Groundwater – Environmental Encyclopedia

Groundwater is water that is found below the surface of the earth and occupies the pores and cracks of the strongest rocks. In general, it maintains a temperature very similar to the annual average for the region, therefore, in arctic regions, it can freeze.

The deepest groundwater can remain hidden for thousands or millions of years. However, most reservoirs are shallow and play a discrete but constant role in the hydrological cycle.

At the global level, groundwater represents about twenty times more than the total surface water of all the continents and islands, hence the importance of this water as a reserve and as a freshwater resource. In addition, it has an important role in nature. The effect of the large water reserve on the annual flow is essential for maintaining the base flow of many rivers and soil moisture in banks and low areas.

Groundwater is of essential importance for our civilization because it represents the largest reserve of drinking water in the regions inhabited by human beings. It can appear on the surface in the form of springs or it can be extracted by sinks. In times of drought, it can be used to maintain the flow of surface water, but even when there is no shortage, it is better to use groundwater because it does not tend to be contaminated by debris or microorganisms. Although groundwater is less polluted than surface water, contamination of this resource has also become a concern in industrialized countries.

Currently, in Spain, groundwater extraction provides an approximate volume of 5,500 cubic hectometres, of which a maximum of 1,500 are used to supply drinking water to populations and the rest to agricultural irrigation.

It should be noted that the role of groundwater in the supply of drinking water is relevant, because the supply of a population of more than 12 million inhabitants depends on it. The use of groundwater for irrigation in Spain is no less spectacular, as it has brought significant benefits which, in many cases, have exceeded those obtained with the use of surface water.

Underground waters

Due to a series of uncontrolled actions by certain individuals, the aquifers have been overexploited. This has deteriorated sharply, causing problems of various kinds, such as the depletion of reserves, the deterioration of water quality, economic and environmental impacts, etc.

The problem of overexploitation affects a large number of aquifers used as a source of drinking water supply, it is estimated that at least 58 hydrological units used for this purpose suffer from this problem, this means that at least between 20 and 30 % of groundwater resources see their sustainability threatened.

Groundwater mobility depends on the type of rocks underground at each location. Saturated permeable layers capable of providing a useful supply of water are called aquifers and are usually made up of sand, gravel, limestone or basalt. Other layers, such as clays, shales, glacial moraines and silts, tend to reduce groundwater flow. Impermeable rocks are called water-repellent rocks or basement rocks.

In permeable zones, the surface layer of the water saturation zone is called the water table. When excessive groundwater is extracted from the ground in densely populated places or arid areas with heavy irrigation, the water table can drop very quickly, making it impossible to access it, even using very deep wells.

Systematic control and monitoring of groundwater are recent. Let us see, for example, the evolution in Catalonia.

In 1995, the actions of the Sanitation Board in the field of groundwater began to manifest themselves. The consolidation of the team of hydrogeologists that make up the groundwater unit has made it possible to expand the study of areas and aquifers of interest, as well as to give strong impetus to the quality control network, which now has more than 900 wells.

Direct use of groundwater in Spain

Regarding the study of areas and aquifers of interest, it is issued:

  • Insightful and binding reports, in accordance with the provisions of Law 19/1991, on the reform of the Sanitation Office, in the concessions and authorizations of hydraulic resources and uses, and other issues of a similar nature.
  • Reports on environmental conditions referring to specific contamination episodes or at risk of conditions, response to allegations, opinions and specific studies.

With regard to quality control, there are control networks in the areas or aquifers of the Pía de la Plana de Vic, the Ter in its middle part, the Aubí and Calonge streams, the Ridaura, the Pía d ‘Urgell, of the Congost river, of the Mogent, the middle and lower Tordera, the Conca de Barberá, Carme-Capellades, the Llobregat (Abrera and Sant Andreu basins, lower part and delta), the Camp de Tarragona, the Sorres from Santa Oliva and the Ebro.

These networks include more than 900 wells. The physico-chemical composition of the water (ion balance) is determined annually. Volatile organohalogens, non-halogenated solvents and metals are selectively determined on a quarterly, half-yearly or annual basis, depending on the vulnerability and the initial state of the various risk areas. In addition to this primary control network, a secondary network is operated in relation to specific pollution episodes, such as organohalogens in the Llobregat delta, dioxanes in Tordera, gasoline in the Baix Llobregat or others.

Also worth mentioning is the salinity network due to marine intrusion in coastal aquifers, a control network of some 300 wells from the Gulf of Roses to the mouth of the Sénia River. This network is part of the primary control network and is sampled annually.

Like Catalonia, other Spanish regions have advanced in the study and control of groundwater, having more and more technical knowledge about them and making their control and better use possible.

Groundwater use has been the traditional source of our consumptionGroundwater use has been the traditional source of our consumption

Wastewater treatment group. Polytechnic University School. University of Seville.

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