PANAJI: The folly of allowing intensive mining activities in close range of Selaulim dam have come to haunt the consumers and water suppliers alike, and the intense summer may have triggered the effects earlier than usual.
The National institute of oceanography (NIO) scientists attribute the high concentrations of manganese being detected in the early part of summer season to the unusually hot summer this year. "It may be due to the warm conditions in the early part of the year," said NIO director S W A Naqvi.
February had started the trend with the mercury soaring to 36.5 degrees celsius on February 14. In March, the blazing sun scorched a record at the beginning of this summer after the meteorological department recorded temperatures +6 degrees above normal (37+ degrees C) for seven consecutive days from March 1 to March 7, 2013.
"Hence, due to the excessive heat, stagnation and the consequent oxygen-depleted conditions in Selaulim dam may have set in earlier," Naqvi explained.
A study by NIO scientists has shown that intense summer heat separates the surface and bottom layers in the water columns of Selaulim and Tillari dams, triggering depletion of oxygen and dissolution of iron and manganese from the solid or particulate form. During summer, as in larger water bodies anywhere else in the world, the deeper layer, hypolimnion, is cut off from the top surface, epilimnion in both reservoirs.
Degradation of organic matter due to bacterial action depletes the oxygen, which is not replenished from the atmosphere due to stagnant conditions in the water. The water in the reservoirs separates to two or there distinct layers, the upper one is well oxygenated and the lower one becomes anoxic by March and continues to be so till the end of summer. The monsoon brings about the mixing of the water once again.
The NIO scientists chose the reservoirs of Tillari and Selaulim dams to study phytoplankton and bacterial pigments in Indian freshwater lakes. The study by Naqvi's group led by Siby Kurian and titled 'Seasonal occurrence of anoxygenic photosynthesis' in Tillari and Selaulim reservoirs, Western India was published in the journal, Biogeosciences of the European Geosceinces Union recently.
A process of proper mixing of water occurs naturally in the upper surface. The oxygen is derived from the atmosphere and also produced by the algae in the sunlit zone.
But Naqvi said he would not blame mining activity for the high manganese content in the Selaulim water. "The sediments are rich in iron and manganese because of the composition of the source rock in the region," he said.
South Goa's lifeline has been long threatened by intense mining activity. A total of 17 mines are operating within 500m and 12 mines within 200m of the dam. This was admitted by PWD minister, Ramkrishna 'Sudin' Dhavalikar in the assembly. He also admitted that there are around 17 mining dumps within 200m of the Selaulim dam.
The NIO study states that the levels of dissolved oxygen are low in the deeper layer. 'The lower layers in a water body are marked by low temperatures and there is insufficient light for photosynthesis," the study states. The layer of water in a thermally stratified lake that lies below the thermocline, is noncirculating, and remains perpetually cold. "Levels of dissolved oxygen are low here,' the study says.
By April/May, all the oxygen in Selaulim and Tillari is gone. In Tillari, the NIO scientists studied data throughout the year while in Selaulim, the water samples were analyzed during May in 2010 and 2012.
'The oxygen content in the water varies between five and six milliliters of oxygen per litre of water at the surface,' states the study. During the stagnation periods, the oxygen concentration is zero in the bottom layer.