Hydrogeology is the science that studies the groundwater, it´s origin and formation, it´s movement and it´s physical, chemical and biological characteristics.

Geology affects the operation of the aquifers because the groundwater moves inside these geological formations.

In Spain, these geological formations identifies four different types of aquifers.

  1. Zones formed by loose or semiconsolidated materials such as gravel, sands and silt that cover the valleys of the major rivers like the Ebro and Guadalquivir, and deposits of similar nature that extend into the great plateaus of the Duero and Tajo , and coastal areas and deltas of the Llobregat and Ebro or in the Planas of Castellón and Valencia, among others.  Detrital aquifers are exploited and provide water to industries and especially irrigated agriculture, occupying an area of 99,000 km ² approximately.
  2. Zones with carbonated rocks, generally limestone. This areas arise in the Eastern and southern sector of the peninsula and the Balearic Islands. These aquifers carbonated cover an extension of 69,000 km ².
  3. In the western sector of the peninsula appear igneous rocks (slates and similar), materials classified as waterproof materials or with very low permeability, but with aquifers of local interest. There are tens of thousands of springs and wells in those areas that supply to small towns and to the agricultural industries.
  4. In the CanaryIslands the aquifers are linked to volcanic rocks.A great part of the water used in the islands is of underground origin. The extension of these aquifers is of 7,800 km ². Inside of the peninsula appear little volcanic aquifers in Olot and Campo de Calatrava



Soil permeability represents the speed at which a fluid can penetrate in it. When the degree of permeability of a soil is high, the rain water will penetrate easily by the pores. When the permeability is low, the rainwater will tend to accumulate on the surface or will move by the same, if the field is inclined.

Permeable outcrops are part of the aquifers geological formations that are on the surface of the soil, and whose permeability is sufficient to allow recharge by infiltration. The characteristics of these materials influence in the rate of aquifer recharge and in the chemical properties of groundwater.

The following map shows the classification of the permeability in the Spanish territory according to the lithostratigraphic map on scale 1:200.000 developed in 2006 by the Geological Institute of Spain (IGME).

Mapa litoestratigrafico

Figure1. Lithostratigraphic map on escale 1:200,000 (Geological Institute of Spain (IGME).Source:Sistema Integrado de Información del Agua. SIA



An aquifer is a geological formation, formed by one or more layers of rocks (gravel, sand, limestone ...) located in the saturated zone, able to store and transmit to the free water in important quantities. Is characterized to own a significant permeability and a considerable extension and thicknes.



Aquiferscan be classified according to different criteria:

  • According to the lithology: detrital and carbonated
  • According to the type of holes: porous, karst, fissured
  • According to the hydrostatic pressure: free, confined and semi-confined

1.-  According to the lithology or type of holes

  • Detrital aquifers: are aquifers of rocks or detrital sediments. It´s permeability is due to the intergranular porosity (of primary type). If the rocks are partially consolidated or cemented, the porosity can be besides of secundary type, by cracking or dissolution.Are all the materials with grain size of sand: sands, arkoses, arenaceous, gravel, conglomerates, etc..
  • Aquifers cracked or karst:are aquifers in carbonated rocks (limestone / dolomite) or other types of rocks with fracturing or dissolution (igneous, metamorphic, detrital).These aquifers have permeability due to fissures from mechanical and dissolution origin. Are the limestone, dolomite, gypsum, granite, basalt.., being the first the most important types.
  • Aquifers mixed: the porosity is due to a several causes. An example are the calcareous sands.

Acuiferos caracteristicas litologicas

Figure 2. Types of aquifers according lithology characteristics: Sistema Integrado de Información del Agua. SIA



2.- According to the Hydrostatic Pressures

  • Free Aquifers: also called phreatic or unconfined. Amongthem there is a real free surface and stored water in contact with air and atmospheric pressure.Between the surface and the phreatic level is the unsaturated zone. The surface until the water arrives, is the phreatic surface; when this surface is cut by a well can we talk of  the phreatic level in that point.
  • Confined Aquifers: Also calledcaptive.In them, the water is subjected to a high atmospheric pressure and totally occupies pores or hollows in the geologic formation, saturating it totally.If we extract water, the water pressure decreases, but the pores don´t empty. When pressure decreases, there may be land subsidences. In this type of aquifers there isn´t non saturated zone. In case of this type of aquifers were perforated, the water level would rise a certain position which coincides with the saturation level of the aquifer in the recharge area. This is the piezometric level. Uniting all the piezometric levels, we will obtain the piezometric surface (virtual surface formed by the points that would reach the water if we make infinite perforations)
  • Semiconfined aquifers:This type of aquifers is more frequent than the confined aquifers, affirming that are pressure aquifers, but some confinantes layers are semipermeables, called aquitards.
  • Hung Aquifers: produced by a strong effect of the water recharge, in wihch, the phreatic level increases and a little portion of water is retained by a impermeable level.
  • Multilayer Aquifers:  special case of aquifers with different levels of permeability.

Acuiferos según estructura y funcionamiento

Figure 3.Types of Aquifers according to the structure and functioning (free / confined). Source: Sistema Integrado de Información del Agua. SIA

Acuiferos caracteristicas hidrodinámicas

Figure 4. Types of aquifers according to the hydrodynamics characteristics. Source: Sistema Integrado de Información del Agua. SIA



The environmental objectives determined by the Directive by underground waters are:

  • Avoid the entry of contaminants and the deterioration of groundwater
  • Protect, improve and regenerate the groundwater and ensure a balance between extraction and recharge
  • Reduce progressively  the contamination of groundwater


The A.G.U.A. program has identified a total of 740 groundwater bodies, that cover around the 70% of the Spanish area. Many of these masses are associated with aquatic ecosystems that there are to consider in the water extraction.

Aguas subterráneas

Figure 5. Groundwater. Sistema Integrado de Información del Agua. SIA


Tabla 1. Groundwater in Spain. Sistema Integrado de Información del Agua. SIA



The pressures on the groundwater are caused by human activity.

  • Puntual pollution
  • Water extractions
  • Seawater intrusion
  • Water pollutionof diffuse origin
  • Artificial rechargeof aquifers

These pressures can affect the chemical or quantitative state of groundwater Good quantitative state is reached when the groundwater level does not change and there are no changes in flow direction. The chemical state depends on the conductivity and the concentration of pollutants, and must fulfill the legislation.



The impacts according to the Hydrologic Plan designed by the A.G.U.A Program, are classified as:

  • "Comprobados": in waters in which the law is not fulfill or bodies of water overexploited.
  • Probable: possible when the environmental objectives are not reached in the 2015.

In Spain, the percentage of waters in which impacts have been found  is around 36% , the probable impacts supose the 12.43%, 27.43% of the waters don´t have any impacts and 23.24% of the masses have not data.

The southeast of the peninsula is the zone with greater hydric stress, where the extractions have increased.



Methods of contamination

The pollutants can reach groundwater by various means: The pollutants can reach groundwater by various means:

  • Infiltration of substances deposited in surface
  • Filtration of substances stored under the soil or dissolved  by groundwater.
  • Filtration from other rivers
  • Accidental spills from tanks or conductions
  • Through abandoned deposits (from surface)
  • From another aquifer
  • Directly in wells

Polluting activities

  • Deposit of urban solid residues
  • Waste water discharges
  • Agricultural activities
  • Cattle activities
  • Industrial and mining activities
  • Nuclear activities

Reaction of aquifers to pollution

  • Dilution
  • Mechanical filtration
  • Precipitation
  • Oxidation-redution process
  • Adsorption
  • Disintegration or descomposition

Prevention and control

  • Control in the origins of the contamination
  • Control in the catchments

Corrective measures

  • Activities in the origin of the contamination
  • Isolation of residues
  • Removal of residues
  • Diminish the phreatic level (with barriers or bulges), in case of this level reaches the residues
  • Actions in the aquifer
  • Use of bacteria to degrade the organic pollutants
  • Intense pumping of the contaminated aquifer
  • Pumping of the aquifer in case of non-miscible products
  • Use ofair or steamto volatilizethe contaminants



The following graphs and tables represent the concentrations of nitrate in the Spanish groundwater in the period of time between the years 2003-2007. These values  are represented also by hydrographic demarcations.


Figure 1. Nitrate concentration. Sistema Integrado de Información del Agua. SIA


Tabla 2. Nitrate concentration by hydrographic demarcations:Sistema Integrado de Información del Agua. SIA




An spring can be defined as a point of soil surface  in which, naturally, an appreciable amount of water from an aquifer.

The composition of spring water varies with the nature of soil or rock. The flow of the springs depends on the season and the volume of rainfall.



 Catalogue of soundings of the Ministerio de Medio Ambiente y Medio Rural y Marino


  • Aquiclude: porous and impermeable formation that contains water, but doesn´t transmit it and therefore it isn´t possible exploitation. They may constitute the limits of the aquifers.
  • Aquitard: semipermeable geological formation that contain water and transmit it very slowly, is not suitable for the location of water catchment, but under special conditions this formations allow an important vertical recharge of aquifers.
  • Acuifugo: geological formation with null porosity and therefore without capacity to store or transmit water. This formations may constitute the limits of the aquifers.
  • Phreatic level: limit of saturation, differentiating two zones: saturated and unsaturated.
  • Piezometric level: level of free water over the sea, in unconfined aquifers, and in the confined aquifers is defined as the level that would reach the water inside of a well until balanced with the atmospheric pressure.
  • Piezometric surface: geometric place of the points that indicate the piezometric level of each one of the portions of a aquifer to a certain deep. Are represented by lines of equal piezometric level, called isopiezas lines.