Artificial recharge of groundwater by injection well

摘要

The land area of the Kathmandu valley is 600 square kilometers (sq. km) in which about 400 sq. km. is the valley floor towards the center, the remaining 200 sq. km. area being the surrounding hills. The hills and the basement beneath the floor are composed of intensely folded, faulted and fractured igneous and metasedimcntary rock. Upon this basement complex rests unconsolidated to partly consolidated sediments which range from thin deposits along the valley floor margins to over 600 meters thick in the central portion forming the principal groundwater reservoir of the valley. The average annual rainfall in the valley is about 1580 mm, 80 per cent of which takes place during the monsoon, Jun-Sep. All the surface flow of the valley join the river Bagmati which drains out of the valley in the south at Chobhar. The maximum average monthly flow of Bagmati at Chobhar is 53.43 m3/sec in Aug and the minimum is 1.55 m3/sec during March. With the use of most of the available surface sources of the valley, the development of deep groundwater began since the early eighties. About 50 million liters of water per day is being withdrawn from the deep aquifer of the valley. This is above the safe yield, as there have been lowering of deep water level by about 20 meters and reduction in the well discharge by about half. With the utilization of all the surface sources and over exploitation of groundwater, Melarnchi Water Supply Project is being implemented through which 510 million liters water per day will be diverted to the Kathmandu Valley in three phases from the adjoining Indrawati river basin. Along with the implementation of this new project it has also become imperative to protect and wisely use the groundwater of Kathmandu Valley, it would at least be needed during emergency time and the peak of the dry months when the Melamchi sources may not be able to meet the demand of Kathmandu valley as some water will need to be released downstream in the river to meet the river's ecological needs. As such monitoring the level and quality of the groundwater has been started, and a program on registration and licensing of groundwater use is being developed. Along with these program on management of groundwater it was put forward to launch a program entitled artificial groundwater recharge pilot project with the objectives of injecting the deep aquifer with the surplus surface water available during the monsoon in order to improve the water supply condition and recover the depleted groundwater level. In the injection method of recharge water is allowed to pass directly into the deep aquifer through the screens provided in the well due to the pressure caused by rise in water level in the well due to injection of water into it. This method has been chosen because the aquifers to be charged are deep and confined and their recharge area is not known precisely. However given the high sophistication and cost of such a program the government wanted to drop the program, but later it was scaled down to a capacity of 10 lps only and to testing the feasibility at a more preliminary leve! with very limited cost as described below.