Hydraulic and hydro-salinity behavior of skimming wells under different pumping regimes
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Skimming wells are meant to extract top fresh water layer in the fresh-saline aquifer. Their development in the Indus basin occurred through private sector in a technological vacuum. As a result, these wells have some technical, environmental and social constraints, which hinder the sustainability of these wells. As an initial step to improve the well technology, the hydraulic and hydro-salinity responses of the fresh-saline aquifer under different pumping regimes need to be monitored. The present paper reports the hydraulic and hydro-salinity behavior of the Indus basin aquifer in Pakistan under field conditions at farmers' wells. Two sites, having 6- and 16-strainer wells were monitored during July 2000-December 2001. The 6-strainer well was operated for 4 h with single-, 4- and 6-strainer arrangements and the spatial behaviors of specific drawdown were observed under these arrangements. The 16-strainer well was monitored continuously for the above period. The well discharge, pumped water quality and pumping duration was recorded of every pumping event under farmer's practice to extract groundwater. The rainfall and temporal water table fluctuation was also recorded at this site. The impact of 24 years of well operation on groundwater quality was observed by comparing the hydro-salinity profiles of 1974 and 1998 under 3-strainer well. The results showed that the specific drawdown was higher for single-strainer and it decreased with the increase in number of strainers in skimming wells and hence reduced the chances of saline-upconing. Each strainer in multi-strainer well contributes equally in well discharge provided the horizontal distances among the strainers are equal. The pumped water quality in fresh-saline aquifer was a very sensitive function of fresh water recharge and pumping duration. It was observed that with the increase in daily operation from 2 to 12 h per day, the pumped quality deteriorated three-folds and there was also 30% reduction in well discharge due to high suction lift. It was observed that continuous operation of a 3-strainer well having discharge of 141ps over the 24 years had raised the fresh-saline interface (iso-concentration line of 1.5 dS/m) to 9 m. Keeping in review the observations, it is recommended that the daily operation of 4-6 h keep the water quality within marginal limit (<1.5 dS/m) and the pumping operation is also cost-effective with only 15-20% reduction in well discharge for the study area. (C) 2003 Elsevier Science B.V. All rights reserved.
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