M Salgot A Torrens

Institut de l'Aigua de la Universit├Ąt de Barcelona, Laboratori d'Edafologia, Facultat de Farmacia, Institut de Recerca de l'Aigua, Universitat de Barcelona, Joan XXIII, s/n. 08028 Barcelona, Spain (e-mail: M. Salgot: [email protected])

Abstract: In an era of increasing contest for limited water resources, the wise joint management of conventional and non-conventional water resources must be considered. Water scarcity is aggravated in coastal zones which are often characterized by high population densities and intense economic activities making heavy seasonal water demands. In this context, the use of non-conventional water increases the availability of water supplies. Non-conventional water resources of lower quality could be directed to meet additional needs. As a consequence, significantly more potable water would be available to meet human demand for safe water.

Non-conventional water resources are described: waste water reclamation and reuse, and its potential application for increasing groundwater resources, as well as several practical applications.

Water demand is increasing in the south of Europe due to the economic development. The southern parts of Portugal, Spain, France, Italy and Greece are strongly associated with tourism activities. Coastal resorts, golf courses, and other water-consuming facilities are spreading throughout the tourism-related areas. Agricultural water demands as well as industry and services in localized areas are also growing in several parts of these countries. Population all around the Mediterranean borders is experiencing additional population growth because of foreign and European immigration (the latter comprise mainly retired persons).

Nevertheless, a difference arises when comparing coastal areas with the interior ones. Population has a strong tendency to inhabit coastal areas for several reasons. These include a mild climate compared to inland, important economic activities related to transportation, and other benefits such as landscaping aesthetics, flat areas for living and access to bathing waters. Also, due to the ease of transport by ship, road or railway, a lot of industries are located near harbours on the coastline.

Among the richest agricultural lands, deltaic and similar areas occupy a prominent site. Vegetables, rice, corn and other rewarding crops can be grown in such areas, using at the same time huge amounts of water. Citrus and other orchards also need to be located in the vicinity of the sea or in places with mild climate.

Mass or select tourism demands sunny, politically safe areas with good transportation facilities, located close to tourist and cultural sites. The people involved, and the associated golf courses, parks and the like, are demanding water in large quantities. An important part of the Mediterranean, including its islands, is meeting all or almost all of these conditions.

Considering all these activities in terms of water resources, we notice a concentration of demand in time and space. This happens for the majority of cases mainly during summertime and in a fringe reaching a few kilometres inland from the sea. Demands in the inland areas are usually more evenly distributed during the year, although it depends on the existence of irrigated areas and agricultural patterns.

Climatic changes will affect the pattern of water resource availability, thus making it more difficult to cope with water demands and increasing health hazards (McMichael 1997). Molles (2002) indicates that the global circulation models predict an Earth more subject to drought, particularly in the interior of continents. Water availability is expected to be limited for populations in many regions because of changes in precipitation and increases in evaporation (Findlay 2003).

These circumstances make it even more attractive to find non-conventional solutions to fulfil the water demands. Among the solutions, treated wastewater reuse through groundwater recharge presents a number of advantages, namely the increase of water availability, the use of aquifers for water storage, and the long hydraulic detention times which contribute to the advanced treatment.

It is important to recognize that there is consumption of recycled water from both non-intentional and intentional recharge (Jimenez 2003).

Water resources

The usual way to cope with water demand is to use conventional water sources, namely surface and groundwater. Since surface waters are not evenly distributed along the coastline and groundwater resource is limited, water quantity must be increased through water transport or by using non-conventional resources. A classification of the different types of water resources based on their origin is shown in Table 1.

Although it seems that a balance between demand and supply should be obtained by increasing supply, the most sustainable way to do it is through the implementation of water-saving measures. This sometimes means a reduction in demand increasing efficiency, or a reduction in the extraction of natural water through the use of several non-conventional water resources.

In coastal arid and semi-arid climate locations, usually corresponding to areas where agriculture and tourism are located, surface waters (streams and lakes) can offer a limited amount of water resources. The limitation is due to the uneven distribution of rivers, a lack of streams on islands, a coincidence of minimal stream flows with peak demands, the need to maintain a minimal environment-related flow, and several other factors. Groundwater can offer a limited water supply because of low flow velocity, the need for pumping, and the need to maintain water barriers to control seawater intrusion. The storage capacity of aquifers must not be forgotten (Dillon et al. 2005).

New conventional resources can be obtained through infrastructures for water transportation from other basins. Nevertheless, this solution becomes difficult because of non-availability of resources, non-acceptance from donor areas, or financial limitations. Additionally, all existing conventional water resources are nowadays fully allocated in the developed countries.

As indicated above, when the availability of conventional water resources reaches a limit, additional supplies can be obtained from other water sources, mainly non-conventional ones, which include seawater and brackish water desalination, rainwater and runoff water, and reclaimed wastewater. Other possibilities are transportation using trucks, railways or ships. Socio-economic features are a key issue for implementing such alternatives. Transportation costs, energy demand and availability (desalting), and socio-economic factors when dealing with wastewater reuse are the limiting factors.

Water balance

From the viewpoint of balancing supply and demand of water resources, excess demand can be dealt with either by increasing supply or by limiting demand. Increasing supply will not create a balance, because there is a false sensation that authorities will facilitate an ever-increasing amount of water, which is by definition impossible.

On the demand side, water authorities can act by pricing (increasing prices, which are in relation to what is called 'demand elasticity') or by supporting water demand reduction through the use of water-saving devices and education campaigns (Griffin 2001) or through the use of taxes. It seems that important water savings can be obtained in agriculture, since this activity is the main water consumer in arid and semi-arid climates. Nevertheless, reduction in water uses in agriculture is not as clear a practice as it seems, because excess water in the agricultural fields contributes to the leaching of salts from the soils. When insufficient water is applied, salts can accumulate, several crops cannot be grown anymore, and soils can finally become completely unproductive.

Industry in developed countries can be persuaded to reduce water use through pricing and other easily implemented measures, while savings in domestic/urban demand are practicable in specific cases (e.g. improving distribution networks,

Table 1. Classification of water resources depending on the origin

Inside the basin External to the basin*

Conventional Non-conventional

Surface water Brackish water Classical transfers among basins

Groundwater Seawater Other movements (ship, railway, etc.)

Runoff

Reclaimed wastewater Dew, frost, fog, etc.

From Salgot & Folch (2003).

*The classification among conventional and non-conventional can also be performed here, if necessary.

installing in-house devices), although the savings are not as important as those of agriculture.

Coastline specificities

Because water-consuming activities on the coastline are located at the end of basins, there is almost always an association of scarce flows (in relation to demand) with scarce quality of existing flows, because of previous use and disposal in the basin. Apart from this, it is difficult to build water storage facilities near the points of use on the coastline. There is a tendency to cope with demand by first using surface waters (easy to obtain from the physical and economic aspects) and afterwards using groundwater, especially when peak demands are to be covered. Nevertheless, a lack of planning procedures for water extraction is usual in these cases, especially while existing resources can cover the demand easily. Planning efforts start when drought or scarcity appears, which in most cases cannot solve the problems.

Water resources and wastewater need to be conveyed to the point of use or to the point of treatment/disposal. As the territory is usually fully occupied by housing developments along the coastline, the only space usually free to install infrastructures is the shore. It is therefore typical to find water transportation infrastructures (pipelines, channels) along the coastline, especially for sewerage. The water for distribution reaches the inhabited areas, the industries and the agricultural areas in a localized way. Afterwards, there is the need to build sewerage facilities for non-consumptive uses, mainly urban and industrial.

Agriculture can also generate water-related pollution problems. As water needs to be applied in excess of water demand, infiltration or irrigation tails are at all times heavily charged with salts, nutrients and pesticides. This generates a negative impact on existing water resources, either ground-water or surface water (diffuse pollution), and occasionally seawater if the agricultural area is located in the vicinity of the shoreline.

Sewerage facilities on the coastline present additional problems because of the relationships with seawater. Seawater can enter sewers through discontinuities or through old outfalls, as tides or strong winds force seawater into the conduits. Building activities on the shoreline tend to pump water (mainly seawater under the sand) and throw it into the sewerage.

When groundwater is extracted to excess in the vicinity of the interface with seawater, the consequence is seawater intrusion into the coastal aquifers (Custodio & Bruggeman 1987). This situation is typical in locations with water stresses all over the world. In recent years, several solutions have been proposed, from an integrated management of aquifers to groundwater recharge using reclaimed wastewater. Aquifers can play a key role for water storage in coastal areas, as it is impossible to build important surface storage infrastructures near the coastline due to the lack of suitable locations.

Water use in coastal areas normally has uneven patterns throughout the year. Peak demands from different users (agriculture, tourism, year-round and seasonal inhabitants) tend to concentrate during the summer months, thus exerting a strong pressure on water distribution facilities and on water authorities. It could happen that several users exert demand in two or more different sites, if they own two or more permanent or temporary residences along the coast in the same country or in different ones.

Urbanization of the coastline (resorts, cities, industries, surface and ground urban infrastructure) disturb natural water circulation in three different ways. The first is related to 'natural structures' for groundwater circulation from mainland to the sea; underground railways, roads, etc., create artificial barriers. The second is the implementation of infrastructures that modify the surface water circulation (high buildings, urbanization of inundation areas and wetlands) or are built over rivers and small stream beds. The third is the occupation of infiltration surfaces with impervious structures (car parks, buildings, streets). Then, groundwater and surface flows are reduced, and the normal circulation of water is not allowed, creating a different water flow model at the land-sea interface, which can extend to thousands of metres and even kilometres towards the interior. It needs to be managed if specific events or natural hazards occur. Then, so-called 'geohazards' can appear and generate 'natural disasters', which are to some extent facilitated by human infrastructures (Coch 1995).

As a result, wastewater from industries and towns and stormwater are concentrated at the end of the pipes and need to be evacuated, usually to the sea, because there are no other solutions except reuse. Submarine outfalls are the classic solution for wastewater, but stormwater needs different solutions. Urban runoff coming from rainwater does not necessarily imply clean water, because runoff is effectively cleaning the entire town, from streets to sewerage systems, including roofs and other impervious surfaces. Since there is no possibility for water to infiltrate, such water reaches the sewerage if unitary systems are implemented, or the rainwater elimination systems if two separate networks exist (Metcalf & Eddy, Inc. 2003).

In the case of heavy rains, runoff reaches the sea along the coastline through alleviating devices or by surface infrastructures, thus polluting seawater. This creates problems in bathing areas, because of water quality problems during several days or hours (depending on the tides and currents). The problems are due not only to faecal contamination, but to suspended solids that can include dead animals or huge suspended solids. Apart from the aesthetic problems, health-related risks appear temporarily.

The existence of harbours and marinas in the coastal environment should not be forgotton. Those facilities can exert a certain influence through the dissemination of wastewater and loss of paints and other chemicals from ships. Then, neighbouring systems (bathing water, sand) become heavily contaminated, but the contamination is not of land origin. Also, the facilities located on the coastline modify the circulation of solids along the coast, and can affect the extension or form of beaches.

Mainland specificities

Usually, water users in the upper and middle parts of the basins experience fewer problems than users located at the end of the basins. The reasons are numerous, from the better quality (water has not been used before) to higher amounts of water available. There are other features to be considered, such as availability of specific sites for building dams and diverting water to the presence of wildlife. Groundwater is also used when water needs are located away from the surface water sources. Irrigated polygons also appear in these areas, where water is diverted from rivers, dams or lakes to usually open channels for water transportation and distribution. Usually, the demands are covered with the existing resources.

As in the coastal areas, agriculture can generate pollution, usually through non-point sources. There are also point source pollutions, derived from cattle facilities, towns and industry. In comparison with coastal areas, the territory is not fully occupied, except in the case of big cities.

Water uses in inland areas have a pattern of use derived from water demands; but scarcely related to tourism. Main demands are associated with irrigated agriculture and existing towns. Peak demands are not as strong as in the case of coastlines. Infrastructures are distributed along a more extended territory and so their impact is 'diluted' in some way.

In any case, there is a need to find a place for the final disposal of effluent generated in domestic, industrial or cattle wastewater treatment facilities. Rivers or lakes are the most usual disposal sites, but this creates point pollution. Alternatively, soil-plant systems can be used, or reuse is a possibility if advanced (reclamation) systems are implemented.

Climate change and groundwater resources

Although effects of climate change are still largely hypothetical, it seems that changes in meteoric water will become evident and affect the amount of water available for human activities. Precipitation patterns will change and it seems obvious that the relations between surface waters and groundwater will experience changes. Modifications in recharge, quantitative and qualitative, are also to be expected, although it is not clear if the amount of groundwater will be affected in a global basis. Local situations will change if precipitation is reduced or augmented in a noticeable way.

The role of groundwater as a storage system for water will be enhanced. Natural recharge will acquire more importance and artificial recharge will be supported and enhanced. Recharge through soil/subsoil systems will be studied further and the possibilities for water contamination through the solid matrix must be considered and managed adequately.

The final result will be an increase in quantity and change in quality in some places. Because up to now we are referring to theoretical models, it is not possible to establish clear patterns. As a consequence, groundwater recharge could play an important role in specific places, especially where non-conventional resources are available and soil and geological formations are adequate for artificial recharge.

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