Study on groundwater recharge and discharge characteristics over a reach of ganges river

Abstract

In this study groundwater recharge and discharge characteristics are investigated over a selected reach of the Ganges river from Hardinge Bridge to Rarnpur Boalia of Rajshahi district. Four transacts were selected on the Ganges river such that maximum number of wells fall along these transact lines. For each transact line the groundwater levels were compared with the river stage. The range of influence of groundwater movement was obtained by observing the relative levels of the groundwater wells with the river stage. Then the river was termed as affluent or influent depending on whether the groundwater levels are higher or lower than the river stage. Ganges river shows a high seasonal variability of flows and levels. Annual maximum, minimum and average flows of the Ganges were estimated as 75,800 m3/sec, 261 m3/sec and 10,700 m 3 /sec at Hardinge Bridge. Annual maximum, minimum and average water levels were calculated as 19.35 m(PWD), 7.81 m(PWD) and 12.06 m(PWD) at transact 4-4 in Rampur Boalia and 14.97 m(PWD), 4.48 m(PWD) and 9.00 m(PWD) at transact I-I near Hardinge Bridge. Groundwater levels show a variation of around 10 meters between the maximum and minimum values in a given year. Correlation of river stage and groundwater level hydrographs were investigated for the period 1974- I994 for all the four selected transacts. This study reveals that the groundwater levels respond with the seasonal variation of river stages. In the dry months of a year, the groundwater levels were found to be higher than the river stage and the flow direction is from aquifer to the river. As monsoon approaches, the river stages rise at a faster rate than the groundwater levels and the flow direction changes from river to the aquifer. The computed hydraulic gradient of groundwater movement towards or away from the river varied from values as low as 0.005 mIkrn to as high as 13 mIkrn. Transmissivity of aquifer ranges from 875 m 2 /day to 6000 m 2 lday in ,the selected reach. The estimated transmissivity compared well with the values that were available from previous reports. For the purpose of studying the groundwater level fluctuation and trends, piezometric surfaces were generated for the years 1974, 1980, 1985, 1990 and 1994 for the month of February-March and July-August, considering all the groundwater observation wells and the river stages for corresponding weeks. During the dry periods, higher potential lines are seen to be away from the river and lower potential lines are near the river, meaning that flow is from aquifer to the river. In monsoon, this trend reverses with the higher potentia! lines being closer to the river and low potential lines lying further from the river. This means that flow is from river to the aquifer. Groundwater flow is generally from north-west to south-east direction. Higher values of groundwater flow occur towards the river in the lower part than the upper part of the study reach. The quantity of groundwater flow was estimated, and found to have a maximum value of 57 m3/day/m at transact I-I and 3.0 m3/day/m at transact 4-4 respectively. The effluent and influent effect extends upto about 300 m to 20 km in a direction perpendicular to Ganges flow, depending on the relative levels.

Net annual maximum groundwater recharge from the river was estimated as 640 m 3 /day/m and a minimum of II m3/day/m in the 70 km study reach. Net discharge from the groundwater aquifer towards river was estimated as 1519 m3/day/m to 117 m3/day/m. Trend of groundwater movement towards the river decreases annually at a rate of 13.80 m3/s during the period of 21 years.