Fertile agricultural lands in the Krishna Godavari basin are turning saline, raising questions on what is primarily responsible for the phenomenon. Is land subsidence the result of reduced sedimentation flow in dammed rivers, oil and natural gas extraction, or other factors?
Retired geologist from Andhra University, G. Krishna Rao, who has worked in the area for over 30 years, and trained in field surveys, told The Hindu that land subsidence could be a greater threat to the eastern coast with its fertile river deltas than sea-level rise of a few millimetres per year expected due to climate change.
Field surveys done in the KG basin by Mr. Rao revealed land subsidence ranging from 1.5 ft to 5.4 ft over a 30-year period, leading to seawater ingress. The resulting increase in soil salinity affected farms. Increased coastal erosion had also damaged structures.
Land subsidence was verified by studying changes in the gauge levels for flood sinking in the Gannavaram aqueduct, which is connected to the Dowleswaram barrage in East Godavari district.
“The aqueduct, which sank under flood waters at a gauge level of 23.6 ft. in 1986, sank at a level of 18.2 ft in 2013. This confirmed that the land had sunk by about 5.4 ft here.” Surveys of farms around agricultural canals in Uppalaguptam tehsil showed lands earlier at higher elevation experiencing water logging, while canal locks in Bhimaravam, Modalpur rock and Sakhinetipalli showed subsidence.
Venugopal Rao, former Executive Engineer of Godavari Delta, confirmed the changes to the flood water sinking level at the Gannavaram aqueduct. The mean flood water sinking level changes at the Ganavaram aqueduct between 1986 and now would be not more than 1.2 metres (about 4 ft). He said the sinking levels could be due to tidal action though, and was “only minimal”. Yet, it has had a significant impact on agricultural lands nearby affecting about 3,000 acres of farm land.
Contentious factors
Mr. Krishna Rao blames oil and natural gas extraction for the land sinking in the KG basin.
He is member of the Krishna Godavari Deltala Parirakshana Samithi, an NGO, which moved the High Court of Hyderabad to fix responsibility on oil and gas majors, including the ONGC. He contends that extraction of hydrocarbons from deep under the Earth’s surface decreased the effective stress that kept rock formations in balance, resulting in their destabilisation.
However, when contacted, ONGC denied “any substantial evidence in terms of scientific data to conclude that the activities of ONGC are causing any disturbance in the areas studied [KG basin].”
ONGC declined to share copies of the two interim reports of investigations on land subsidence conducted by the Delta Studies Institute on the directions of the High Court. The Hindu accessed extracts of the two reports which noted “noticeable land changes had occurred” in the KG Basin based on remote sensing studies (SAR interferometry), but did not establish the reason.
A part of the second interim report submitted to the court says: “Among three zones it was noticed that one particular zone indicates pronounced changes.
The studies further indicate that these areas are either surrounded by aquaculture ponds and [in] some areas are close to the oil field areas where drilling must have been carried out.”
T.Y. Naidu, Director, Delta Studies Institute, Visakhapatnam, said subsidence was not a continuous phenomenon. A third study was underway to establish the factors responsible.