The study, which involved careful examination of a sediment core collected off Goa’s shores “provide a clear record of historical changes in terrestrial mining induced sedimentation in the area.”
“The results indicate that the sharp increase in the sedimentation rate along with an increase in mineral magnetic parameters at a depth of 41 cm corresponding to an age of 1982 CE is a result of the increased iron ore mining activities in Goa,” the study led by researchers B Nagender Nath, Tyson Sebastian and others has noted.
In a bid to reconstruct the sediment history, the researchers collected a 1.03m long gravity core from the Arabian Sea off Goa from a water depth of 24.6 m. The top core was then sub-sampled at one centimetre intervals to study the sediments.
According to researchers, the column of sediment they sampled indicated two distinct phases -- the bottom phase showed a sedimentation rate of 0.19 centimetre per year with the top phase showing rate of 1.57 centimetre per year with 1982 being the year separating the two distinct phases.
“Our study proves that open cast iron ore mining in the state of Goa, India is affecting the sedimentation in the offshore region and may be detrimental to the oceanic environment. This study also indicates that environmental magnetic properties of coastal sediments can reflect the land use changes in the source areas,” the researchers noted in their conclusions.
“The reason for the very sharp rise in linear sedimentation rate (0.19 to 1.57 cm/yr) at 1982 may be due to this shifting of mining sites to the catchment area of Mandovi River,” the study notes referring to the transition of the State’s mining industry from Bauxite mining to mainly iron ore mining.
The findings of this latest study are consistent with a similar study conducted on the sediments in the estuaries of Goa where the rate of sedimentation is higher. A similar study on the Mandovi mudflats found that a higher sedimentation rate of 1.42 cm/yr in the top 35 cm of the column (roughly corresponds to year 1980 CE) is reported with a slower sedimentation rate (0.14 cm/yr) in the bottom section of the record.
An increase in suspended particulate matter (SPM) content in the Mandovi estuary with time after 1980 has been attributed to the increased supply of mining induced sediments to the coastal regions of Goa.
Open cast iron ore mining in Goa covers almost 8 per cent of its total geographical area. Though the mining in Goa started well before its liberation in 1961 from Portugal, it gained its momentum by the 1970s and increased thereafter. The open cast mining produces a lot of rejects which are transported by wind or water to the estuary and then possibly to the offshore areas. The transportation of the ore and pellets by mechanised barges along Mandovi and Zuari estuaries to Mormugao Harbour for export, also contribute significant amount of material to the river and estuarine system.