Distinguishing seawater from geologic brine in saline coastal groundwater using radium-226; an example from the Sabkha of the UAE Thomas F. Kraemer a,c, ⁎, Warren W. Wood b , Ward E. Sanford c a U.S. Geological Survey, Woods Hole Coastal and Marine Science Center, 384 Woods Hole Rd., Woods Hole, MA 02543, United States b Department of Geological Sciences, Michigan State University, East Lansing, MI 48824, United States c U.S. Geological Survey, 12201 Sunrise Valley Dr., Reston, VA 20192, United States abstract article info Article history: Received 1 May 2013 Received in revised form 24 January 2014 Accepted 27 January 2014 Available online 6 February 2014 Editor: J. Fein Keywords: Sabkhat Radium-226 Geologic brine Seawater Sabkhat (Salt flats) are common geographic features of low-lying marine coastal areas that develop under hyper- arid climatic conditions. They are characterized by the presence of highly concentrated saline solutions and evap- oritic minerals, and have been cited in the geologic literature as present-day representations of hyper-arid regional paleohydrogeology, paleoclimatology, coastal processes, and sedimentation in the geologic record. It is therefore important that a correct understanding of the origin and development of these features be achieved. Knowledge of the source of solutes is an important first step in understanding these features. Historically, two theories have been advanced as to the main source of solutes in sabkha brines: an early concept entailing seawa- ter as the obvious source, and a more recent and dynamic theory involving ascending geologic brine forced upward into the base of the sabkha by a regional hydraulic gradient in the underlying formations. Ra-226 could uniquely distinguish between these sources under certain circumstances, as it is typically present at elevat- ed activity of hundreds to thousands of Bq/m 3 (Becquerels per cubic meter) in subsurface formation brines; at exceedingly low activities in open ocean and coastal water; and not significantly supplied to water from recently formed marine sedimentary framework material. The coastal marine sabkha of the Emirate of Abu Dhabi was used to test this hypothesis. The distribution of Ra-226 in 70 samples of sabkha brine (mean: 700 Bq/m 3 ), 7 sam- ples of underlying deeper formation brine (mean: 3416 Bq/m 3 ), the estimated value of seawater (b 16 Bq/m 3 ) and an estimate of supply from sabkha sedimentary framework grains (b~6 Bq/m 3 ) provide the first direct evi- dence that ascending geologic brine contributes significantly to the solutes of this sabkha system. Published by Elsevier B.V. 1. Introduction Sabkhat (salt flats) are extensive geographic features in many arid and hyper-arid coastal environments of the world and are typically associated with highly concentrated brines occurring in the shallow sur- ficial aquifers beneath the surface. As a result, contemporary sabkha environments have become geological analogs for evaporitic environ- ments in the sedimentary record and therefore it is important to identi- fy and correctly understand the processes responsible for accumulation of solutes in these systems. Early models of sabkha brine formation assumed that the solutes were concentrated by evaporation of seawater (Kinsman, 1969; Butler, 1969; Patterson and Kinsman, 1977, 1981, 1982) or seawater and shal- low groundwater (Hsü and Siegenthaler, 1969; Hsü and Schneider, 1973; McKenzie et al., 1980; Müller et al., 1990). Later, a fundamentally different model was presented whereby the majority of the water making up these brines was supplied by local rain- fall, but solutes originated to a large extent from upward migration of geologic brine from deeper formations, with seawater contributing only initially and in a minor way to the composition (Sanford and Wood, 2001; Wood and Sanford, 2002; Wood et al., 2002; Wood et al., 2005; Wood and Sanford, 2007; van Dam et al., 2009). This ascending brine model (ABM) overcame difficulties inherent in sea water-based models, and was in agreement with solute mass bal- ance, water isotope data, solute ratios and positive hydraulic head in the underlying formations. However, clear direct evidence that the model functioned as described, and is currently active, was lacking. Here we use the presence of Ra-226 to provide such primary evidence based on 1) its elevated activity of several hundred to thousands of Bq/m 3 (Becquerels per cubic meter) in subsurface brines (Kraemer and Reid, 1984; Vengosh et al., 2009), 2) near absence in open ocean and coastal seawater (b 16 Bq/m 3 , Okubo et al., 1979; Moore et al., 1985; Liu et al., 2010; Su et al., 2010; Ohta et al., 2011; Charette et al., 2013), and 3) lack of supply in significant amounts (as compared to for- mation brines) by sedimentary grains (b~6 Bq/m 3 ). The presence of radium in significant amounts (greater than a few hundred Bq/m 3 ) in the sabkha brine would therefore be strong evidence for the ultimate source of solutes being from geologic brines, whose accumulation in the sabkha brine, additionally, would have to be recent owing to the geologically short half-life of Ra-226 (t 1/2 = 1601 y). Chemical Geology 371 (2014) 1–8 ⁎ Corresponding author at: U.S. Geological Survey, Woods Hole Coastal and Marine Science Center, 384 Woods Hole Rd., Woods Hole, MA 02543, United States. E-mail address: tkraemer@usgs.gov (T.F. Kraemer). 0009-2541/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.chemgeo.2014.01.018 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo