Significance of hydraulic head gradients within horizontal wells in unconfined aquifers of limited saturated thickness

Abstract

• Significant hydraulic head gradients often occur in horizontal wells. • Field results are obtained for a horizontal well in a shallow aquifer. • Computational models successfully reproduce field behaviour. • Inflows vary significantly along the horizontal well. • The effectiveness of horizontal wells of different length is explored. Horizontal wells can be used to withdraw water from shallow aquifers. Frequently there is a substantial hydraulic gradient within the horizontal well. This paper considers the significance of the hydraulic head distribution for horizontal wells in shallow aquifers and estimates the inflows of groundwater along the length of the well. Conceptual and computational models are developed to represent time-variant regional groundwater flow. Convergent flows towards and into the horizontal well and hydraulic conditions within the well are also included. A case study of a 300 m long horizontal well, curved in plan, in an aquifer of limited saturated thickness is used to illustrate the approach. Field monitoring indicates significant drawdowns in groundwater heads in the vicinity of the well. Substantial difference in hydraulic head between the pump end and the far end of the well are also identified. Successful comparisons are made between field and model results. From the numerical models, groundwater inflows along the well are estimated; inflows at the pump end are about four times the inflows towards the far end. The effect of reducing the length of the well is explored.