Maximum Water-Table Drawdown at a Fully Penetrating Pumping Well

Abstract

The magnitude of water-table drawdown achievable by a fully penetrating pumping well in an unconfined aquifer commonly is limited to a small fraction of the initial aquifer thickness. The maximum potential water-table drawdown can be estimated based on the aquifer thickness, the pumping well effective radius, and the estimated radius of influence in the aquifer during maximum steady-state pumping (Kozeny, 1953). The maximum steady-state flow rate into a well can be predicted if the aquifer hydraulic conductivity is known based on observed water-table drawdown during a pumping period.

To achieve the maximum potential pumping rate from a fully penetrating well in an unconfined aquifer, the water level inside the well must be maintained at the bottom (Kozeny, 1953; this paper). This condition creates a seepage face along the inside of the well screen. The hydraulics of the seepage face control the removal of water from the aquifer and the size of the resulting cone of depression. The area of the seepage face, which is a direct function of the effective radius of the pumping well, strongly influences the maximum potential water-table drawdown and the maximum steady-state pumping rate.

The vertical component of the hydraulic gradient in the formation is downward and increases toward the pumping well. The hydraulic potential, therefore, decreases with increasing depth below the water table. Only shallow observation wells that are screened across the uppermost fraction of the saturated zone are adequate for delineating the actual water-table position during pumping. Observation wells screened across deeper portions of the aquifer, including fully penetrating, fully screened observation wells, exhibit potentiometric drawdown in excess of true water-table drawdown.