Understanding wetland sub-surface hydrology using geologic and isotopic signatures

Abstract

Abstract. This paper attempts to utilize hydrogeology and isotope composition of groundwater to understand the present hydrological processes prevalent in a freshwater wetland, source of wetland groundwater, surface water/groundwater interaction and mixing of groundwater of various depth zones in the aquifer. This study considers East Calcutta Wetlands (ECW) -a freshwater peri-urban inland wetland ecosystem located at the lower part of the deltaic alluvial plain of South Bengal Basin and east of Kolkata city. This wetland is well known over the world for its resource recovery systems, developed by local people through ages, using wastewater of the city. Geological investigations reveal that the sub-surface geology is completely blanketed by the Quaternary sediments comprising a succession of silty clay, sand of various grades and sand mixed with occasional gravels and thin intercalations of silty clay. At few places the top silty clay layer is absent due to scouring action of past channels. In these areas sand is present throughout the geological column and the areas are vulnerable to groundwater pollution. Groundwater mainly flows from east to west and is being over-extracted to the tune of 65×10 3 m 3 /day. δ 18 O and δD values of shallow and deep groundwater are similar indicating resemblance in hydrostratigraphy and climate of the recharge areas. Groundwater originates mainly from monsoonal rain with some evaporation prior to or during infiltration and partly from bottom of ponds, canals and infiltration of groundwater withdrawn for irrigation. Relatively high tritium content of the shallow groundwater indicates local recharge, while the deep groundwater with very low tritium is recharged mainly from distant areas. At places the deep aquifer has relatively high tritium, indicating mix-ing of groundwater of shallow and deep aquifers. Metals such as copper, lead, arsenic, cadmium, aluminium, nickel and chromium are also present in groundwater of various depths. Therefore, aquifers of wetland and surrounding urban areas which are heavily dependent on groundwater are vulnerable to pollution. In the area south of ECW isotope data indicates no interaction between shallow and deep aquifer and hence this area may be a better location to treat sewage water than within ECW. To reduce the threat of pollution in ECW's aquifer, surface water-groundwater interaction should be minimized by regulating tubewell operation time, introducing treated surface water supply system and artificial recharging of the aquifer.

Groundwater potential zoning of a peri-urban wetland of south Bengal Basin, India

“…8 Spatial distribution of groundwater potential of the area in and around the ECW sand mixed with occasional gravel. In some places, along with silty clay, sticky clay is also present at the top of the lithological column (Sikdar and Sahu 2009). Lithologs of deep exploratory boreholes, drilled by various agencies suggests the existence of underlying Tertiary clay/silty clay at an average depth of 296 m (Sikdar 2000;Mukherjee et al 2007).…”

“…The Quaternary aquifer of the area is sandwiched between these two aquitards and comprises a sequence of fine to coarse sand horizon mixed occasionally with gravel. The continuity of the sand layer that forms the aquifer is broken by occasional clay lenses of limited lateral extent (Sikdar 2000;Sikdar and Sahu 2009). An exhaustive discussion on the subsurface geological setup of Kolkata and ECW can be found in Sikdar (2000), and Sikdar and Sahu (2009), respectively.…”

“…The continuity of the sand layer that forms the aquifer is broken by occasional clay lenses of limited lateral extent (Sikdar 2000;Sikdar and Sahu 2009). An exhaustive discussion on the subsurface geological setup of Kolkata and ECW can be found in Sikdar (2000), and Sikdar and Sahu (2009), respectively. The subsurface geological set up indicates that the groundwater occurs under confined to semiconfined condition.…”

“…Many wetland hydrology studies have described the wetland water table or developed an annual water budget (Owen 1995;Cooper et al 1998) to illustrate the link between wetland hydrology and ecology (Drexler et al 1999) or have utilized hydrogeology and isotope composition of groundwater to understand the hydrological processes prevalent in the wetland (Sikdar and Sahu 2009). These studies often fail to represent the ever-increasing humaninduced pressure on groundwater for drinking, agricultural, and industrial purposes due to rapid increase in population.…”

Can mussels be used as sentinel organisms for characterization of pollution in urban water systems?

“…Anthropogenic nitrogen and organic pollution of water systems, including the interaction between surface and groundwater, have been successfully investigated using a range of stable isotopes (Sikdar and Sahu, 2009;Yang et al, 2012;Lutz et al, 2013). In addition, stable isotopes have been widely used in purely hydrological studies focused on flow paths, hydraulic residence time, and other hydrological dynamics (Clay et al, 2004;Rodgers et al, 2005;Volkmann and Weiler, 2014).…”