Interpretation of pumping from aquifer overlain by a low permiability zone using a numerical model.

Abstract

An understandmg of flow mechanism near an abstraction well is important for successful exploitation of groundwater resources. Properly planned and carefully conducted pumping tests provide basic information for the solution of many groundwater flow problems. Pumping test data are normally analysed using conventional curve-matching techniques. Although little information about the aquifer behaviour can be gained from this analysis, yet a far greater understanding can be achieved using numerical models. The two zone numerical model described by Rathod and Rushton (199'1) has been further developed and applied to interpret three pumping tests conducted in an alluvial aquifer in central Bangladesh. The model could reproduce the significant features of the aquifer behaviour and the overall agreement between the observed drawdowns and model results was very good. The important features of the flow mechanisms include: vertical components of flow in the vicinity of the well, leakage from overlying layer with fall of water table, well storage and well losses. The values of transmissivity and storage coefficient, as derived from numerical analysis, for the three test sites ranged from 940 to 1510 m2/day and 0.0006 to 0.001 respectively. Thk vertical hydraulic conductivity of the overlying layer was found to . . . vary from 0.005 to 0.007 m/day. The model has also been used to predict the response of the aquifer to pumping over a typical growing season. The results show significant drawdowns both at the pumped well and at outer boundary. Unless sufficient recharge takes place during the monsoon, the groundwater head will fall year by year. Since these predictions were made based on a model which had been verified only for a short period the results should be treated as an indication of the likely trend. Nevertheless, such information are quite useful for successful development of groundwater resources in the study area.