Extracting relatively-fresh groundwater from aquifers underlain by salty groundwater

The native groundwater in the central regions of the doabs of the Indus basin of Pakistan is deep and salty because of the marine origin of the hydrogeologic formation. Percolation of fresh irrigation waters has formed a relatively-fresh groundwater lens above the underlying native salty groundwater layer. The thickness of this relatively-fresh groundwater lens varies from a few meters to 150 m. The costs involved in the installation of horizontal subsurface drains to extract the water are prohibitive. Alternatively, shallow wells with small discharge rates, commonly known as the skimming wells, can be used to extract the relatively-fresh groundwater. Extraction of this water from inappropriate depths and at inappropriate rates will cause upconing' of the interface' between the relatively-fresh groundwater lens and salty groundwater layer, and draw marginal quality water to the root zone, resulting in an increase in salinity and sodicity. In this paper, two numerical models, MODFLOW (McDonald and Harbough, 1988) and MT3D (Zheng, 1990) were used to model the interface movement in an unconfined aquifer in Punjab, Pakistan. Data collected by Kemper et al. (1976) was used to calibrate and validate the models. Subsequently, the sensitivity of the depth to watertable, discharge rate, thickness of the relatively-fresh groundwater lens, well penetration ratio, and daily operating hours on the salinity of pumped groundwater was studied. The results show that skimming wells of 10-18 l/s discharge rates can be installed and operated successfully with 60-70% well penetration ratio for an operating time of 8-24 h per day from an unconfined aquifer with 15-18 m thick relatively-fresh groundwater lens.