ISOTOPE HYDROCHEMICAL TECHNIQUES FOR ASSESSING THE HYDRODYNAMICS OF A TROPICAL WETLAND

Environmental isotope and hydrochemical methods along with the hydrological and hydrogeological information were employed to assess the source of water, wetland-aquifer interactions and the hydrodynamics of Vembnand Wetland (area: 2030 km²), one of the largest wetland ecosystems in the southwest coast of India. Seasonal water sampling was carried out from the various locations of the wetland components such as the four rivers, shallow aquifer, canals, paddy fields / marshes and the Vembanad Lagoon during July 2007 (summer monsoon), November 2007 (winter monsoon) and May 2008 (dry period) and the samples were analyzed for ¹⁸O, ²H, ²²²Rn and major cations & anions.

Isotope results indicate that the hydrodynamics of the wetland is governed by the river flows during the summer and the winter monsoons while the groundwater flow and the tidal conditions control the flow during the dry period. Because of the steep topography of the river catchments, the time of concentration is too short and the peak flow reaches the wetland within a day after a rain event. Two groundwater flow regimes are identified: (i) a regional groundwater flow through the weathered and fractured aquifers that discharges into the rivers at the low lands and ultimately joins the wetland as surface flow (ii) a local groundwater flow that discharges directly into the wetland through its fringes. Presence of thick clay layers beneath the wetland impedes the hydraulic exchange between the wetland and the underlying aquifer.

Hydrochemical data shows restricted flow / stagnation and pollutant build up in the wetlands during the dry period while flushing take place during the monsoon season because of the flood flow. Spatial distribution of various water types of the canals, paddy fields and marshes during the dry period show a systematic trend along the flow path with (a) Ca-HCO₃ types near the fringes of the wetland which is due to the river and direct groundwater inputs (b) Na-HCO₃-Cl & Na-SO₄-Cl types at the central part because of the restricted flow/stagnation & contamination and (c) Na-Cl types close to the Vembanad lagoon which is due to the tidal influence. These findings are further confirmed from the spatial distribution of δ¹⁸O in the wetland.

Thus the study demonstrates the usefulness of isotope and hydrochemical techniques to understand the hydrological processes of wetlands.