Why seawater intrusion has not yet occurred in the Kaluvelli-Pondicherry basin, Tamil Nadu, India

Abstract

Worldwide, coastal aquifers are threatened by seawater intrusion. The threat is greatest when aquifers are overexploited or when recharge is low due to a semi-arid or arid climate. The Kaluvelli-Pondicherry sedimentary basin in Tamil Nadu (India) presents both these characteristics. Groundwater levels in the Vanur aquifer can reach 50 m below sea level at less than 20 km inland. This groundwater depletion is due to an exponential increase in extraction for irrigation over 35 years. No seawater intrusion has yet been detected, but a sulphate-rich mineralization is observed, the result of upward vertical leakage from the underlying Ramanathapuram aquifer. To characterize the mechanisms involved, and to facilitate effective water management, hydrogeological numerical modelling of this multi-layered system has been conducted. Existing and acquired geological and hydrodynamic data have been applied to a quasi-3D hydrogeological model, NEWSAM. Recharge had been previously quantified through the intercomparison of hydrological models, based on climatological and surface-flow field measurements. Sensitivity tests on parameters and boundary conditions associated with the sea were performed. The resulting water balances for each aquifer led to hypotheses of (1) an offshore fresh groundwater stock, and (2) a reversal and increase of the upward leakage from the Ramanathapuram aquifer, thus corroborating the hypothesis proposed to explain geochemical results of the previous study, and denying a seawater intrusion. Palaeo-climate review supports the existence of favourable hydro-climatological conditions to replenish an offshore groundwater stock of the Vanur aquifer in the past. The extent of this fresh groundwater stock was calculated using the Kooi and Groen method.

Groundwater in catchments headed by temperate glaciers: A review

“…Offshore fresh groundwater stocks in littoral areas are being increasingly recognised (Post et al, 2013;Vincent and Violette, 2017), but their origins are still speculative.…”

“…Studies on such systems are developing e.g. the offshore groundwater stock suggested by observations and numerical modelling and flux estimation in the Vanur aquifer, south-east India (Vincent and Violette, 2017), or the geophysical profiling establishing evidence of an offshore fresh groundwater system in the southwestern waters of Hong Kong (Jiao et al, 2015;.…”

“…-Emerging research lines are then set out, which will allow to consider the future evolution of catchments headed by temperate glaciers (Sections 3, 4 and 5): -What are the effects of glacial meltwater runoff on offshore T groundwater stocks (see definition in Table 1) or the supply of groundwater springs? Those offshore stocks are being increasingly recognised (Post et al, 2013;Vincent and Violette, 2017), but their origins are still speculative. One hypothesis is that a massive recharge happened just after the Last Glacial Maximum (LGM) during the melting of the ice caps.…”

Wetlands for a Sustainable Urban Future: Insights from Pondicherry, South India

“…If the major resource of this development model is land, the increasing human population causes a surge in demand for drinkable water (Figure 3). As groundwater is overexploited threatening these fragile coastal aquifers by saline intrusion and water depletion, surface water is becoming a potential source for a complementary drinkable water supply (Sridharan and Nathan, 2017;Vincent and Violette, 2017). A survey on this subject conducted during 2002 by Water and Power Consultancy Services (India) Ltd. (WAPCOS) emphasized that the surface water available in Oussudu tank can be utilised for domestic water after being given proper treatment.…”

Hydrochemical, Isotopic, and Geophysical Studies Applied to the Evaluation of Groundwater Salinization Processes in Quaternary Beach Ridges in a Semiarid Coastal Area of Northern Patagonia, Argentina

“…Freshwater lens salinization is a global issue that deteriorates water supply sources, therefore limiting the population development of coastal areas. This salinization may be due to seawater intrusion due to the intensive exploitation of freshwater lenses [22], by upward flowing of saline groundwater [23,24], or by past marine transgressions [25][26][27][28]. In arid and semi-arid areas, salinization problems are accentuated due to scarce and erratic rainfall and the high rate of evaporation that condition aquifers recharge [29][30][31].…”