The role of groundwater in the management of water resources in the World

Abstract

Groundwater forms about 98% of all the fresh liquid water available in the World. Even so, groundwater is the forgotten and most ignored part of the water cycle. Any time a drop of water infiltrates into the soil, it enters an occult world where still today many mysteries need to be solved. Studies can sometimes prove the journey of water under the soil, from the infiltration point to a discharge point, but it is practically impossible to know the exact path of this drop along the subterranean journey. The same is true for groundwater quality. The infiltrating water begins to interact chemically with the soil particles immediately it enters the soil, within the unsaturated zone. When it enters the saturated zone (aquifer) it interacts with the rock and weathered materials, dissolving minerals, interchanging ions, depositing in other places previously dissolved substances, in a permanent change that includes interaction with human activities, be it capture by a pump in a well or the contamination caused by infiltrating residual water from agriculture or a sewage system. Interaction with surface water systems is basic for life on Earth, for both plants and animals depending on water. With its capacity to regulate the water flow in the World, groundwater is responsible for the presence of surface water in many arid and semi-arid regions of the World during the driest season. In fact, the water in rivers flows at velocities much higher than in most parts of the aquifers, and sometimes the groundwater flow is so slow that rainwater infiltrated in the soil returns to the superficial water system months or even years after infiltration, sometimes bringing water to rivers during periods of severe dryness. Such regulatory control needs more study and attention from the regulators and users, since any disequilibrium in these fragile systems can affect all dependent life. This is particularly true for countries with a Mediterranean climate influence. In relation to quantity, groundwater is normally under stress in countries with lack of surface water, a high imbalance between the rainy season and the dry season, and strong rates of evapotranspiration. The stress is also more common in highly developed countries, and in less developed countries with high rates of current development. Of these countries, there are some where groundwater is still renewable and where two types of situation can occur: those where the quantity of abstracted water is less than the average infiltration rate and the resource is still sustainable, and those where the abstraction rates have long exceeded the infiltration rates. In the latter case, the resource is not sustainable in the future and other related issues can happen, such as land subsidence problems following the depletion of groundwater resources. But there are countries in desert areas in which the rain is so scarce that practically no recharge is possible. In these climatic environments, the abstracted groundwater is actually fossil water, infiltrated thousands of years ago, and this abstraction must be considered “water mining”. In this case, groundwater must be considered as any other geological resource, non-renewable for many generations to come. In some of the less developed countries the situation is not so desperate, but it is just a matter of time. Currently, the actual economic conditions do not permit the use of drilling techniques, and deficiencies in the supply of energy do not permit the high costs of abstraction. Once this situation is solved in these countries, the pressure on the aquifers will begin, as happened before in other countries. In relation to quality, a fraction of groundwater does not have natural quality adequate for water supply or for use in agriculture. This can be the result of several processes, such as the water retained in the geological formations when the rock was formed (many of the rocks that form aquifers were