Tsunami and the soils of South Andaman

By: Dr S Ghoshal Chaudhuri
The 2004 tsunami in coastal areas of Andaman and Nicobar Islands has not only devastated lives but also ecosystems rendering the soil and water resources salt affected. Periodical soil and water sampling in selected locations of South Andaman reveals that post tsunami, the soluble salt concentration increased markedly. This was, however, offset by subsequent high rainfall of 3774 mm in 2005 resulting in drastic reduction in the salinity levels at these sites to almost close to pre tsunami levels. The gradual recovery process of the salt affected sites can be augmented by adoption of appropriate location specific engineering and agronomic management strategies.
Disaster Events
Article 11 Image 1
Image: 1. The author collecting samples from the low lying coastal areas where sea water intruded only during tsunami and then receded permanently – Situation I 2. Low lying coastal areas where sea water reaches with every high tide and recedes with low tide – Situation II 3. Low lying coastal areas where there is permanent stagnation of sea water and the depth of impounding increases with high tide – Situation III 4. Saline sea mud deposition by the tsunami on agricultural lands

 

Soils usually turn saline due to weathering of parent materials – causing fossil or primary salinity, or from anthropogenic activities involving the improper management of land and water resources – contributing to human or secondary salinity. Until recently, the occurrence of large scale soil salinity due to natural disasters like the tsunami was understood as a rare phenomenon. However, with complete submergence of coastal areas of Andaman and Nicobar Islands the soil and water resources have turned saline. The direct environmental impact of the tsunami varied according to factors such as bathymetry and geomorphology of the coastline. Thus, areas adjacent to the relatively steep continental shelves were generally less damaged than coasts with shallow continental slopes. The coastal areas of the islands affected by the tsunami present highly diversified human activities, from inland, freshwater, rice based systems to mangrove and fisheries. The tsunami also affected the cultivated lands surrounding the coastal areas, especially the eastern coast.

After tsunami, a preliminary survey and soil sample collection was carried out in the islands to assess the extent of damage on the agricultural lands. Based on the survey, the affected lands were categorised into three different situations.  Situation I – ‘Low lying coastal areas where sea water has intruded only during tsunami and then receded permanently’, Situation II ‘Low lying coastal areas where sea water reaches with every high tide and recedes with low tide (areas affected with fluctuating sea water table)’ especially during new moon and full moon days and Situation III ‘Low lying coastal areas where there is permanent stagnation of sea water and the depth of impounding increases with high tide’, were analysed.

Salt accumulation due to sea intrusion resulting in decline in soil fertility was one of the chief impacts. In order to measure the extent of soil damage, soil and water sample collection cum analysis was undertaken at periodical intervals, i.e. immediately after the tsunami in February and March 2005, one rainy season after the tsunami in February and March 2006, and two rainy seasons after the tsunami in February and March 2007.

Likewise, soil sampling was done at periodical intervals in the tsunami affected sites from the surface layer (0-15 cm) and sub surface layer (15-30 cm). Samples were also collected from the tsunami affected agricultural lands. The pH of surface soil under Situation I indicated that there were several changes in pH and soluble salt content between pre tsunami and post tsunami conditions and the surface soil has become saline. But the sub surface soil had comparatively lesser amount of soluble salts. This trend clearly indicates that sea water intrusion during the tsunami did not affect the sub surface soil because the waves intruded in a flash and receded completely, leaving a layer of sodium and other soluble salts on the surface soil. Under Situation II, the results revealed that irrespective of soil series and initial salinity level, the surface soil has become highly saline due to the percolation of soluble salts.

However, rains caused an appreciable reduction in soluble salt content. The reason for appreciable reduction in soil salinity in surface and sub surface soils may be attributed to the leaching of soluble salts by the high rainfall received during the 2005 rainy season. Though the salinity level has reduced from the initial (immediately after tsunami) value but it has remained above 4 dSm–1 in almost all the locations. This may be attributed to the soil texture of the different soil formations such as sandy clay loam which might have facilitated the removal of soluble salts through leaching, as against the clay loam texture which does not allow leaching of soluble salts. A larger fraction of rainwater also tends to either run off or evaporate from stagnant water on the surface of the soil due to the low infiltration rates of fine textured soils having high clay content. This reduces the water available to displace the salts. After two rainy seasons, there has been a drastic reduction in soluble salt concentration of tsunami affected agricultural lands although excess water has also resulted in the water stagnation in many places due to lack of drainage outlets. The results of the water samples collected immediately after the tsunami have revealed that the waves have contaminated both well and pond water.

Reclaiming saline soils

Soil salinity reclamation requires analysis of sensitivity parameters that affect interactions between salinity and crop yield. Methodologies developed elsewhere for reclaiming saline and saline sodic soils of arid and semiarid regions due to irrigation may not be suitable for the islands. Hence, rehabilitation and management of salt affected soils of these islands require a combination of engineering and agronomic measures depending upon different situations. The first step towards reclamation of any salt affected soil should be on the basis of assessment of the soil, including soil profile, which will in turn establish whether the soil is saline/saline sodic in nature, or is not affected by salts. Thus, application of amendments like gypsum is not required instantly and leaching through rainwater impounding alone will be effective to reclaim the tsunami affected agricultural lands. Hence, agricultural lands under situation I can be easily reclaimed considering the higher annual rainfall which can be effectively used for leaching out the accumulated salts. However, the areas under Situation II require construction of raised embankments along with sluice gates, which will regulate the ingress of sea water in these areas. It will restrict the entry of sea water into the field during high tide and will allow the drainage of rainwater from the field, which may collect during the rainy season during low tide. In case of Situation III, it has been envisaged that brackish water aquaculture would be an alternative livelihood pattern for the agricultural lands affected by tsunami.

Besides these, a set of agronomic management practices may be followed for effective rehabilitation of salt affected soils. Salt tolerant varieties of crops may be raised like rice, sugarcane, sorghum, watermelon and forage crops like karnal and para grass and green manure crop. Suitable crop rotation like rice-watermelon, rice-maize, rice-sorghum, rice-vegetables, rice-sugar beet and rice-forage crops may be selected. Adoption of broad bed and furrow system of land manipulation may be practiced in the affected areas. Application of higher dose of farmyard manure (FYM) to improve the physical condition of the soil and drainage can be envisaged. In case of rice, transplanting of aged seedlings of salt tolerant variety and increased number of seedlings per hill can be planted. For wide spaced crops like vegetables, adoption of the pit system of planting by replacing the salt affected soil with a mixture of normal soil and FYM can be practiced along with adoption of frequent light irrigation such as drip or pitcher, for high value crops. Also adoption of auger hole technique for planting tree species in salt affected areas may enhance productivity.

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