Study on barind Aquifer - Ganges river interaction

Abstract

In this study Barind Aquifer- Ganges River interaction has been investigated over a 55km reach of the Ganges River from Godagari to Charghat, having an area of 916 km2. The study area has two aquifers overlain by a thick clay layer which are interconnected in many places. The thickness of the upper clay layer varies from 15 to 30m. Within the study area, the thickness of the aquifer varies from about 5 to 10m for the higher topographic area and 10 to 30m for lower topographic area. Within the study area the value of transmissivity (T) varies from 600-1600m2/day and of specific yield varies from 0.06-0.30. The low permeability of the upper clay layer limits the recharge to the aquifers. Heavy abstraction of groundwater for agricultural, municipal and domestic uses has been occurred in this area. This results in excessive lowering of water table making the shallow and hand tubewells inoperable during the dry period. Ganges River shows a high seasonal variation of water level. Average annual maximum, minimum and mean Ganges water level during the study period were estimated as 19.90 mPWD, 10.63 mPWD and 14.20 mPWD at reach_l near Godagari and 17.06 mPWD, 8.18 mPWD and 11.75 mPWD at reach_5 near Sardah at Charghat Upazila. The average annual fluctuation of river stage in Ganges is about 10m. The study reach was segmented into five reaches. Correlations of river stage and groundwater level hydro graphs were investigated from 1996 to 2005 for all the five reaches. This study reveals that the groundwater levels respond with the seasonal variation of river stages. In the dry season, the groundwater levels were seen to be higher than the river stage and the flow direction is from aquifer to the river. As monsoon approaches, the river stages raise at faster rate than the groundwater levels and the flow direction changes from river to aquifer. In this study the annual lateral inflow or outflow of the aquifer, abstraction and annual recharge were calculated for the period of 1996 to 2005. Darcy's law has been used to calculate the lateral flow of groundwater due to interaction of Barind aquifer with the Ganges River. The number, capacity and operating hour of the deep tube well (DTW) and shallow tube well (STW) were considered for estimation of abstraction due to irrigation and number of population for estimation of abstraction due to municipal and domestic use. The infonnation about DTW and STW were obtained from Bangladesh Agricultural Development Corporation (BADC). It has been observed that the gain of groundwater from river to aquifer occurs only for a short period from July to September. On the contrary loss of groundwater from aquifer to river occurs for a longer period from October to June. Highest rate of groundwater outflow was observed for the reach_ 2 which represent 51-61 km and lowest for reach_ 4 which represent 71-81 km. It was also observed that the magnitude and duration of groundwater loss from aquifer to river is higher in upper part than in lower part of the study reach. Within the study reach, during the study period the yearly average lateral groundwater outflow from aquifer to river was estimated as 0.29 Mm3 per kilometer varies from 0.20 Mm3 to 0.45 Mm3 . It has also been observed that the trend of lateral outflow from groundwater is increasing for the recent years. It has also been observed that groundwater table goes below the suction limit (7.0m) for STW and HTW for most of the area in dry season due to excessive withdrawal of groundwater for irrigation and domestic use. As a result, the HTWs and STWs become inoperable in those areas during dry season. From the analysis of historical groundwater level data, it has been observed that there is a continuous lowering trend after 1999, in some places within the study area.