Geothermics 42 (2012) 32–46 Contents lists available at SciVerse ScienceDirect Geothermics journal homepage: www.elsevier.com/locate/geothermics Correlations between radium and radon occurrence and hydrogeochemical features for various geothermal aquifers in Northwestern Romania Carmen Andreea Roba a,∗ , Dan Nit ¸˘ a a , Constantin Cosma a , Vlad Codrea b ,S ¸ tefan Olah c a Babes ¸ -Bolyai University, Department of Environmental Science and Engineering, Romania b Babes ¸ -Bolyai University, Department of Geology-Palaeontology, Romania c S.C. Transgex S.A., Romania article info Article history: Received 12 May 2010 Accepted 5 December 2011 Available online 9 January 2012 Keywords: Western Romania Geothermal waters Hydrogeochemical survey Radionuclides migration Geochemical processes abstract Geothermal waters originating from three aquifers in northwestern Romania have been analyzed for several physico-chemical parameters namely, pH, temperature, electrical conductivity, total dissolved solids, hardness, redox potential, and chemical oxygen demand. In addition, major dissolved ions and organic matter content and radium and radon occurrence have been analyzed. The three aquifers are the fissured Middle Triassic limestone/dolomite aquifer from Oradea, the Lower Cretaceous lime- stone aquifer from 1Mai–Felix Spa, and the Lower Pontian (Late Miocene) granular (sand) aquifer from S˘ acuieni. The first two aquifers belong to the Inner Dacides and the last to the Pannonian Basin. We have examined the relationships between the occurrence of radium and radon in the thermal waters and the hydrochemical and geothermal features of the aquifers, which may impart a secondary control on the activity of these radionuclides. According to their chemistry, the thermal waters were clas- sified as sulfate–bicarbonate–calcium–magnesium (Oradea), bicarbonate–sulfate–calcium–magnesium (Felix–1Mai Spa), and bicarbonate–sodium–chloride (S˘ acuieni). The activities of radon and radium were higher in S ˘ acuieni (up to 22.88 and 1.40 Bq/L) and Oradea (up to 34.82 and 1.82 Bq/L) than in Felix–1Mai Spa (up to 16.76 and 0.19 Bq/L). The data shows that the thermal processes may control the migration and distribution of radionuclides, and that the mobility of radionuclides may be influenced by several physico-chemical parameters, such as pH, total dissolved solids, and redox potential, or by some chemical processes, such as adsorption on humic acids or partitions into the organic (hydrocarbons) phase. The data illustrate no significant differences between the parameters analyzed from February to July 2009, proving that the production regime of the aquifers has a minor impact on their chemical and physical parameters. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Radium ( 226 Ra) and radon ( 222 Rn) are two of the most common naturally occurring radionuclides found in groundwater. Radium (with half-life T ½ = 1620 years) is produced by the alpha decay of thorium ( 230 Th with half-life T ½ = 75,200 years) in the uranium ( 238 U) decay series. Radon (T ½ = 3.8 days) is produced by the alpha decay of its parent radium. Some geothermal waters can have high concentrations of radium and radon. For example, in some thermal springs used as spas in Spain, radon activities up to 1868 Bq/L and radium activities up to 29.03 Bq/L were reported (Ródenas et al., 2008). The release of these waters into the surrounding environment may raise concerns about the aquatic ecology and health risks for local residents. As a consequence, the continuous monitoring of the thermal waters’ ∗ Corresponding author. Tel.: +40 264 307030; fax: +40 264 307032. E-mail address: robaacarmen@yahoo.com (C.A. Roba). radioactivity is necessary for the evaluation and prevention of pos- sible harmful health effects caused by the use and disposal of these waters. It is known that radium and radon activities in the groundwa- ter are mainly controlled by uranium and thorium contents of the aquifer (Zukin et al., 1987; Isam et al., 2002). The hydrochemical features of the aquifer (Szabo et al., 1992; Sturchio et al., 1993, 2001; Vinson et al., 2009; Vengosh et al., 2009) or the presence of high geothermal heat flow (Kruger et al., 1977) may impart a secondary control on radium and radon distribution in the ground- water. Radon is almost chemically inert. Its behavior is basically determined by the physical processes and not by the chemical interactions, and it reflects the presence of its parent—radium. Because of its gaseous state and inert chemical properties, radon is very mobile and can move either as a distinct gas or dissolved in geofluids, as cold or hot water or petroleum (Dickson, 1990; Etiope and Martinelli, 2002). In contrast, radium is highly chemi- cally reactive. Radium mobility or its adsorption may be influenced by several physico-chemical parameters like low pH (Szabo et al., 0375-6505/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.geothermics.2011.12.001