3.7. seawater intrusion in coastal aquifers

3.7. Seawater intrusion in coastal aquifers

The exploitation of coastal aquifers always produces a lowering of the water table levels which are protected by the effects in the coastal strip of the sea level. When the extracted volumes are greater than the recharge, even on a local basis, a salinisation process begins in the aquifer as the seawater flows upon the land. In the light of this, the management of coastal aquifers is conditioned by the need to determine the maximum permissible penetration limit for each particular aquifer (MOPTMA-MINER, 1984).

The problem of aquifer contamination by seawater intrusion, together with the extent and seriousness of the problem, are mainly conditioned by three factors: the difference between the respective densities of the fresh and salt water, the hydrodynamic properties of the aquifer and the flow that the aquifer discharges into the sea.

The first two factors are intrinsic to the seawater intrusion problem regardless of the climate in the region. Furthermore, these two parameters are fixed, and cannot be modified by the usual anthropogenic activity. The flow that the aquifers discharge into the sea is conditioned by natural conditions (from rainfall) or artificial recharge (mainly from irrigation) and by pumping. Therefore, anthropogenic actions can bring about some modifications.

The main salinity problems in the semi-arid zones of Europe are caused by a scarce recharge by rainfall. This problem is further aggravated by the fact that the climates in these areas attracts an influx of European tourism, especially in the summer months. Such a situation causes imbalances, water shortage problems and costly and complicated management mechanisms.

Furthermore, the solutions that have been usually adopted in the fight against saline intrusion, are rather inappropriate to the semi-arid zones of Europe. The most common technique is to increase the groundwater flow from the aquifers to the sea. Another is to recharge water reserves in technically strategic coastal zones. However, it is difficult to obtain freshwater recharge sources in semi-arid areas. Slightly diluted wastewater constitutes a further problem when attempting to fight seawater intrusion and, finally, given that the recharge installations would have to cover an extensive surface area, the availability of land for such purposes is both scarce and costly and carries its own environmental impact.

3.7.1. Spain

A total of 58% of the 82 coastal hydrogeological units in Spain and the Balearic Islands show some evidence of seawater intrusion as a direct result of over-exploitation of the freshwater resources. In some cases (7%) this is a local effect around the pumping area, while in others (33%) this encroachment covers a larger area. In the rest (18%) this intrusion affects all the aquifer or most of it (MOPTMA-MINER, 1994).

Figure 3.7-1 shows the aquifers with saline intrusion problems in the Segura and Jucar catchment areas. There are some others in the Balearic Islands and in the Catalan area, but these are not regarded as semi-arid areas in the context of this report.

Figure 3.7.-1 Marine intrusion in aquifers in semi-arid areas of Spain

Notes:

All of them are used for human purposes. The mean annual Cl concentration are expressed as the lower limit.

* Guadiana basin:e1: Ayamonte-Huelva
* Guadalquivir basin:e2: Rota-Sanlucar-Chipiona; e3: Puerto de Santa Maria; e4: Puerto Real-Conil; e5: Vejer-Barbate
* Sur basin:e6: Bajo Almanzora; e7: Campos de Nijar; e8: Andarax-Almeria; e9: Campo de Dalias; e10: Albulñol; e11: Carchona; e12: Rio Verde; e13: Velez; e14: Bajo Guadalhorce; e15: Fuengirola; e16: Marbella-Estepona
* Segura basin:e17: Campo de Cartagena; e18: Mazarron; e19: Aguilas
* Jucar basin:e20: Plana de Vinaroz; e21: Plana de Oropesa; e22: Plana de Castellon; e23: Plana de Sagunto; e24: Plana Valencia-Norte; e25: Plana Valencia Sur; e26: Plna Gandia-Denia; e27: Peñon-Montgo-Bernia; e28: Orcheta.

3-7.2. Portugal

Problems in Portuguese aquifers are of little extension and are not serious, these appear in Coast-Central Algarve and Coast-Oriental Algarve, as Table 3.7-1 shows.

3.7.3. Italy

In the Island of Sardinia, there are four aquifers having sea water intrusion as Table 3.7-1 shows; all of them are used for human supply purposes.

Table 3.7.-1 Sea water intrusion in Portugal and Italy (Sardinia)

Notes:

(*) R: Mean annual recharge in 10⁶ m³/year
(**) E: Mean annual exploitation in 10⁶ m³/year
(***) Cl: Mean annual Cl concentration in mg/l

3.7.4. Greece

Marine intrusion problems have been detected in ten hydrologic departments, particularly in coastal areas and in the Aegean islands.