Impact of tropical cyclone on groundwater response
Description
Physico-chemical impact of extreme climatic events on coastal groundwater is not well-studied. We delineate the impact of recent (2017-2020) tropical atmospheric depressions (AD) and cyclones (TC) on shallow (SW, 15 and 24 meter below ground level (m bgl)) and deeper (DW, 115 m bgl) groundwater level (GWL) in one of world’s most EC-affected region at the Ganges river delta-front. Multi-depth piezometry and hydrogeochemical profiling suggest that GWL have responded to each of the studied EC events, with highest GWL rise during the Cyclone Bulbul (November 2019). The instantaneous depth-dependent groundwater response, even in highly heterogeneous and low hydraulically connective delta front aquifers, suggests the potential vulnerability of drinking water quality deterioration during EC events, especially in the present trend of increasing frequency of intense cyclones. The study was conducted in coastal aquifers of the Ganges river delta front in the Sundarbans, South 24 Parganas, West Bengal, India. The studied area is about 16 Km2, lies within 21°33’22.87” N and 88°14’43.47” E to 21°35’54.64” N and to 88°17’13.98” E. The River Bhagirathi-Hooghly passes through the west and the extreme southern part of the area extends till the Bay of Bengal (BoB). Absolute groundwater and atmospheric pressure were recorded at half-hourly intervals for three consecutive years (2017 to 2019) using water level sensors (Solinistlevelogger 3001) for SW and DW wells (accuracy of ± 0.05% Full Scale) and by barologger with an accuracy of ± 0.05 kPa. Two closely placed observation wells are screened at depth of 15 (SW15) and 115 (DW115) m bgl, and other two shallow observation wells for monitoring of GWL are placed at a depth of 14 (SW14) and 24 m (SW24). The piezometers were suspended approximately 5 to 10 m below the static water level, whereas a barometric logger was installed in the air-filled section of a shallow well with the same data recording interval. Wells are located differently from nearshore (SW14 and SW24) and 3.5 km inland (SW15 and DW115) from the shoreline (Figure 1b). The SW15 and DW115 have recorded data from May, 2017 to December 2019, SW14 and SW24 have recorded data from May, 2017 to January, 2018. Wells were installed by rotary drilling and permanent casing is made by PVC pipes, and temporary casing was made by bentonite during drilling process. Observation wells were flushed after installation for 8 hours in one-hour interval. To find out the correlation and for model establishment, a six-hour interval of GWL data is used at 0, 6, 12 and 18 hours for selected cyclonic periods as geopotential height (GPH) was available only at the same time interval. Atmospheric model data for geopotential height (GPH) was obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) with a grid size of 0.25° × 0.25° at pressure level of 500 hPa.