INTRODUCTION
Land subsidence is obviously one of those problems humanity has not yet managed neither to estimate properly nor to find out how to stop it. A reason for that is the fact that land subsidence may happen in virtue of multiple reasons, both natural and anthropologic ones.
Land subsidence is the result of numerous subterranean processes consisting in deformation and displacement of underground materials leading to the disfiguration and the change of level of the earthly surface. The most frequent natural causes of land subsidies are compaction, movements of tectonic layers, collapse of karst caverns, thermokarst depression, drying up of underground materials etcetera. To the contrary, the most common anthropological causes of a land subsidence are removal of solids, disruption of permafrost and pumping or removal of subterranean fluid. The following table illustrates the processes of subsidence in different parts of the world as well as compares their estimates in figures.
In this paper I will undertake an attempt to analyze how water pumping influences the process of land subsidence using two specific examples – the one of Mexico city and san Joaquin valley. I will draw a comparison on how processes differ in two places and try to provide some prognosis given the existing evidence.
The state of California is a good example of a place where land subsidies began to happen in the wake of an anthropological influence. Due to the great volumes of water extracted from the ground from 1925-1926 up to now for mostly irrigational but also industrial and domestic purposes land subsidies began to happen in the San Joaquin valley. The primary problem with the valley is that underlying materials consist mostly of alluvial deposits which in the circumstances of dehydration start to destruct and make the surface compress. Given that water is taken from as far to the depth as 200 meters dehydration of the subterranean materials is so significant that it led to a subsidence of more than nine meters so far. The figure is really huge as compared to many analogies (Carbognin, 1985: 26).
Excessive pumping of water for irrigational purposes started in the 1930s drastically increasing after the Second World War, in mid-40s. According to the estimates already in the following decade San Joaquin valley water pumping volume was considered to constitute one quarter of the entire United States volume of pumping (Carbognin, 1985: 27).
The results have been impressive. 20 billion square meters are considered to have disappeared in the course of the last fifty years.
MEXICO CITY LAND SUBSIDENCE.
Mexico-city subsidence is very similar to that happening in the San Joaquin valley in terms of the scope since the move of land has constituted 9 meters so far. However, whereas compaction in case of San Joaquin valley happened to numerous aquitards in Mexico city the compaction happened majorly in the upper half centennial of meters right beneath the surface. The direct reason why subsidence began is connected with the rapid growth of the city in the last century which led to the need of better and vaster water supply. Accordingly, wells were drilled in large numbers whereas increasing population of the emerging mega polis demanded more agricultural irrigation as well. 1925 was the year when subsidence is confirmed to have begun. The rate at which subsidence happened was first estimated at 4 cm per year, within the period of 1925-1938. The following decade however, the rate tripled and spilled over into the sixfold speed in early 1950s. However, in 1970 the rate of subsidence slowed down to initial 4 cm per year and has not increased since that time (Carbognin, 1985: 27).
Nowadays subsidence in Mexico city is considered to be in recession if not ended. However, this recurrently leads to problems in architecture planning. The level of subsidence was uneven in different places in the city, transport logistics is very hard to elaborate and drainage systems are hard to construct (Carbognin, 1985: 27).
CONCLUSION
In the last less than one hundred years land subsidies have become to be initiated mostly due to anthropological than natural reasons. In both of the cases that were considered in this paper pumping water for irrigation, domestic and technical uses led to underground materials destruction. The reason for that is the fact that subterranean deposits are mainly constituted of unconsolidated sediments that have the high level of porosity. When the underlying aquifers that include mainly gravel or sand characteristic with high permeability lose water they become less cohesive and are more subject to slow destruction. For instance, this is especially the case with Arctic and sub-Arctic regions where thermokarst deprived of water gets very frail (The Human Impact on the Natural Environment).
The Mexico city and the Joaquin valley examples have very much in common but they also have some differences. The primary common characteristic is the reason why land subsidence happens which has already been commented on before. The second characteristic is that the level of subsidence has reached a similar level – 9 meters. However, differences are also serious. First of all, in case of the valley we talk about the area with a far more diverse relief. Therefore, the subterranean processes have their peculiarities given that. Mexico city is another case. Another difference is that land subsidence has stopped for now in Mexico but still is an ongoing actual problem in San Joaquin valley.
Potential risks are huge not only for the two areas concerned but for any spot on the planet. The first worst-case scenario is the damage for architecture all over the world, since relief of cities and densely-populated areas in general are going to disfigure by the compressing processes. The second worst-case scenario is the humanity will lack water since it will disappear incrementally in all the comparatively accessible regions in the world.
Works Cited
Carbognin, L., 1985. “Land Subsidence: Worldwide Environmental Hazard.” Nature and Resources, vol. 21, pp. 2-12.
Delta-Mendota Canal: Evaluation of Groundwater Conditions and Land Subsidence, N.d. California Water Science Center. Web. 26 Feb. 2016.
Galloway D. and Riley, Francis, S., 2011. San Joaquin Valley: Largest Human Alteration of the Earth’s Surface. US Geological Survey. PDF.
Goudie, A., 2000. The Human Impact on the Natural Environment. Fifth edition. Cambridge: MIT Press.
The Human Impact on the Natural Environment. UCLA Course Reader Solutions. Print.
