity intrusion. Another permanent solution is to construct a dam to minimize the upward salinity,” Dr. Piyadasa observed.
During the drought, when the rainfall is scarce, the salt water intrusion peaks with the low water levels in the river.
“In 2004, the heavy drought resulted in very low water levels in the river. As a result, saline water intruded inland passing over the sand barrier. At that time the solution was to release the upper catchment from the reservoir to flush down the salt water,” he recalled.
However, this could only be carried out when there is enough rainfall in upper catchment areas. When the rainfall is low, the limited water supply cannot be released to flush away the saline water in the river.
“It’s important to protect the Ambatale water intake right now as there is a risk of salt water intrusion, due to the low water levels in the river,” Dr. Piyadasa emphasized.
National Water Supply and Drainage Board, General Manager, Engineer GA Kumararatne commented that there is no risk of salinity to the water supply in Colombo. According to him, the salt water intrusion along the Kalu and Kelani rivers are under control at the moment. “Depending on the tidal characteristics and the prevailing rainfall the salt water intrusion varies. At the moment we have managed to control the situation by the effective use of sand barriers,” he said.
Kumararatne said that if the drought becomes frequent, there is the need for a more permanent solution. One example is a salt water purifying system. “Already the total cost to produce one cubic metre of water costs between Rs.500,000 to 750,000 which includes the cost for pipes, plants and purification. If we are to implement the use of a water purifying system, this cost would increase further,” he elaborated.
According to Dr Piyadasa, a salt water purifying process would be extremely costly. “The purification of saline water gives rise to waste products which are not environment friendly,” he opined.