Unsaturated and Saturated Flow in Response to Pumping of an Unconfined Aquifer: Field Evidence of Delayed Drainage

This paper revisits a long-standing problem in aquifer analysis. In particular, it provides a physically consistent explanation of the characteristic time-drawdown response of unconfined sand or gravel aquifers to pumping. It also provides a physical explanation for the low values of specific yield commonly obtained by type-curve matching procedures. Tensiometers and piezometers distributed throughout the saturated thickness were used to monitor, in great detail, the hydraulic response of an unconfined sand aquifer to pumping. A gamma moisture gauge and core samples were also used to obtain measurements of water content in the region of the declining water table. The results showed the early period of time-drawdown graphs to correspond with the period of increasing gradients. Excess storage of water was observed above the water table and was shown to be a necessary consequence of the vertical hydraulic gradients and the water-content/pressurehead relationship for the aquifer material. The excess storage decreased during the period of decreasing vertical gradients. The period of delayed drawdown was shown to be the result of the decline in excess storage. Contrary to most of the recent literature, the results show that drainage processes above the water table have a very significant effect on the response of unconfined aquifers to pumping. The results indicate further that the normal methods of analyzing time-drawdown curves for unconfined aquifers give excessively high values of storativity and values of specific yield that are unrealistically low.