Experts flag issues in removal of reservoir sand

The move by the district panchayat to desilt the Aruvikkara reservoir and use the extracted sand for construction has led to concerns about the impact of the project on public health and the environment.

Experts fear that desilting the reservoir, the only drinking water source for the greater Thiruvananthapuram area, is fraught with danger, unless preceded by an extensive scientific investigation and done with proper safeguards and regulations. They point out that the copious quantity of water required for washing the sediment and separating the sand from silt and clay will be unaffordable for a city that faces an acute shortage of drinking water, especially during summer.K.P. Thrivikramji, former Head of the Department of Geology, University of Kerala, says the potential sand reserve in the reservoir has not been assessed scientifically. The estimated sediment deposit of 80,000 cubic metres is based on 10-year-old data. “Sand makes up only 20-30 per cent of the total accumulated sediment deposit in a reservoir. This works out to sand deposits of 16,000 to 24,000 cubic metres (2,666 to 4,000 tipper-lorry loads) only,” he said.Dr. Thrivikramji feels that manual removal of silt, as proposed by the Aruvikkara panchayat, is unsuited for the reservoir.

“Manual extraction is time-consuming and raises the risk of contaminating the drinking water source for an extended period. Suction dredgers which cause the least or minimum disturbance to the water should be used for desiltation,” he says.He points to the problem in disposing of contaminated water used for washing and removing the sand from the silt and clay deposits. “How do we keep this water away from the distribution network? How will the residues be disposed of,” he asks.R. Sridhar, activist of the city-based environmental group Thanal, says the absence of regulation in extraction of sand from reservoirs and its sale will result in a free-for-all. “The government will have to decide who is going to get the sand and how it is to be apportioned,” he says.Mr. Sridhar fears that even a government company entrusted with the operation is likely to outsource the job, making regulation difficult. “Controlled dredging will become impossible,” he points out.

Another issue pointed out by Dr. Thrivikramji is the likely public health hazard caused by the release of chemical ions present in the sediment. “A large portion of the chemical fertilizers and pesticides applied in farms in the catchment area of the reservoir will have joined the waters as dissolved ions and been washed down into the reservoir to bind with the fine sediments at the bottom. When the sediment is disturbed, the chemical ions are released into the water piped to the city or washed downstream,” he says. He advocates a study of the nature of the ions and their impact on public health before embarking on the project.Yet another point raised by Dr. Thrivikramji is the likely presence of the gem chrysoberyl (Vaiduriyam) in the sediment. The area is rich in the gem and illegal mining for it is rampant. “The authorities will have to decide on who will have the right on the chrysoberyl extracted from the reservoir,” he says.

Mr. Sridhar highlights the need for a detailed environment management plan before launching the project. “The plan should also be subjected to a public hearing,” he says.

The Hindu, 27th October 2009