what is the method of extraction of surface water and ground water?
Answers
Groundwater extraction has taken place under a number of large cities and has resulted in subsidence. Notable subsidence occurred in central London between 1865 and 1931 due to water abstraction from the Chalk. Beneath Mexico City, there is an aquifer between 50 and 500 m below ground level. Progressive pumping from this aquifer for over 100 years has resulted in much of the old city settling by 4 m, and in the north-east of the city, settlement of 7.5 m has been recorded.
Problems with settlement have been noted in the Shanghai area, where up to 300 m of Quaternary deposits are present in the Yangtze delta. These deposits contain five aquifers that were extensively pumped during the twentieth century. In the early years (1921–48), only some 9 million m3 of water was pumped, but it was noted that a ground settlement of 23 mm per annum occurred and a 19-km2 area experienced settlement of more than 0.5 m. With the increase in population of the city, between 1949 and 1956, the extraction rose to 140 million m3 per year, resulting in an average settlement of 43 mm and an area of 93 km2 experiencing in excess of 0.5 m subsidence. Between 1957 and 1961, the abstraction rose to 200 million m3 per year and the ground settlement was 100 mm per year. Having recognized the problem that the water extraction was creating, the rate of pumping was reduced to 72 million m3 per year between 1966 and 1989, and the settlement dropped to 2 mm per year. However, in the period 1990 to 2001, the rate of extraction increased to 113 million m3 per year and the settlement increased to 16 mm per year. As a result of the groundwater extraction, the ground in the central part of Shanghai has settled by over 2.5 m. As the urban sprawl has extended, the settlement has expanded out such that now the whole of Shanghai has effectively suffered some ground settlement due to water extraction. This has caused major problems and damage to the infrastructure-sewers, roads, subway tunnels, and buildings.
Groundwater extraction near Pixley in California caused some 0.75 m of settlement between 1958 and 1963, the surface depression being mainly over the area pumped, with the effect decreasing with distance. In the Houston/Galveston region, the presence of faults has restricted the area affected by the dewatering such that a face up to a metre high has developed over a length of almost 17 km.