Dissolved uranium, radium and radon evolution in the Continental Intercalaire aquifer, Algeria and Tunisia Elliot, T., Bonotto, D. M., & Andrews, J. N. (2014). Dissolved uranium, radium and radon evolution in the Continental Intercalaire aquifer, Algeria and Tunisia. Journal of Environmental Radioactivity, 137, 150-162. https://doi.org/10.1016/j.jenvrad.2014.07.003 Published in: Journal of Environmental Radioactivity Document Version: Publisher's PDF, also known as Version of record Queen's University Belfast - Research Portal: Link to publication record in Queen's University Belfast Research Portal Publisher rights This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). 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Sep. 2021 Journal of Environmental Radioactivity 137 (2014) 150e162 Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad Dissolved uranium, radium and radon evolution in the Continental Intercalaire aquifer, Algeria and Tunisia * Trevor Elliot a, , Daniel Marcos Bonotto b, John Napier Andrews c, 1 a Environmental Tracers Laboratory (ETL), School of Planning, Architecture & Civil Engineering (SPACE), Queen's University Belfast, David Keir Building, Stranmillis Road, Belfast BT9 5AG, Northern Ireland, UK b Departamento de Petrologia e Metalogenia, IGCE-UNESP, Av. 24-A, 1515, CP 178, CEP 13506-900 Rio Claro, Sao~ Paulo, Brazil c School of Chemistry, University of Bath, Bath, UK article info abstract Article history: Natural, dissolved 238U-series radionuclides (U, 226Ra, 222Rn) and activity ratios (A.R.s: 234U/238U; Received 16 April 2014 228Ra/226Ra) in Continental Intercalaire (CI) groundwaters and limited samples from the overlying Received in revised form Complexe Terminal (CT) aquifers of Algeria and Tunisia are discussed alongside core measurements for 2 July 2014 U/Th (and K) in the contexts of radiological water quality, geochemical controls in the aquifer, and water Accepted 7 July 2014 residence times. A redox barrier is characterised downgradient in the Algerian CI for which a trend of Available online 26 July 2014 increasing 234U/238U A.R.s with decreasing U-contents due to recoil-dominated 234U solution under reducing conditions allows residence time modelling ~500 ka for the highest enhanced A.R. ¼ 3.17. Keywords: fi Uranium isotopes Geochemical modelling therefore identi es waters towards the centre of the Grand Erg Oriental basin as 14 36 234 238 Radiogenic helium palaeowaters in line with reported C and Cl ages. A similar U/ U trend is evidenced in a few of Noble gas recharge temperatures the Tunisian CI waters. The paleoage status of these waters is affirmed by both noble gas recharge Groundwater ages temperatures and simple modelling of dissolved, radiogenic 4He-contents both for sampled Algerian and Continental Intercalaire & Complexe Tunisian CI and CT waters. For the regions studied these waters therefore should be regarded as “fossil” Terminal aquifers waters and treated effectively as a non-renewable resource. & Algeria Tunisia © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). 1. Introduction In Algeria, Amrani (2002) has measured bottled mineral waters (for 8 samples: 2.6e14 Bq/l 222Rn; 12e37 mBq/l 226Ra; 15e39 mBq/l Low-level activity, naturally-occurring uranium- and thorium- 232Th; 0.4e1.6 Bq/l 40K). Amrani et al. (2000) have published 222Rn series radionuclides and their isotopes in groundwaters can give measurements as 0.5e19.37 Bq/l (15 samples, only one insight into reductioneoxidation (redox) and geochemical controls, sample > 11 Bq/l) in groundwater from springs and deep wells of watererock interactions, aquifer mixing, and subsurface residence the Tassili N'Ajer high sandstone plateau in southeast Algeria. Most times of sampled waters in aquifer systems (Andrews, 1991; recently, Aït-Ziane et al. (2012) have reported groundwater 222Rn Bonotto, 2004; Porcelli, 2008). They also have significance for data (21 samples: 0.1e34.4 Bq/l, mean ¼ 11.5 Bq/l) and 226Ra (11 health in terms of their alpha (a-)radioactivity, and many countries samples: 0.25e5 Bq/l) for the Chott El Hodna (Algeria) region. Much have adopted regulatory standards for water use (cf. Chau et al., higher concentrations of 222Rn may be associated with oil and gas 2011). production (Hamlat et al., 2003) as radon is also a noble gas which In semi-arid and arid zones in Algeria, groundwater is the naturally and preferentially partitions to non-aqueous liquid and principal source of drinking water; and in Tunisia in 2010 the gas phases. High natural radium concentrations have been reported Ministry of Agriculture developed a strategic survey for sustainable associated also with palaeowaters, e.g. in the Disi sandstone aquifer water usage by 2050 which could include the use of non-traditional in Jordan (Vengosh et al., 2009). sources of water like desalination of seawater or salty groundwater. The Continental Intercalaire (CI) formation in North Africa hosts an extensive, regional, internally-drained (endorheic), sedimentary aquifer which underlies Algeria, Tunisia, and Libya. Castany (1981) originally emphasised the deep-basin nature of this aquifer system * Corresponding author. of the northern Sahara such that “development and management of E-mail addresses: [email protected] (T. Elliot), [email protected] …… ‘ ’” (D.M. Bonotto). water stored in aquifer [is] groundwater mining . Puri et al. 1 Deceased author. (2006), Mamou et al. (2006) and Edmunds (2012) have asserted http://dx.doi.org/10.1016/j.jenvrad.2014.07.003 0265-931X/© 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). T. Elliot et al. / Journal of Environmental Radioactivity 137 (2014) 150e162 151 that in the Saharan basins the water resource often can be shown to been attempted to date. Preliminary results for CI waters in the be ‘fossil’ or palaeowater, almost entirely non-renewable in terms of Tunisian aquifer by Petersen et al. (2013) and Frohlich€ (2013) sug- both their water resources management and in International Law gest residence times ~500 ka based on an apparent decreasing 234 238 234 3 (Eckstein and Eckstein, 2003, 2005). Semi-arid/arid regions like trend of U/ U activity ratios ( U t1/2 ¼ 244.5 Â 10 a) with Libya and Algeria are heavily dependent on groundwater as their increasing U-contents (see also Bonotto, 2006). In the current only water resource; coordinated management of this trans- study, the evolution of U, 234U/238U A.R., 226Ra (including boundary, shared water resource led to the creation of a Consul- 228Ra/226Ra A.R.), and 222Rn systematics particularly along a flow tation Mechanism Unit for the North Sahara Aquifer System e SASS line in the Algerian CI are discussed in terms of 238U-series sys- (Systeme Aquifere du Sahara Septentrional) in July 1999 (UNESCO, tematics and groundwater dating (including also 4He ages). 2010). Previous attempts to date the geochemical residence times of 2. Study area 14 the Algerian CI waters using C(t1/2 ¼ 5730 a: Gonfiantini et al., 1974; Sonntag et al., 1978; Guendouz, 1985; Elliot, 1990; The Continental Intercalaire (CI) aquifer underlies continuously Guendouz et al., 1997; Edmunds et al., 2003) identified that, apart an area ~600,000 km2 in Algeria and Tunisia (Castany, 1982) and from aquifer margins, the sampled CI waters all have low radio- ~100,000 km2 overall (including Libya). In Algeria, the M'Zab Ridge carbon activities (<5 per cent modern carbon, pmc). Towards the running NeS(Fig. 1) structurally provides a watershed divide be- centre of the basin, waters are very close to the limit of age tween (to the west) the Grand Erg Occidental and (to the east) the discrimination by the radiocarbon method (~25e30 ka). This along Grand Erg Oriental hydrogeological basins. The shallower, overlying with past climatic signatures archived in the waters through their Complexe Terminal (CT) aquifer covers ~35,000 km2. In the Grand stable isotopes of water signatures (d2H, d18O) and dissolved noble Erg Occidental the two aquifers are hydraulically connected (cf. gas contents (Elliot, 1990; Guendouz et al., 1997) appear to confirm Moulla et al., 2012), whereas in the Grand Erg Oriental the two their palaeowater status. Moreover, Guendouz and Michelot (2006) aquifers are separated by semi-permeable/impermeable layers and 36 5 report Cl (t1/2 ¼ 3 Â 10 a) dates for CI waters suggesting mini- confined, artesian conditions exist for the CI aquifer towards the mum model groundwater ages of 0e134 ka and maximum ages centre of this basin. 49e223 ka for relevant samples on the M'Zab ridge (Berriane, The CI formation comprises permeable