Estimating Natural Recharge by Means of Chloride Mass Balance in a Volcanic Aquifer: Northeastern Gran Canaria (Canary Islands, Spain)

Water 2015, 7, 2555-2574; doi:w10.3390/7062555 water ISSN 2073-4441 www.mdpi.com/journal/water Article Estimating Natural Recharge by Means of Chloride Mass Balance in a Volcanic Aquifer: Northeastern Gran Canaria (Canary Islands, Spain) Gema Naranjo 1,*, Tatiana Cruz-Fuentes 1, María del Carmen Cabrera 1 and Emilio Custodio 2 1 Departamento Física (GEOVOL), Universidad de Las Palmas de Gran Canaria, Campus Universitario de Tafira, Las Palmas de Gran Canaria ES-35017, Spain; E-Mails: [email protected] (T.C.-F.); [email protected] (M.C.C.) 2 Groundwater Hydrology Group, Department of Geo-Engineering, Technical University of Catalonia (UPC), Jordi Girona, 1–3, Barcelona 08034, Spain; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +34-928-458-080; Fax: +34-928-452-922. Academic Editor: Miklas Scholz Received: 9 April 2015 / Accepted: 19 May 2015 / Published: 27 May 2015 Abstract: The chloride mass balance method was used to estimate the average diffuse groundwater recharge on northeastern Gran Canaria (Canary Islands), where the largest recharge to the volcanic island aquifer occurs. Rainwater was sampled monthly in ten rainwater collectors to determine the bulk deposition rate of chloride for the 2008–2014 period. Average chloride deposition decreases inwardly from more than 10 g·m−2·year−1 to about 4 g·m−2·year−1. The application of the chloride mass balance method resulted in an estimated average recharge of about 28 hm3/year or 92 mm/year (24% of precipitation) in the study area after subtracting chloride loss with surface runoff. The average storm runoff was estimated to be 12 hm3/year (9% of precipitation) for the 1980–2014 period. Runoff was sampled during scarce rainy periods, which produce surface water flow. Average recharge varies from less than a few mm/year near the coast up to 270 mm/year in the highlands (about 33% of average rainfall), with a close-to-linear increase inwardly of about 18 mm·year−1·km−1. Recharge rate uncertainty corresponds to an estimated CV of 0.3–0.4 because of the short data series available. Keywords: recharge; volcanic aquifer; chloride mass balance; Gran Canaria Island OPEN ACCESS Water 2015, 7 2556 1. Introduction Groundwater often forms a dominant part of the total water resources in many volcanic islands and may play a key role in local economic development. In many cases, intensive irrigated agriculture uses the largest fraction of available water resources, in concurrence with other water uses, which may be needed economically and socially as, for example, a source of employment. Accordingly, by taking into account water quantity and quality aspects, natural aquifer recharge evaluations are crucial for evaluating aquifer water resources and their management. Recharge is a complex natural phenomenon and one of the most difficult hydrological variables to measure and/or estimate [1], considering the: unavoidable simplifications to calculate it; wide temporal and spatial variability; paucity of observations; gaps in data series; and lack of measurements of the hydraulic parameters needed to apply recharge calculation codes and models. All this explains the commonly large uncertainty of the results. Several approaches have been developed to quantify groundwater recharge from precipitation, such as direct measurement, water-balance methods, and tracer techniques [2–5]. In order to improve recharge estimations, combining different techniques that are as independent as possible from each other is advisable [6–9]. The detailed application of many of these methods is costly and time consuming, and a long monitoring time is needed to obtain reliable results when direct methods are used [2]. For this reason, natural tracer techniques based on environmental components are widely and successfully used. Regional recharge is one of the most difficult hydrological components of the water balance to estimate [10], especially in aquifers where rock fracturing plays a key role, which is the case of volcanic terrains [11]. This is compounded by adequate water table data records and accurate outflow measurements or evaluations not being available to calibrate water-balance models. This is a frequent circumstance in volcanic islands due to a regularly profound water table depth, and also to the fact that groundwater outflow is produced in large water bodies, such as the sea. For realistic and sound groundwater management and planning, recharge estimates have to be accompanied by an estimation of uncertainty, which is often significant. This estimation derives from error propagation analyses supported by sensitivity analyses to obtain weighting coefficients. The chloride mass balance (CMB) method of atmospheric bulk deposition is the most widely used tracer technique to estimate long-term groundwater recharge under steady-state conditions [6]. It has been applied in many studies worldwide [12–15]. Knowledge of atmospheric chloride deposition is a prerequisite for applying this method, which is never straightforward given a range of processes that control atmospheric deposition [16–19] as they fluctuate temporally and are spatially variable, which makes extrapolating point measurements difficult. This has often been recognized as the main source of uncertainty when applying the CMB method [6,15,20,21], along with the right evaluation of the chloride concentration of recharge water [5]. Long-term average CMB data taken at different points are necessary to estimate long-term average recharge values. Long-term records of CMB variables are rare, so they have to be estimated from available short-term records, which are often only 1–5 years long. This period is shorter than the minimum 10-year period required to minimize uncertainty, especially for atmospheric chloride deposition [15,19], although this uncertainty is still intrinsically significant. Short-term records may not adequately represent long-term values, and may increase the uncertainty of recharge evaluation. Water 2015, 7 2557 Knowledge of the tracer concentration in recharge water is just as important as average deposition. By assuming that tracer transport through the unsaturated zone is conservative, which it is for chloride, and under steady-state conditions, the tracer concentration at the upper part of a saturated zone can be used as a good proxy of recharge water concentration, provided that recharge is active enough to safely neglect any diffusion effects from below. This is not the case in arid areas, especially if the water table is deep, as diffusion effects are strong; in particular, recharge water from old periods may be in transit. Since the tracer concentration in the groundwater from springs and deeply penetrating wells is a mixture of locally recharged water and groundwater upflow [5], corrections need to be made to the tracer concentration in recharge water. Adequate proxy selection significantly contributes to uncertainty. On the Canary Islands, the CMB method to evaluate recharge, in addition to the case presented herein for the northeastern Gran Canaria Island, has been successfully applied in: (1) Betancuria Massif, on the Fuerteventura Island, under arid conditions; average recharge was calculated from four open rainwater collectors in operation for 422 days and the calculated average recharge ranged from 9 to 33 mm/year (90% to 27% of precipitation) [22] with quite a large uncertainty; (2) Las Cañadas aquifer, on the central Tenerife Island, when five rain collection stations were in operation in hydrologic year 2005–2006; the average recharge was estimated to be 215 mm/year (44% of precipitation) [23]; (3) La Aldea aquifer [24], under semiarid conditions; (4) on La Gomera Island with a detailed groundwater chloride map, corrections were applied for mixing due to slope effects and after assuming deposition rates that had been extrapolated from other islands [5]; (5) the Amurga Fonolitic Massif on the southeastern part of Gran Canaria Island using chloride deposition [25] and 13C-corrected 14C data in a sloping aquifer to obtain the average recharge rate. This resulted in an areal average of about 4 mm/year, from less than 1 mm/year on the coast to 12 mm/year at the top of the wedge-shaped massif [5]. In all these cases, the recharge process was assumed to be under steady-state conditions, even in arid areas, provided that no contribution from deep-seated formations took place. This paper presents the estimated groundwater average recharge on the northeastern Gran Canaria Island (Canary Islands, Spain) by means of the atmospheric chloride mass balance method, and also its uncertainty. This area is considered the main recharge area of the island given its heavier rainfall, lower temperature and favorable soil conditions. 2. Study Area The Canary Islands, in the northeastern Central Atlantic Ocean close to the African continent, comprise seven main islands and several islets (Figure 1). The archipelago is in the Saharan dry belt, with regional rainfall averaging 70–150 mm/year, which is the case of the eastern islands. Nevertheless, rainfall depends on the island altitude effect on their north faces, where humid trade winds are intersected and pushed upward. Thus rainfall increases orographically and reaches average values of up to 1000 mm/year. Water 2015, 7 2558 Gran Canaria, located in the central part of the archipelago, is almost coned-shaped, with a diameter of 50 km and a maximum elevation of 1949 m a.s.l. (above sea level). It is dissected by deep radial gullies. Figure 1. Study area, location of rainfall stations, where samples were collected, and runoff sampling points. Groundwater head contours (2008–2009), modified from [26,27]. The island aquifer is conceptualized as a single, stratified heterogeneous water body, with groundwater flowing from recharge areas at high altitude in the central part of island toward the coast. Natural discharge is produced along the coast, which took place through springs at gully bottoms in the past, but currently occurs by means of wells and water galleries. Groundwater is recharged and flows through different rock formations on the island, which are hydraulically connected and actually form a single, but heterogeneous, island aquifer system [28]. There are some exceptions, such as the La Aldea aquifer in the western part of the island, which is hydrogeologically isolated from the rest of the island [29] and only receives surface water inflow from three storage reservoirs upstream through a deep narrow canyon. Water 2015, 7 2559 The northeastern area of the Gran Canaria Island considered herein covers 312 km2 of surface area, and includes areas N3 and N4 and part of area N2 of the hydrological zoning of Gran Canaria (Figure 1). The study area limits are the sea and the watershed divides of the major gullies. The study area is considered the most favorable one for groundwater recharge on the island due to the relatively high rainfall, the high steep relief that intercepts prevailing trade winds, the young permeable materials on the surface, low water retention soils and moderate-density vegetation. Climate varies from the coast to the highlands, with conspicuous changes in temperature and rainfall. The annual average temperature ranges from 14 °C in the highlands to 21 °C on the coast, with an average value of 18 °C. Weighted average rainfall (1980–2014) is about 490 mm/year, and exceeds 750 mm/year in wet years, and remains below 200 mm/year in dry years. Rainfall increases with altitude, and ranges from an average annual rainfall (1980–2014) of 250 mm/year near the coast to 750 mm/year in the highlands, and close to 900 mm/year locally. In the study area, groundwater follows the insular pattern that flows radially from summit to coast (Figure 1). The water table varies from a depth of 137 m in coastal areas to 200 m in the middle and high areas on interfluve flats. Groundwater flows predominantly through the Pliocene basanitic lava flows and ignimbrites (Roque Nublo group), and the Miocene trachytic and phonolitic pyroclastic deposits and lava flows. Recharge is mainly the result of rainfall, but some irrigation return flows from the banana crop areas and contributes to this recharge on the coastal fringe. 3. Materials and Methods 3.1. Atmospheric Chloride Mass Balance (CMB) Method The chloride mass balance method compares total chloride deposition (wet and dry) on the land surface with chloride concentrations in groundwater. It is assumed that chloride is a conservative ion, and that the only chloride (Cl) source is atmospheric deposition through rainwater (wet deposition) and aerosols and dust (dry deposition). Their addition is bulk deposition. Under long-term steady-state conditions of atmospheric deposition, given the evapoconcentration (concentration of salts in water due to evaporation and plant transpiration) in soil, and flow through the unsaturated zone profile—These being the expected conditions in the study area—Groundwater receives an average mass flow that matches the contribution made by average atmospheric deposition, minus the average output produced by direct runoff [3,5,20,30]. In other circumstances, corrections have to be made or the method cannot be applied. The basic equation for quantifying recharge using the CMB method is: R ·Cl=D−E·Cl (1) where R is recharge, E is runoff, DP is total atmospheric chloride deposition (subindex P refers to precipitation), and ClR and ClE are the chloride concentrations of recharge and runoff, respectively. The equation refers to the total monitoring period, or divided by the length of the time period to the rates. DP is obtained from the accumulation of the successive depositions that correspond to each sampling period. If i is one of the sampling periods of a total of n periods, the total deposition rate can be calculated as: Water 2015, 7 2560 D=1 dCl ·P    (2) where ClP = chloride concentration in the water collected during the sampling period (mg·L−1); P = precipitation during the sampling period (mm/year); d = ∑   total number of days for n sampling periods; to avoid seasonal effects as much as possible, d should comprise a number of complete years. A discussion of the variables in Equations (1) and (2) is provided in [15,20]. 3.2. Rainwater Sampling In order to determine the total chloride deposition in rainfall, precipitation samples were collected from 10 rainwater open collectors located at different altitudes (Figure 1) for 2008–2014 or for 2010–2014, depending on the collectors (Table 1). Table 1. Rainfall sampling stations’ designation and sampling period. n = number of samples. Station Sampling Period n 036 6 November 2008 5 November 2014 48 037 31 October 2008 5 November 2014 52 055 30 October 2008 5 November 2014 54 098 * 31 August 2010 5 November 2014 36 174 * 31 October 2008 1 July 2010 18 203 31 October 2008 5 November 2014 52 002 4 February 2010 4 November 2014 37 088 4 February 2010 4 November 2014 30 136 4 February 2010 4 November 2014 35 199 4 February 2010 4 November 2014 30 213 4 March 2010 4 November 2014 41 Note: * Stations 174 and 098 are considered the same because the collector was moved to a nearby location in July 2010. Collectors were placed close to existing rainfall stations, which belong to the Gran Canaria Water Council, where daily precipitation data were measured. A floating paraffin-oil layer was added to the collector pot to avoid evaporation. Samples were collected monthly or after dry months, which usually coincided with summer, over a longer period, which was needed to obtain a sufficient water volume for the chemical analyses. 3.3. Runoff Water No permanent surface runoff exists on Gran Canaria Island. It occurs sporadically and is associated with short, heavy storms. Surface runoff is produced mainly in highland areas where the larger amount of precipitation is recorded, and is retained and stored in the reservoirs located in high and middle areas. Surface runoff is scarce, or even null, in coastal areas. Runoff gauging stations do not exist. Runoff has been calculated by applying the curve-number method of the US Soil Conservation Service [31,32] for the 1980–2014 period [33]. This is an empirical method to estimate the runoff Water 2015, 7 2561 produced by storms in a watershed according to precipitation by taking into account current water storage in the soil of the basin as a result of rainfall infiltration. A geographical information system (GIS) allowed the discretization of the area in different zones according to slope, soil use and soil typology. By overlapping these factors, a map of approximately the uniform areas was produced, which was suitable to estimate runoff by rainfall storms in the different zones of the study area, as well as the runoff coefficient (runoff/precipitation). Using the data from three reservoirs located in the area, a validation attempt was made with good results (R2 > 0.75). However, as the reservoirs are located in the high part, it was not possible to reliably apply the results to the remaining area. The runoff estimation results are shown in Figure 2. Figure 2. Area of influence of each collector and spatial distribution of the estimated runoff and runoff coefficients for 2008–2014, modified from [33]. Bold-italic numbers correspond to the rain-gauge station code (rain collectors). Due to the droughts in 2011–2012 and 2012–2013 hydrologic years, and also to the surface runoff behavior noted in the aforementioned study area, only six surface runoff water samples were collected Water 2015, 7 2562 in four different locations in different gullies (Figure 1). Three of these samples were collected during the same storm that took place in February 2013. 3.4. Recharge Water (Groundwater) In order to obtain an approach to the chloride concentration of recharge water in the different study areas shown in Figure 2, a map of chloride iso-concentration lines was devised with the data from a field campaign done in 1997 by the Island Water Authority. Samples with >10 mg/L of nitrate are assumed to be affected by irrigation return flows or by other recharge sources, and were consequently eliminated. This map was supplemented with data from 32 springs and galleries sampled during the 2008–2012 period, which were highly coherent with previous data. The intention of this map is to compensate for scarce permanent springs, most of which lie in the highlands. Some bias is expected given the mixing from the up-slope recharge (slope effect), especially from wells. However, in this case, most of them are large-diameter shaft-wells with a relatively small penetration into the saturated zone in which groundwater caption takes place by means of horizontal boreholes at the well bottom. 4. Results 4.1. Chloride Concentration of Rainfall Rainfall chemistry varies conspicuously in space and time. Figure 3 shows the modified Stiff diagrams of the representative rainwater for each rainfall station, weighted by the amount of rainfall in each collector for the whole study period. Rainwater is mainly of the sodium-chloride type. Salinity increases from highlands to the coast, which is characteristic of coastal areas with a great relief and a major wind component from the coast. The rainwater chemical composition depends also on the relative location of the collector: in the bottom of gullies or in divides; on the leeward side or the windward side. Concentrations are generally higher at lower altitudes and correspond to drier periods (March to October). Table 2 offers the average precipitation and atmospheric chloride bulk deposition for rainwater, which are used to estimate recharge at each rainfall station site. The average atmospheric chloride deposition in each collector during the sampling period (October 2008 to November 2014) ranged from 4.2 g·m−2·year−1 in collector 002 at an altitude of 1365 m a.s.l., where the highest rainfall and the lowest dry deposition were recorded, to 9.2 g·m−2·year−1 at 443 m a.s.l. 4.2. Runoff Chloride Concentrations Runoff water is of the Na-Mg-HCO3-Cl type (Figure 3). It presented a significant chemical difference with rainwater due to a water-rock interaction in soil. It was not possible to collect a runoff sample in all the areas where the rainfall collectors were located. So the chloride concentration in these stations was estimated from a regression line (R2 = 0.91) obtained between the altitude and chloride concentration of the available samples (Figure 4). The estimated chloride concentration values and the average runoff estimated with the curve-number method for the area of influence of each rainfall station are shown in Table 3. Water 2015, 7 2563 Figure 3. Modified Stiff diagrams of the representative rainwater chemistry of each rainfall station, weighted by the amount of rainfall in each collector. Runoff chemistry is also represented (average values when more than one sample was available at the same sampling point). Average isohyets (mm/year) for the 1970–2011 period are also shown. Table 2. Results of atmospheric chloride bulk deposition in the area. Z = elevation; P = precipitation; CP = precipitation-weighted average chloride concentration in rainfall during the study period; DP = average atmospheric chloride deposition during the study period. Zone Station Z m a.s.l. P mm/year CP mg/L DP g·m−2·year−1 N2 036 375 264 21.9 5.3 199 443 361 28.9 9.2 055 577 581 12.9 8.8 203 645 467 16.9 8.4 037 990 598 11.9 7.1 098-174 1370 739 7.8 6.0 N3 088 315 296 20.6 5.9 136 841 491 18.2 7.5 002 1365 715 6.7 4.2 N4 213 485 310 28.9 9.0 Water 2015, 7 2570 recharge nor to the CMB method to estimate net recharge (total recharge minus groundwater evapotranspiration by deep-rooted plants or direct evaporation from shallow water tables) as there is a thick non-saturated zone and no direct groundwater discharge to the land surface. The relative average rainfall during the considered period in relation to the long-term average was not taken into account. To increase the recharge estimation by daily water balance, the maximum soil water reserve used should lower and/or some preferential recharge through soil cracks and fissures must be allowed, and in accordance with field observations. The atmospheric chloride deposition balance estimates the long-term average net recharge when the groundwater chloride concentration from the top of the saturated zone is used, which was done. However, classical (Canarian) wells and the drainage crown of horizontal water-galleries and drills (“catas”) somewhat penetrate into the saturated zone; springs mix groundwater and drilled wells penetrate deeply. The groundwater sampled and used as a proxy for recharge water does not represent locally recharged water, but is actually a mixture of local recharge with water from the up-flow. This results in dilution, which implies recharge overestimations. This overestimation is slight in high elevation areas, but may be somewhat relevant at medium altitudes, and quite significant in low areas. Forestland may also trap atmospheric salinity by forest leaves’ interception of dry particulate matter, which will be incorporated later on into local recharge through leaf fall, increasing chloride deposition with respect to what is collected as bulk deposition in open rainwater samplers. This may overestimate long-term recharge to some extent, and in an unknown quantity, mainly in lower parts of the study area. Direct studies of this effect have not been carried out. 6. Conclusions The average total recharge volume in the considered area was estimated by contemplating that surface runoff was about 28 hm3/year (92 mm/year), almost 24% of precipitation. This percentage would be 30% of precipitation should surface runoff not be taken into account. This result shows that accurate knowledge of runoff in the area would help improve recharge estimation by reducing the uncertainty of the calculated recharge, which cannot be well constrained with available data. The atmospheric chloride balance yielded higher recharge results than those derived from the water balance in soil. However, the latter values were non-calibrated, and may consequently be biased. The former may be overestimated to some extent, even after the major correction of taking runoff into account, and the latter may underestimate recharge as preferential recharge or lighter soils were not considered. This shows how recharge estimations are uncertain and how different methods may yield non-coincident or overlapping results, which are subject to their own uncertainties given the simplifications introduced and the use of biased proxies and parameters. A first approach of the uncertainty is provided, which is quite high. It should be refined by extending the monitoring series and complementary data in the middle and high areas where recharge was mostly produced, especially in the eastern part. Studies should be extended to other areas of the island where recharge was potentially high. The fact that rainfall and recharge present quite a good relationship with distance to the coast helps correct the island surface slope effect on chloride concentration in the water samples from large springs and deeply penetrating wells. Yet such a linear Water 2015, 7 2571 variation is not so clear for atmospheric chloride deposition, so new sampling stations would help improve trend variability estimations with radial orientation on the island. According to the European Water Framework Directive and the Spanish Water Act, water plans have to be reviewed every 6 years. The Water Council of Gran Canaria is in charge of maintaining the monitoring network and should provide, operate and maintain the monitoring network needed to estimate recharge by the atmospheric chloride deposition balance method, which would reduce uncertainty. The same can be said of applying other methods and the data needed for calibration and validation. These other methods are necessary to make separate independent estimations of difficult-to-quantify recharge rates. Acknowledgments This work forms part of the REDESAC (CGL2009-12910-C03-02) and CONSOLIDER-TRAGUA (CSD2006-00044) Programs, a Spain Ministry of Science and Innovation project. We thank EMALSA (Empresa Mixta de Aguas de Las Palmas), and volunteer observers of the Insular Water Council of Gran Canaria rainfall gauges Network, for their selfless collaboration in sampling. The visit to the galleries and springs in the Guiniguada gully was possible thanks to the collaboration of the Heredad de Aguas de Las Palmas, Dragonal, Bucio y Briviesca. Access to the wells and springs in part of the area was facilitated by the Heredad de Aguas de Arucas y Firgas. Part of the supporting information comes from the provisional draft of the dissertation by Pilar Hernández-Quesada. The authors also thank Helen L. Warburton for improving the English and the anonymous reviewers for their constructive criticism and suggestions, which significantly improved the quality of the manuscript. Author Contributions Gema Naranjo performed the fieldwork, sample and data collections, and recharge calculations, and she prepared the manuscript with the help of the other authors; Tatiana Cruz-Fuentes performed sample collection and the curve-number method implementation. MCarmen Cabrera and Tatiana Cruz-Fuentes reviewed the manuscript. MCarmen Cabrera and Emilio Custodio conceived the original idea, and supervised the research, the manuscript and the results. Conflicts of Interest The authors declare no conflict of interest. References 1. De Vries, J.J.; Simmers, I. Groundwater recharge: An overview of processes and challenges. Hydrogeol. J. 2002, 10, 5–17. 2. Lerner, D.N.; Issar, A.S.; Simmers, I. (Eds.) Groundwater Recharge, a Guide to Understanding and Estimating Natural Recharge; International Association of Hydrogeologists: Hannover, Germany, 1990; Volume 8, p. 345. 3. Custodio, E.; Llamas, M.R.; Samper, J. (Eds.) La Evaluación de la Recarga a los Acuíferos en la Planificación Hidrológica; Asociación Internacional de Hidrogeólogos—Grupo Español/Instituto Geológico y Minero de España: Madrid, Spain, 1997; pp. 1–455. Water 2015, 7 2572 4. Scanlon, B.R.; Healy, R.W.; Cook, P.G. Choosing appropriate techniques for quantifying groundwater recharge. Hydrogeol. J. 2002, 10, 18–39. 5. Custodio, E. Estimation of aquifer recharge by means of atmospheric chloride deposition balance. Contrib. Sci. 2010, 6, 81–97. 6. Scanlon, B.R.; Keese, K.E.; Flint, A.L.; Flint, L.E.; Gaye, C.B.; Edmunds, W.M.; Simmers, I. Global synthesis of groundwater recharge in semiarid and arid regions. Hydrol. Process. 2006, 20, 3335–3370. 7. Nolan, B.T.; Healy, R.W.; Taber, P.E.; Perkins, K.; Hitt, K.J.; Wolock, D.M. Factors influencing groundwater recharge in the eastern United States. J. Hydrol. 2007, 332, 187–205. 8. Coes, A.L.; Spruill, T.B.; Thomasson, M.J. Multiple-method estimation of recharge rates at diverse locations in the North Carolina Coastal Plain, USA. Hydrogeol. J. 2007, 15, 773–788. 9. McMahon, P.B.; Plummer, L.N.; Bohlke, J.K.; Shapiro, S.D.; Hinkle, S.R. A comparison of recharge rates in aquifers of the United States based on groundwater-based data. Hydrogeol. J. 2011, 19, 779–800. 10. Allison, G.B. A review of some of the physical, chemical and isotopic techniques available for estimating groundwater recharge. In Estimation of Natural Groundwater Recharge; Simmers, I., Ed.; NATO ASI Series; Springer: Berlin, Germany, 1988; Volume 222, pp. 49–72. 11. Custodio, E. Groundwater in volcanic hard rocks. In Groundwater in Fractured Rocks; Krásný, J., Sharp, J.M., Eds.; Taylor & Francis: London, UK, 2007; pp. 95–105. 12. Eriksson, E.; Khunakasem, V. Chloride concentrations in groundwater, recharge rate and rate of deposition of chloride in the Israel coastal plain. J. Hydrol. 1969, 7, 178–197. 13. Wood, W.W.; Sanford, W.E. Chemical and isotopic methods for quantifying ground-water recharge in a regional, semiarid environment. Ground Water 1995, 33, 458–468. 14. Scanlon, B.; Mukherjee, A.; Gates, J.; Reedy, R.; Sinha, A. Groundwater recharge in natural dune systems and agricultural ecosystems in the Thar Desert region, Rajasthan, India. Hydrogeol. J. 2010, 18, 959–972. 15. Alcalá, F.J.; Custodio, E. Natural uncertainty of spatial average aquifer recharge through atmospheric chloride mass balance in continental Spain. J. Hydrol. 2015, 524, 642–661. 16. Richter, D.D.; Ralston, C.W.; Harms, W.R. Chemical composition and spatial variation of bulk precipitation at a coastal plain watershed in South Carolina. Water Resour. Res. 1983, 19, 134–140. 17. Keywood, M.D.; Chivas, A.R.; Fifield, L.K.; Cresswell, R.G.; Ayers, G.P. The accession of chloride to the western half of the Australian continent. Aust. J. Soil Res. 1997, 35, 1177–1190. 18. Gustafsson, M.R.; Hallgren Larsson, E. Spatial and temporal patterns of chloride deposition in Southern Sweden. Water Air Soil Pollut. 2000, 124, 345–369. 19. Alcalá, F.J.; Custodio, E. Atmospheric chloride deposition in continental Spain. Hydrol. Process. 2008, 22, 3636–3650. 20. Alcalá, F.J.; Custodio, E. Spatial average aquifer recharge through atmospheric chloride mass balance and its uncertainty in a large varied territory. Hydrol. Process. 2014, 28, 218–236. 21. Bresciani, E.; Ordens, C.M.; Werner, A.D.; Batelaan, O.; Guan, H.; Post, V.E.A. Spatial variability of chloride deposition in a vegetated coastal area: Implications for groundwater recharge estimation. J. Hydrol. 2014, 519, 1177–1191. Water 2015, 7 2573 22. Herrera, C.; Custodio, E. Groundwater flow in a relatively old oceanic volcanic island: The Betancuria area, Fuerteventura Island, Canary Islands, Spain. Sci. Total Environ. 2014, 49, 531–550. 23. Marrero-Díaz, R.; Pérez, N.M.; Alcalá, F.J.; Melián, G.V.; Padrón, E.; Padilla, G.D. Estimación de la recarga mediante balance de masas de cloruro atmosférico en la Caldera de Las Cañadas, Tenerife, Islas Canarias. In Estudio, Aprovechamiento y Gestión del Agua en Terrenos e Islas Volcánicas; Cabrera, M.C., Cruz-Fuentes, T., Mendoza-Grimón, V., Palacios-Díaz, M.P., Eds.; Instituto Geológico y Minero de España and International Association of Hydrogeologists-Spanish Group: Las Palmas de Gran Canaria, Spain, 2015; pp. 207–214. 24. Cruz, T. Contribución de la Modelación Numérica e Hidroquímica al Conocimiento del Acuífero de La Aldea (Gran Canaria). Ph.D. Thesis, Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain, 2008; pp. 1–283. 25. Gasparini, A.; Custodio, E.; Fontes, J.C.; Jimenez, J.; Núñez, J.A. Exemple d’étude géochimique et isotopique de circulations áquiferes en terrains volcanique sous climat semi-aride (Amurga, Grande Canarie, Iles Canaries). J. Hydrol. 1990, 114, 61–91. 26. Benavides, A.; Galindo, E. Evolución de la piezometría de la zona norte de la isla de Gran Canaria. In El conocimiento de los Recursos Hídricos en Canarias Cuatro Décadas Después del Proyecto SPA-15; Cabrera, M.C., Jiménez, J., Custodio, E., Eds.; Las Palmas de Gran Canaria: Las Palmas, Spain, 2010; pp. 53–59. 27. Estévez, E.; Cabrera, M.C.; Molina-Díaz, A.; Robles-Molina, J.; Palacios-Díaz, M.P. Screening of emerging contaminants and priority substances (2008/105/EC) in reclaimed water for irrigation and groundwater in a volcanic aquifer (Gran Canaria, Canary Islands, Spain). Sci. Total Environ. 2012, 433, 538–546. 28. MOP-UNESCO. Estudio Científico de los Recursos de Agua en las Islas Canarias (SPA/69/515); Centro de Estudios Hidrográficos-Servicio Geológico de Obras Públicas, Las Palmas de Gran Canaria-Madrid: Las Palmas, Spain, 1975; Volume 4. 29. Cruz-Fuentes, T.; Heredia, J.; Cabrera, M.C.; Custodio, E. Behaviour of a small sedimentary volcanic aquifer receiving irrigation return flows: La Aldea, Gran Canaria, Canary Islands (Spain). Hydrogeol. J. 2014, 22, 865–882. 30. Guan, H.; Love, A.J.; Simmons, C.T.; Hutson, J.; Ding, Z. Cathcment conceptualization for examining applicability of chloride mass balance method in an area with historical forest clearance. Hydrol. Earth Syst. Sci. 2010, 14, 1233–1245. 31. United Stated Soil Conservation Service (US-SCS). National Engineering Handbook, Section 4; U.S. Department of Agriculture: Washington, DC, USA, 1972. 32. United Stated Soil Conservation Service (US-SCS). National Engineering, Handbook, Section 4: Hydrology; U.S. Department of Agriculture: Washington, DC, USA, 1985. 33. Cruz-Fuentes, T.; Naranjo, G.; Cabrera, M.C.; Custodio, E. Estimación de la escorrentía superficial mediante la generación automática del número de curva con un SIG. Aplicación al norte de Gran Canaria (Islas Canarias, España). In Proceedings II Congreso Ibérico de las Aguas Subterráneas, CIAS2014; Gómez-Hernández, J., Rodrigo-Illari, J., Eds.; Universidad Politécnica de Valencia: Valencia, Spain, 2014; pp. 217–232. Water 2015, 7 2574 34. Custodio, E. Coastal aquifer salinization as a consequence of aridity: The case of Amurga phonolitic massif, Gran Canaria Island. In Study and Modeling of Saltwater Intrusion into Aquifers; Custodio, E., Galofré, A., Eds.; CIMNE: Barcelona, Spain, 1993; pp. 81–98. 35. Cabrera, M.C.; Naranjo, G.; Hernández-Quesada, P.; Benavides, A.; Estévez, E.; Cruz-Fuentes, T.; Martín, L.F.; Custodio, E. Estimación de la recarga natural a los acuíferos del Norte de Gran Canaria a partir del balance de cloruros. In Temas Actuales de la Hidrología Subterránea; González, N., Kruse, E., Trovatto, M.M., Laurencena, P., Eds.; Provincia de Buenos Aires: La Plata, Argentina, 2013; pp. 247–254. 36. Boletín Oficial de Canarias (BOC). Decreto 82/1999, de 6 de Mayo, por el que se Aprueba el Plan Hidrológico Insular de Gran Canaria; Consejería de Obras Públicas, Vivienda y Aguas, Gobierno de Canarias: Canary Islands, Spain, 1999; pp. 8218–8436. 37. Naranjo, G.; Cabrera, M.C.; Cruz-Fuentes, T.; Martín, L.F.; Custodio, E. Cuantificación de la recarga natural al acuífero del norte de Gran Canaria mediante el balance diario de agua en el suelo. In Estudio, Aprovechamiento y Gestión del Agua en Terrenos E Islas Volcánicas; Cabrera, M.C., Cruz-Fuentes, T., Mendoza-Grimón, V., Palacios-Díaz, M.P., Eds.; Instituto Geológico y Minero de España and International Association of Hydrogeologists-Spanish Group, Las Palmas de Gran Canaria: Las Palmas, Spain, 2015; pp. 247–254. 38. Custodio, E.; Cabrera, M.C.; Poncela, R.; Cruz-Fuentes, T.; Naranjo, G.; Puga de Miguel, L.O. Uncertainty in groundwater governance. General comments and application to the volcanic Canary Islands, Spain. Water, submitted. © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).