Feasible design and operational guidelines for skimming wells in the Indus basin, Pakistan

Abstract

This paper reports work done to assess the status of groundwater extraction technologies and practices in the Indus basin of Pakistan and hence to improve these technologies for sustainable groundwater extraction. A socio-technical approach was used which involved a field survey using participatory rural appraisal (PRA), monitoring of existing farmers’ wells for hydraulic and hydrosalinity behavior of these wells, and simulating hydrosalinity behavior under skimming wells using appropriate groundwater and solute transport models. The data collected in PRA shows a variety of wells designs, which reflects the absence of design code for these wells. Consequently, farmers have to choose one of the design options provided by the local drillers. We monitored a farmer's operation of a multi-borehole skimming well to extract groundwater to supplement canal water. Most of the time the well was operated daily, and occasionally on alternate days. The number of operating hours varied from 2 to 12 h per pumping event. We also conducted pumping tests in two wells, one with a single-borehole, and another with six boreholes. The data obtained at these two wells were used to develop guidelines for well design and operation using a flow model, MODFLOW and a solute transport model, MT3D. The parameters considered in the model studies were as follows: perforated well depth with respect to depth of the freshwater layer in the aquifer (i.e. well penetration ratio), number of boreholes and spatial distance between them in a multi-borehole well system, well discharge rate and daily operational hours. The results indicate that a single borehole well operated at a discharge rate of ranging from 32 to 180 m 3/h (9–50 l/s) can be operated successfully with a 30–60% well penetration ratio for an operating time of 4–8 h/day where the thickness of the freshwater layer ranges from 20 to 30 m. Multi-borehole wells consisting of four to eight boreholes at a spacing of 3 m can be installed where the thickness of the freshwater layer ranges from 10 to 20 m without compromising the quantity and quality of pumped water.