Radon in Groundwater of the Northeastern Gran Canaria Aquifer

Water 2015, 7, 2575-2590; doi:10.3390/w7062575 water ISSN 2073-4441 www.mdpi.com/journal/water Article Radon in Groundwater of the Northeastern Gran Canaria Aquifer Héctor Alonso, Tatiana Cruz-Fuentes, Jesús G. Rubiano *, Jonay González-Guerra, María del Carmen Cabrera, Miguel A. Arnedo, Alicia Tejera, Alejandro Rodríguez-Gonzalez, Francisco J. Pérez-Torrado and Pablo Martel Physics Department, Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria 35017, Spain; E-Mails: [email protected] (H.A.); [email protected] (T.C.-F.); [email protected] (J.G.-G.); mcarm[email protected] (M.C.C.); [email protected] (M.A.A.); [email protected] (A.T.); [email protected] (A.R.-G.); franciscojos[email protected] (F.J.P.-T.); [email protected] (P.M.) * Author to whom correspondence should be addressed; E-Mail: jesus.ga[email protected]; Tel.: +34-928-454-495; Fax: +34-928-452-922. Academic Editor: Miklas Scholz Received: 10 April 2015 / Accepted: 19 May 2015 / Published: 27 May 2015 Abstract: 222Rn has been detected in 28 groundwater samples from the northeast of Gran Canaria (Canary Islands, Spain) utilizing a closed loop system consisting of an AlphaGUARD monitor that measures radon activity concentration in the air by means of an ionization chamber, and an AquaKIT set that transfers dissolved radon in the water samples to the air within the circuit. Radon concentration in the water samples studied varies between 0.3 and 76.9 Bq/L. Spanish radiological protection regulations limit the concentration of 222Rn for drinking water to 100 Bq/L, therefore the values obtained for all the analyzed samples are below this threshold. The hydrogeological study reveals a significant correspondence between the radon activity concentration and the material characteristics of the aquifer. For a selected group of samples with high radon concentrations, gross alpha activity has been determined to have values higher than the prescriptive screening level (0.1 Bq/L). Keywords: radon activity; gross alpha; groundwater tracer; volcanic terrain; normative OPEN ACCESS Water 2015, 7 2576 1. Introduction Radon, a natural byproduct of the radioactive decay of uranium, radium and thorium, is an alpha-emitting noble gas with a half-life of 3.8 days. Radon gas is soluble in water and consequently the gas may be incorporated into groundwater flows. Radon is extracted from the volcanic deposits in which the aquifer resides, its transport taking place basically through the fissure network in the fractured system or from mantle degassing. The quantity of radon dissolved in groundwater depends on different factors such as the characteristics of the aquifer, water-rock interaction, water residence time within aquifer, material content of radium, etc. [1–3]. Measurements of radon contents in groundwater have been performed in connection with geological, hydrogeological and hydrological surveys and health hazard studies. On the one hand, the half-life of radon and its solubility have allowed the use of radon gas as a natural groundwater tracer to identify and quantify groundwater discharge to surface waters [4–6] or to attempt to elucidate the type of rocks through which groundwaters flow [5,7]. On the other hand, the presence of high levels of radon in drinking water constitutes a major health hazard [8–10]. The Commission of European Communities (CEC) recommends the monitoring of radon levels in domestic drinking water supplies originating from different types of groundwater sources and wells in different geological areas, in order to determine consumer population exposure. The limit is fixed at below 100 Bq/L [11]. The region studied here is an area of volcanic-origin located in the northeast (NE) of Gran Canaria Island (Canary Islands, Spain). Groundwater in the investigated area plays an important role in guaranteeing water supply for agricultural and domestic purposes, mainly in the higher parts of the island. In coastal areas, desalinated seawater provides for urban needs. The content of radon in water samples must be determined by reliable methods. Radon is a very mobile gas and it can escape from water with ease during the process of sampling and transportation, hence careful sample preparation is necessary. Several procedures can be found in the literature to perform measurements of radon in groundwater using different techniques such as Lucas-cell, ionization chambers, solid-state detector, or gamma spectrometry [12–16]. The aim of this study was to measure radon concentration in groundwater at most populated zone of Gran Canaria as a starting point for monitoring of the quality of groundwater with respect to radon gas in the whole territory as well as making a first attempt at using this gas as a tracer to establish dynamic knowledge of groundwater flow in Gran Canaria. 2. Area of Study 2.1. General Description The Canary Islands, located at the eastern edge of the Central Atlantic Ocean (between 27° N and 30° N and from 19° W to 13° W), close to the north-western continental margin of Africa, comprise seven major volcanic islands (Tenerife, La Palma, La Gomera, El Hierro, Gran Canaria, Lanzarote and Fuerteventura) and six islets (La Graciosa, Alegranza, Montaña Clara, Lobos, Roque del Este and Roque del Oeste). The Canary archipelago extends over approximately 500 km, and the eastern islands are situated about 100 km off the African coast (Figure 1). Water 2015, 7 2577 Figure 1. Location of the study area. Simplified geological map of Gran Canaria (modified from [17]) and geological cross sections of the study area. The sections were made using geological data from wells (modified from [18]). Gran Canaria Island, located at the center of the Canary archipelago (Figure 1), has a conical morphology and a nearly circular shape with a diameter of 45 km and a maximum elevation of 1950 m above sea level (a.s.l.) at its center. It is shaped by a series of radial gullies that originate in the center of the island and flow into the sea. The population of the island is around 900,000 inhabitants mainly concentrated in the NE area where the most populated cities of the island, Las Palmas de Gran Canaria (400,000 inhabitants) and Telde (100,000 inhabitants), are located. Water 2015, 7 2578 2.2. Geological Setting The construction of the Canary archipelago is related to the movement of the African plate above an anomalous or pulsating mantle plume [19,20]. The geology of Gran Canaria is conditioned by its origin (Figure 1). The subaerial formation of Gran Canaria occurred in two main phases: (1) a juvenile stage (about 14.5–8.0 Ma), which includes a basaltic shield volcano, a vertical caldera collapse and a post-caldera resurgence characterized by evolved magmas; and (2) a rejuvenated stage (about 5 Ma to present). Both phases are separated by a period of volcanic inactivity that lasted about 3 Ma [20–22]. The geology of the study area includes volcanic and sedimentary materials (Figure 1). On the surface basaltic, basanitic and trachybasaltic lavas of the Roque Nublo and Post-Roque Nublo volcanism, and sediments of Las Palmas Detritic Formation mainly occur. Miocene trachytic and phonolitic lavas crop out, usually in the coastal areas and at the bottom of ravines, where alluvial deposits are also observed [21,23]. The main mineral phases in Basic volcanic rocks consist of diopside, olivine and labradorite, whereas in Salic volcanic rocks andesine, kaersutite, anorthoclase and hauyne are found [23,24]. On the other hand, secondary mineral phases, such as weathering cover or fillings of fractures and vesicles on volcanic rocks, include calcite, dolomite, gypsum and clays (illite, and smectites, usually montmorillonites), and zeolites (analcime, chabazite, phillipsite). The basic and ultrabasic rocks (peridotites, pyroxenites, gabbros, basalts, etc.) do not contain unstable elements in the crystal structure of the mineral and therefore have low radioactivity. The more differentiated rocks (syenites, trachytes, phonolites, carbonatites, etc.) contain in its crystal structure some forms of trace elements (La, Ce, Sm, Nd, Sr, Ba, Th, Nb, etc.), some of them radioactive, and potassium that is the sources of higher concentrations of radiological activity present in these rocks. In consequence, it is expected that groundwater flowing through acid geological formations will have higher values of 222Ra concentrations than through basic geological formations. 2.3. Hydrogeological and Hydrogeochemical Characteristics The island aquifer is conceptualized as a single, stratified and heterogeneous water-body, with groundwater flows from the recharge area (at the summits in the central part of the island) towards the coast. There are hydraulic connections between waters occurring in different rocks, which actually form one common hydrogeological system in the island [25,26]. In this framework, groundwater in the study area is part of the northern insular aquifer and flows from the south to the north. In the study area, groundwater is extracted predominantly from the Roque Nublo group rocks and underlying Miocene trachytic and phonolitic lavas. Groundwater exceptionally flows towards the deep gullies (“barrancos”) and generally discharges to the coast. Recharge is mainly a result of rainfall recharge, the study area constitutes the main recharge area of the island, and also from irrigation return flows. Agriculture, which is mainly practiced in the coastal areas, is supplied from both surface and groundwater resources, and locally causes important irrigation return flows [26,27], which promote increases in soil and groundwater salinity. Discharge occurs towards the sea and through withdrawals from wells and galleries. Groundwater is mainly of Mg-Ca-HCO3 and Mg-Ca-Cl type. At the coast, groundwater can be Na-SO4 due to irrigation return flows [25]. Water 2015, 7 2579 2.4. Area of Study The study area, about 323 km2, is bordered by Atlantic Ocean to the north and watersheds to the east and west. The overall average annual rainfall is 375 mm; in the highlands the average rainfall reaches 820 mm/year, while the coastal areas remain dry with an average rainfall of 115 mm/year. Average annual temperature varies from 12 °C in the highlands to 22 °C at the coast, with an average temperature of 18 °C. A total of 28 groundwater points were selected in the northeast of the island of Gran Canaria corresponding to 26 large-diameter wells, one borehole and one spring. In Figure 2 the selected points are shown. Figure 2. Spatial distribution of the groundwater sampling points and groundwater head contours 2008–2009, modified from [28,29]. The names of the gullies are indicated. Coordinates in UTM WGS84 28N. 3. Materials and Methods 3.1. Sample Characteristics and Sampling Procedure Twenty-eight groundwater samples were taken during the field campaign carried out in the summer of 2014. Sampled groundwaters exhibit temperatures of up to 25 °C and/or high concentrations of dissolved volcanic gases. The depth of the wells varied from 125 to 600 m, passing through several geological structures. Water 2015, 7 2580 The samples were taken by pumping using submersible pumps, and so the results obtained can be influenced by the large purging volumes and pumping rates that vary from one sampling point to another. The samples were contained in dark glass bottles of 1 L capacity; the procedure to collect the samples was designed to reduce radon losses due to bubbling. At each location, the bottles were filled to the edge with the sampled water and then immediately closed to avoid loss of radon by degassing during transport to the laboratory. Radon levels were determined within 3–6 h after sample collection in order to minimize the influence of radioactive decay. 3.2. Equipment Setup The concentrations of radon in the groundwater samples were measured using the continuous active radon monitor ALPHAGUARD (Model PQ2000PRO) of Genitron-Saphymo, Germany [30]. This detector was selected due to its proven calibration stability and fast response to concentration gradients, which have been confirmed in numerous studies [31]. It utilizes a pulse-counting ionization chamber of 0.65 liter active volume to detect radon, and it is suitable for long-term monitoring of radon gas concentrations from 2 to 2 × 106 Bq/m3. The measurement procedure is based on the alpha spectrometry of radon and its progeny in the air that enters into the detection volume of the ionization chamber. In addition, the ALPHAGUARD registers the values of the main environmental parameters (temperature, humidity and atmospheric pressure) during the time of measurement. For the measurement of radon concentration in water, an AquaKIT system [32] was used. This system consists of a 500 mL container with a degassing device that works by passing bubbles through the sample, a gas pump (AphaPUMP), and a safety glass bottle which is connected to the detector within a closed loop, as shown in Figure 3. An active carbon filter is also used to reduce background radon concentrations in the system before every measurement. Figure 3. (a) Schematic view of the experimental set-up [30]; and (b) practical setup in the laboratory. Following the prescription of the supplier [33], before every water sample measurement, the system is first purged to reduce radon activity concentrations in the detector to background values for 10 min. After that, the water is introduced into the degassing vessel, and the AlphaGUARD and AlphaPUMP are turned on. After 10 min, the pump is switched off and the AlphaGUARD still measures the radon activity concentration for another 20 min. The AlphaGUARD monitor works in a “flow” mode and the radon concentration is recorded every minute. The flow rate of the pump is 0.5 L/min. Water 2015, 7 2581 The AlphaGUARD monitor indirectly measures the activity of radon in water, since the radon that is expelled is diluted in air within the measurement setup, and a small part determined by the partition coefficient of the radon remains diluted in the aqueous phase. The concentration in water, Cwater, in Bq/L is obtained by using: 0 () 1000 system sample air sample water VV CkTC V C  − +−    × = (1) where Cwater is the radon concentration in the water sample (Bq/L), Cair is the value of radon concentration in the measurement system provided by AlphaGUARD (Bq/m3), C0 is the system background radon concentration, Vsystem is the inner volume of the measurement system (mL), and Vsample is the volume of the water sample (mL). k(T) is an empirical diffusion coefficient which depends on the temperature of the water sample through the following expression [32] 0.502 ( ) ( ) 0.105 0.405 TC kT e ×° =+× (2) To determine if part of the radon in the water sample is generated from 226Ra, which could be dissolved in the own sample [13], a part of each water sample has been stored in their glass containers in order to re-measure the radon content after the time required for 226Ra to reach secular equilibrium with its progeny. 4. Results and Discussion 4.1. Radon as a Natural Radioactive Tracer to Study Aquifer Systems The results of groundwater radon concentrations for the 28 samples measured are shown in Table 1 and the spatial distribution of the results is shown in Figure 4. The average concentrations range from 0.3 to 76.9 Bq/L; the arithmetic mean is 12.8 Bq/L; the median of the distribution is 5.3 Bq/L; and the standard deviation is 17.7 Bq/L. Table 1. Radon activity concentrations in the measured samples. Samples Elevation (m above Sea Level) Depth (m asl) Depth Elevation (m asl) Radon Concentration (Bq/L) Rd1 235 0 235 76.9 ± 8.3 Rd2 108 150 −42 51.9 ± 8.9 Rd3 430 176 254 0.3 ± 0.3 Rd4 354 192 162 5.7 ± 1.9 Rd5 725 151 574 7.4 ± 3.7 Rd6 235 123 112 1.3 ± 1.0 Rd7 679 290 389 12.5 ± 3.1 Rd8 603 340 263 30.8 ± 6.4 Rd9 345 150 195 19.2 ± 4.6 Rd10 515 360 155 3.8 ± 2.4 Rd11 377 125 252 28.2 ± 6.1 Rd12 996 450 546 1.8 ± 1.0 Rd13 499 200 299 14.8 ± 4.1 Water 2015, 7 2582 Table 1. Cont. Samples Elevation (m above Sea Level) Depth (m asl) Depth Elevation (m asl) Radon Concentration (Bq/L) Rd14 452 400 52 0.9 ± 0.8 Rd15 620 315 305 9.7 ± 2.6 Rd16 740 280 460 2.8 ± 1.2 Rd17 260 170 90 0.8 ± 0.8 Rd18 523 230 293 37.3 ± 8.1 Rd19 420 372 48 4.2 ± 1.9 Rd20 625 400 225 4.9 ± 1.7 Rd21 810 302 508 2.6 ± 1.4 Rd22 481 143 338 12.4 ± 3.3 Rd23 750 608 142 1.8 ± 0.9 Rd24 1050 540 510 3.1 ± 1.4 Rd25 445 340 105 3.4 ± 1.5 Rd26 598 394 204 10.9 ± 2.8 Rd27 182 150 32 6.6 ± 2.5 Rd28 827 355 472 3.7 ± 1.5 Figure 4. Spatial distribution of radon activity concentration. Samples are grouped depending on concentration of radon and geographic distribution. The blue group is located in the Guia-Moya basins, the green group in the Moya-Azuaje basins, and the orange group in the Guiniguada-Telde basins. Water 2015, 7 2583 Several studies related to the use of radon as a hydrogeochemical tracer can be found in the literature, [7,34,35]. The main factors that influence the presence of 222Rn in groundwater are the content of 226Ra in the reservoir rock and its emanation coefficient, as well as the feasibility of mixing of various groundwater components [36,37]. 226Ra is present in all rocks and soils in variable amounts and, as mentioned above, it is more abundant in volcanic acid rocks such as phonolites than in basic ones such as basalts. Therefore, higher concentrations of 222Rn would be characteristic of groundwaters flowing through volcanic acid rocks. The behavior of dissolved 222Rn in groundwater is strongly influenced by the properties of the mother rock, in particular by the distribution of 226Ra (precursor of 222Rn), in relation to surface of pores and fissures in the rock where the interchange with groundwater occurs [38]. The half-life of radon is 3.82 days and it disappears in approximately 38 days by radioactive decay. Taking into account the representative velocity of groundwater, a radon particle cannot move more than a few dozen meters away from where it was incorporated [36]. This distance could vary depending on the aquifer transmissivity and 222Rn will reach greater distances (hundreds of meters) when groundwater circulates through fissures or fractures. In consequence, for a porous medium, the radon concentration in a sample is representative of the location where the sample was taken. Nevertheless there are many factors that can affect this ideal behavior, such as the recharging or mixing processes with water with low radon concentrations, or the temperature of the groundwater, since the solubility of radon gas increases as temperature rises. The spatial distribution of radon allows the establishment of three classes or groups (Figure 4) that could explain the similar behavior of groundwater in wells that are near each other, but located in different catchments. (a) Northwestern group (Guia-Moya). This group (blue line in Figure 4) has lower radon concentration values (lower than 10 Bq/L) and it is mainly located in the Guía basin but includes some points associated with the Moya basin. The samples included in this group are Rd10, Rd12, Rd14, Rd16, Rd17, Rd19, Rd20, Rd21, Rd23, Rd24, Rd25, and Rd28. Some statistical parameters are: arithmetic mean, 2.82 Bq/L; geometric mean, 2.47 Bq/L; median, 2.95 Bq/L; and standard deviation, 1.29 Bq/L. According to the deep geology of the area (cross section I-I′ in Figure 1), groundwater in these areas flow through basic rocks with little content in radon precursors. (b) Northwestern group (Guia-Moya). This group (blue line in Figure 4) has lower radon concentration values (lower than 10 Bq/L) and it is mainly located in the Guía basin but includes some points associated with the Moya basin. The samples included in this group are Rd10, Rd12, Rd14, Rd16, Rd17, Rd19, Rd20, Rd21, Rd23, Rd24, Rd25, and Rd28. Some statistical parameters are: arithmetic mean, 2.82 Bq/L; geometric mean, 2.47 Bq/L; median, 2.95 Bq/L; and standard deviation, 1.29 Bq/L. According to the deep geology of the area (cross section I-I′ in Figure 1), groundwater in these areas flow through basic rocks with little content in radon precursors. (c) Northern group (Moya-Azuaje). This group (green line in Figure 4) has the highest radon concentration values (reaching 76.9 Bq/L) and it is mainly located in the Azuaje basin but includes some points from the Moya basin. The samples included in this group are Rd1, Rd2, Rd6, Rd11, Rd13, Rd15 and Rd26. These data have an arithmetic mean of 27.67 Bq/L; geometric mean of 15.64 Bq/L; median of 14.8 Bq/L; and a standard deviation of 27.31 Bq/L. Water 2015, 7 2590 37. Przylibski, T.A.; Gorecka, J. 222Rn activity concentration differences in groundwaters of three Variscan granitoid massifs in the Sudetes (NE Bohemian Massif, SW Poland). J. Environ. Radioact. 2014, 134, 43–53. 38. Eisenbud, M.; Gessell, T. Environmental Radioactivity. From Natural, Industrial and Military Sources, 4th ed.; Academic Press: San Diego, CA, USA, 1997. 39. Naranjo, G; Cruz-Fuentes, T.; Cabrera, M.C.; Custodio, E. Estimating natural recharge by means of chloride mass balance in a volcanic aquifer: North-eastern Gran Canaria (Canary Islands, Spain). Water, in press. 40. International Commission on Radiological Protection (ICRP). Age dependent Doses to Members of the Public from Intake of Radionuclides, Part 5 Compilation of Ingestion and Inhalation Dose Coefficients. In Annals on the ICRP, ICRP Publication 72; Pergamon Press: Oxford, UK, 1996. 41. Aieta, E.M.; Singley, J.E.; Trussell, A.R.; Thorbjarnarson, K.W.; McGuire, M.J. Radionuclides in Drinking Water: An Overview. J. Am. Water Works Assoc. 1987, 79, 144–152. 42. Salonen, L. 238U series radionuclides as a source of increased radioactivity in groundwater originating from Finnish bedrock. In IAHS Publications-Series of Proceedings and Reports-International Association Hydrological Sciences; Great Britain Institute of Hidrology: Wallingford, Great Britain, 1994; Volume 222, pp. 71–84. 43. Suárez-Navarro, J.A.; Pujol, L.; de Pablo, M.A. Rapid determination of gross alpha-activity in sea water by coprecipitation. J. Radioanal. Nuclear Chem. 2002, 253, 47–52. 44. Procedimiento Para la Determinación del Índice de Actividad alfa Total en Muestras de Agua. Métodos de Coprecipitación y Evaporación. Available online: http://www.csn.es/publicaciones/ pro_09.pdf (accessed on 31 March 2015). © 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/).