SALINE WATER IN CENTRAL FUERTEVENTURA ISLAND, CANARY ISLANDS, SPAIN

HERRERA, CHRISTIAN(1)(2) and CUSTODIO, EMILIO(1)(3)

(1) Depart. Geotech. Eng., Technical University of Catalonia (UPC), Barcelona, Spain. [email protected] / [email protected] (2) On leave from Universidad Católica del Norte, Antofagasta, Chile. (3) On leave to the Instituto Tecnológico Geominero de España (ITGE), Madrid, Spain. [email protected]

ABSTRACT

Fuerteventura is the second largest (1650 km2) of the Canary Islands. The relative low relief (maximum altitude of 724 m) respect the other islands and the proximity to the Saharan coast (about 125 km) explains its arid character, with an average rainfall that barely attains 100 mm, and no more than 200 mm in the highlands. Surface runoff is limited to short, flash floods after sporadic, intense downpours. Groundwater recharge is scarce and mostly reduces to an average value of a few mm a–1. This combined with relatively high atmospheric salt accession (closeness to the sea and quite strong winds) produces brackish recharge to the aquifer, as happens in other arid areas of the islands. A few years ago a series of boreholes were drilled to draw water from deep volcanic and intrusive formations. Quite brackish and saline water was found. Studies are being carried out to know the origin of these saline waters. There is an additional component to soil concentration, pointing to a marine origin of salinity, plus gypsum. Preliminary it can be considered the existence of relict marine water in the central part of the area where the old, submarine part of the island basement crops out or is partially covered by Miocene to Recent subaerial volcanics. The paper contributes recent data to a study that is still unfinished.

INTRODUCTION islands or parts of them. This is due to high evapotranspiration of scarce, Fuerteventura is part of the rather saline rainwater, affected by the Canarian Archipelago, Spain. It is closseness to the windy, rough sea. about 125 km away from the Saharan Average rainfall is only slighly higher coast of Africa (fig. 1) It is the second than 100 mm a–1, varying between less largest island of the Archipelago with a than 200 mm a–1 in the highlands 2 surface area of about 1650 km . Its (elevation 700 m) of the central shape is elongated, oriented NE–SW Betancuria massif, to about 80 mm a–1 and with a largest dimension of about near the SE coastal area. Most rain 100 km. events are short but intense downpours between october and Groundwater is dominantly march. There is a large interannual brackish, as happens in other arid variability. To better know the origin of On top of the submarine groundwater salinity and solve recently formations a high, large subaerial introduced, unsupported and doubtfull basaltic shield complex (Gran Tarajal feelings that fresher water may be stratovolcano) of Miocene age developed by deeper wells, a research developped, which was later on deeply study is under way in the central part of eroded and subjected to major the island, around and to the E of the landslides (Stillman, 1999). Betancuria massif (fig. 1). In the central part of the study Previous studies on the area there is a recent and rather hydrogeology of Fuerteventura are extense lava field (malpais), of very scarce. A preliminary survey and Pliocene–Holocene age and small interpretation (finished in 1972) was thickness, with fingers towards the included in the Spanish Government– coast, following the main creeks UNDP programme SPA(69)15, which (barrancos). They rest on older was followed by the MAC–21 (1982) vulcanics or its denudation products, study. The ITGE has maintained and some poorly sorted alluvium. This observation networks for some time. A is a recharge area. wide survey of wells and sampling programme was carried out the Army GROUNDWATER IN FUERTEVENTURA Legion, but largely uninterpreted for chemical data up to now. The Las In Fuerteventura there is no permanent surface water and the barrancos flow only after Palmas Water Authority maintains a intense rainfall events and for a very short rainfall gauging network. The Public time. Local inhabitants try to make use of these Works Geological Survey drilled some waters by creating small dams to spead water exploratory boreholes on the 1980’s. on the infilling materials (gavias) and to divert it The Fuerteventura Water Plan and to areas into which they increase soil humidity (nateros). In the old times some large cisterns related studies do not add new (maretas) collected runoff from slopes, for town significant information. supply. Now most potable water is desalinated ground and sea water. Intense rain events GEOLOGICAL BACKGROUND produce some local diffuse and concentrated recharge of shallow aquifers, which is lately Fuerteventura is the oldest of abstracted by 1.5 to 3 m diametre dug wells. They were mostly fitted with windmills. the volcanic Canary Islands. It is the Average depth is 25 m, but some may be more only island in which the deep marine than 80 m deep. There are a few brackish sedimentary basement of the Eastern water springs (nacientes) which reduce almost islands crops out in some reduced to seepages producing no more than a few areas. On top of it submarine litres per day. vulnanites pile up, intruded by dominantly basic magmatic bodies and From the decade of 1980 some a very dense swarm of mostly basaltic relatively deep boreholes (up to 250 m) dykes. This submarine complex (the were drilled looking for greater flows basal complex) has been uplifted and and fresher water. This was unsuccessful but for a few brackish to intensively eroded. The age ranges –1 from Upper Cretaceous to Lower salty water wells producing a few L s . Miocene. To the end of the guyot stage Commonly they are uncased but for some subaerial vulcanites are found. the few upper metres. Some of them This island, joinly with the Cape Vert are now used for reverse osmosis archipelago are the only ones with desalination. The residual brine is carbonatites outside a pure continental transported by pipeline to the coast. environment (Domény et al., 1999). Major towns and coastal in 1998 and 1999. Sample evaporation touristic centres and hotels use was prevented by a floating layer or desalinated water, but small urban liquid parafine. Only 4 stations have areas and farmers depend on repeated data, and represent the groundwater for human supply, different altitudes and environments. irrigation of small but highly productive orchards and cattle raising. Currently A network of 32 groundwater most dug wells around Tuineje are dry. points (seepages, wells and deep This may indicate interference due to boreholes) were sampled twice a year, the development of the deep plus other oriented samplings, with a boreholes. total a 154 groundwater samples. When possible the samples were taken The study area is the central by means of the well pump. Otherwise part comprising part of the Betancuria a thief sampler was placed at the massif and the rolling plains to the selected depth, sometimes with some East. previous bailing, according with electrical conductivity and temperature STUDY SYSTEMATICS logs. One sample corresponds to a “perched” level seepage into a deep To try to understand the origin water level borehole. and pattern of groundwater salinity in the study area, experience gained in In situ measurements of other arid areas of the islands, mostly electrical conductivity and pH were the Amurga Massif (SE Gran Canaria) often performed, as well as some and Lanzarote island, was considered alkalinity determinations. All chemical (Custodio, 1992). The study analyses have been carried out by the concentrates on chemical and ITGE laboratories in Tres Cantos environmental isotope issues since (Madrid) except for Br determinations, conditions to carry out hydrodynamic which were carried out by the studies were poor, boreholes scarce Barcelona Water Company, and and poorly documented, and actual CIEMAT in Madrid. The isotopic study economic resources do not concentration of 18O, 2H and 13C were allowed drilling and testing. measured at the Cedex, Madrid, and other determinations elsewhere, in a The objective was the chemical reduced number of samples. characterization of rainfall as well as many as possible groundwater points, GROUNDWATER CHEMICAL taking into account sample CHARACTERISTICS representativity of groundwater conditions. Available, unequiped deep Electrical conductivity logs show boreholes were logged for temperature that groundwater is stratified, with and electrical conductivity. Sampling salinity increasing downwards (Herrera has been repeated to be sure of the and Martínez, 1998). This effect is representativity and to identify partly the result of small vertical flows changes. Permanent seepages along the boreholes due to small head (nacientes) are the subject of special changes along it (Custodio, 1995). attention. They may show up in the temperature log if they are large enough for Only 27 rainfall samples of bulk equibration with the solid medium precipitation have been recovered, temperature not taking place. corresponding to six significant events Hydrogeologically it can be differenciated an Upper Unit and a Figure 2 shows the ratio SO4/Cl Lower Unit. The Upper Unit contains vs. Cl. Chemical path studies show a the less saline water: electrical meteoric origing for all of them but for conductivity between 2.5 and 5.5 mS the Lower Unit, which needs a cm–1 and temperature between 18 and separate source of sulphate. In general 25oC, close to the local average terms, as chloride increases the ratio temperature. The Lower Unit contains SO4/Cl first evolves towards the marine brackish to saline, relatively warm value but for higher values of salinity a water: electrical conductivity varies gypsum disolution component is between 5.5 and 24 mS cm–1 and needed further to marine water in temperature between 25 and 30oC, increasing proportion. which can be explained by normal geothermal gradient. 1.4 Legend Precipitation

Springs/seeps (Upper Unit) The Upper Unit contains 1.2 Wells (Upper Unit) recharge water with small water–rock Deep boreholes in Tuineje (Lower Unit) Cl r

/ Deep boreholes in Tesejerague (Lower Unit) interaction. Evaporation of rainfall 4 1 Sea water SO infiltration prior to recharge is the r dominant effect. They are sodium– 0.8 chloride type waters, as does rainfall, have a moderate alkalinity and 0.6 relatively low silica content. High nitrate contents in many samples show 0.4 an anthropic component in water chemistry. They appear dominantly in 0.2 the highlands and along the creeks, but near the creeks it may also 0 0.1 1 10 100 1000 respond to enhanced recharge of rCl episodic surface flows by means of Fig.2.– Plot of rSO4/rCl vs. rCl. –1 runoff retention and small water r=meq L . spreading works (gavias and nateros). To better understand what Below the lava field (malpais) chemically happens the ratio Cl/Br has salinity is relatively low due to the high been considered as well (fig. 3). The soil permeability and small water result is quite surprising since for retention, but it is still brackish due to increasing chlorinity the ratio evolves incorporation of saline atmospheric from high values relative to the marine solid deposition and some unavoidable value to values closer to it. The high evaporation. ratio for rainfall and the less saline groundwater is contrary to what The more saline waters of the generally happens. But this has been Lower Unit are of the sodium chloride– also observed occasionally in a few sulphate type. Silica content is high samples in other of the Canarian and nitrate is almost absent but for islands (Custodio and Herrera, 2000). some contaminantion from shallow Rough sea, windy environment and water seeping into the boreholes. aridity may enhance the production of Sulphate content is controlled (24 to 59 airbone salts rich in halite (poor in Br) meq L–1) by gypsum saturation. by partial precipitation of salts from sea Sodium chloride increases from spray along the relatively rough coastal Tuineje to Tesejerague. strip. The ratio Cl/Br points to a marine component for increasing groundwater evaporated water) cluster around salinity but there is other component as δ18O=–3.2‰ and δ2H=–18‰, about the well, richer in Cl. This may be due to chloride released by the rock. 20 L M W δ18O ( ) SMOW L LM 10 6000

Legend -6 -5 -4 -3 -2 -1 0 1 5000 Precipitation 0 Springs/seeps (Upper Unit) r 4000 B L r Wells (Upper Unit) E Cl/ r Deep boreholes in Tuineje (Lower Unit) ML 3000 -10 Deep boreholes in Tesejerague (Lower Unit)

Wells and shallow boreholes (Malpa s)

Sea water -20 2000 () SMOW () Η 2 δ -30

= Assumed mean local seawater 1000 -40 900 same as rainfall average. 800

700 18 2 600 Fig. 4.– Plot of δ O vs. δ H. WML = 500 World Meteorological line (m=8; d=10); 40 100 1000 10000 20000 Cl (mg L-1) LML = Local Meteorological line (m=8; Fig 3.– Plot of rCl/rBr vs. Cl (mg L–1). d=15); EL = evaporation line of the r=meq L–1. Upper Unit (m=6); ML = freshwater – seawater mixing line. m means slope In a δ2H vs. δ18O plot (fig. 4), the and d excess deuterium (in ‰). See meteoric line is taken as having slope legend in Figure 3. 8 and deuterium excess between +10‰ (average oceanic value) and Most saline groundwaters from +15‰. This may be explained by the the Lower Unit around Tesejerague largest kinetic evaporation effect in the plot along a line pointing to the sea relativelly dry marine environment in value, the closer the higher the salinity. the Saharan belt, which is the same Wells from Tuineje do not follow a effect to explain the greater deuterium clear pattern. An evaporated excess observed in rainfall originated component is possibly included. This in the Mediterranean area. precludes at this stage to define the original freshwater composition. Water samples from seepages Extrapolation to get the isotopic and the Upper Unit in the study area composition of the original freshwater plot along a line of slope 6, shows that it is possibly lighter than characteristic of free water present average rainfall. But rainfall evaporation. This may be linked to isotopic sampling is currently natural or artificial ponding on the land incomplete and short, and probably surface before infiltration, or to does not duly consider the effect of evaporation from bare soil, although in preferential recharge of lighter water in this case a smaller slope can be extraordinarily wet periods. In fact expected. But the slope 6 may be the wells from the lava field produce result of smaller slope displacements groundwater samples which are lighter of waters evaporated from the soil at than the others. different elevations. Non evaporated groundwater (and projected back 0

-1

18 -2 Plotting δ O vs. chloride ) SMOW

content (fig. 5), most samples are near O ( 18 the rainwater–seawater closed system δ -3 mixing line, except for samples showing evaporation. Since chlorinity -4 of the rainfall end member conditions the less saline part of the plot, it is not -5 surprising that samples from the lava ? field and one of the high altitude seepages follow a different mixing line -6 1 10 100 1000 10000 30000 than the others. Lower Unit aquifer Cl (mg L-1) water from Tesejerague adapt to one of sea water with hypothetical recharge of these lines but not those from water. See legend in Figure 3. Tuineje.

0.8 ) -1 Sea 20000

Two seepages and a shallow Cl (mgL well from the lava field show δ34S values of dissolved sulphate from +16 0.6 15000 to +18‰, which is close to the present /rCl 10000 +20‰ of marine water. This shows a 4 rSO dominantly marine origin of airbone 0.4 5000 salinity. The other samples and 0 especially the most saline and sulphate 4 8 12162024 34 δ34 S‰DCT richer ones have a lower δ S of SO4, with values around +8‰. Then the S is 0.2 probably linked to vulcanism, which it Sea is now available to water as gypsum. 34 The value of δ S=+26‰ is probably 0 4 8 12 16 20 24 28 the effect of sulphate reduction; in fact δ34S‰ DCT –1 34 the water sample was blackened by Fig. 6.– rSO4/rCl (r=meq L ) vs. δ S what is assumed reduced sulphur and insert of Cl vs. δ34S. See legend in growths. It is not known if this is natural Figure 3. or the result of organic matter introduced as a drilling component and not removed later on. CONCLUSIONS

34 The plot of δ S (SO4) vs. As preliminary results of the SO4/Cl (fig. 6) shows the two families study under way, the relatively high just commented above, and a few salinity of groundwater in exceptions. The same appears in a Fuerteventura island is due to intense 34 plot of δ S (SO4) vs. Cl. One of the evaporation of rainfall, which seepages seems the discharge of the incorporates marine airbone salts in an Lower Unit. The δ18O value of sulphate arid environment. This explains salinity is under study. of most springs (seepages) and dug wells. Salinity is even rather high below recent lava fields which present a high Fig. 5.– δ18O vs. Cl (mg L–1) for surface permeability but in which precipitation samples and groundwater incipient weathering and air deep of the Betancuria massif. Mixing lines circulation allows for evaporation of rainfall infiltration. But deep boreholes show a higher salinity whose CUSTODIO, E. (1995). The impact of explanation needs to consider other vertical water flow in boreholes: sources of dissolved salts. Relict influence on monitoring marine water in the submarine operations. Hydrogéologie, vulcanites and intrusives is a possibility Orléans 3: 3–12. but some of the Cl may be released by CUSTODIO, E.; HERRERA, CH. the rock itself to explain the relatively (2000). Utilización de la relación high Cl/Br ratio. The release of Na from Cl/Br como trazador the rock to the water obscures this hidrogeoquímico en hidrología effect. Also soluble sulphates are subterránea. Hidrogeología / present which seem to be of volcanic Boletín Geológico y Minero. origin and not seawater related. The ITGE. Madrid (in press). low regional permeability of the DEMÉNY, A.; AHIJADO, A.; formations may explain the persistence CASILLAS, R.; BOYCE, A.J.; of this saline water body above sea FALLICK, A.E. (1999). Crustal level, a even a small seepages. contamination of carbonatites Turnover time is under study. Regional indicated by δ34S–δ13C low permeability values do not correlations (Fuerteventua, preclude small yields from some deep Canary Islands). Rev. Soc. Geol. wells intersecting open fissures. España. 12(3–4): 453–460. Development of these water should HERRERA, CH.; MARTÍNEZ, F. deplete the reserves, combined with (1998). Hidrogeología de la parte large drawdown and replacement with central de la Isla de recent groundwater from above. This is Fuerteventura (Archipiélago de known in other areas of the Canaries. Canarias). Fundación Centro Intern. Hidrología Subterránea. ACKNOWLEDGEMENTS Barcelona (internal). STILLMAN, C.J. (1999). Giant Miocene This paper contain first results landslides and the evolution of form the doctoral thesis under way of Fuerteventura, Canary Islands. J. the first author under the guidance of Volcanology and Geothermal the second. The work is the result of Research, Elsevier. 94: 89–104. an agreement betweing the Local Governement of Fuerteventura and the ITGE. It is carried out inside another agreement between the ITGE and the UPC. The first author works with a grant from the Universidad Católica del Norte, Chili. Chemical analyses have been carried out by the ITGE. The Project office of ITGE in Las Palmas is providing technical support, as well as the Water Authority of Fuerteventura. REFERENCES

CUSTODIO, E. (1992). Coastal aquifer salinization as a consequence of aridity: the case of Amurga phonolitic massif, Gran Canaria Island. Study and Modelling of Salt Water Intrusion (XII SWIM). CIMNE–UPC. Barcelona: 81–98.