Hydrochemical evidences: Vulnerability of atoll aquifers in Western Indian Ocean to climate change

Abstract

The study evaluates movement of the saltwater interface and ionic composition in the atoll aquifer system in response to natural recharge in Western Indian Ocean (WIO). Sea level rise owing to climate change is expected to have substantial impacts on the world's population living on or near the coast over the next century (Vorosmarty et al., 2000; Milly et al., 2005). Change in sea level is predominantly linked to inundation and saltwater intrusion in coastal aquifers (Döll, 2009). However the change in precipitation pattern as a possible clause for seawater intrusion is poorly established and is due for substantial studies. The Intergovernmental Panel on Climate Change (IPCC) report (2007) that projects the global sea level rise of 18 to 59cm from 1990 to the 2090s, along with an unspecified amount that could come from changes in the large ice sheets covering Greenland and Antarctica, depicts a relatively much lower regional sea level rise in the WIO. In addition, the report signifies a reduction in annual precipitation rate in this region in agreement with past historical data. This characterizes the WIO region as a potential critical zone under the influence of climate change (i.e. less sea level rise, high reduction in rainfall pattern) unlike most of its neighboring regions. The WIO region is a home to hundreds of coral islands and is inhabited largely. Despite its purported importance, tracing of the aquifer geometry under the influence of climate change has not been reported much. Application of hydro-chemical analysis to delineate the climate change impacts has even been rare. Androth island of Lakshadweep archipelago has been chosen as the study area. The study establishes that hydro-chemical evidences have a high degree of correlation between groundwater aquifer geometry and rate of precipitation. It depicts that decreasing trend in precipitation in the WIO region would result in seawater intrusion and significant shrinkage of aquifer geometry. The study determines the control of precipitation pattern over the aquifer geometry by systematic analyses of hydro-chemical data. The conceptual analyses include surface plots, correlation analysis, ionic exchange and ionic ratio techniques, and multivariate data analyses. The study comprises two precipitation conditions derived from observed field data: (1) rainfall 30% below yearly average and (2) rainfall 30% above yearly average. The result shows shrinkage of around 54.91% from 82.64% (scenario 2) to 27.63% (scenario 2) in good quality water reserves. The study highlights the importance of aquifer boundary conditions in the context of climate change. The analyses clearly indicate that the decreasing trend in rainfall would severely impact the existence of fragile island aquifer systems of the Lakshadweep archipelago.