radon in groundwater - Mark- och vattenteknik - KTH

Kirlna Skeppström TRITA LWR.LIC 2032water containing 1000 Bq/l. Groundwater isextracted from different sources andgroundwater from bedrock aquifers (drilledwells) has a greater potential of containingan enhanced concentration of 222 Rn thangroundwater extracted from soil aquifers indug wells (Åkerblom and Lindgren, 1997).In bedrock aquifers, groundwater occurs andflows in fractures. Radium-rich mineralspresent along the surfaces of fractures decayand release 222 Rn atoms into groundwater.Since the recoil distance as well as the diffusionlength of 222 Rn is small (Tanner, 1980),it is widely accepted that only those mineralslying on the surfaces of fractures, in directcontact with the groundwater, contribute toincreased 222 Rn levels in the water. Changesin groundwater chemistry can lead to differentmineral fluid interactions (Siegel andBryan, 2004). 238 U is affected by redox conditions.When an oxidising environmentprevails, 238 U enters solution and is transportedalong with the water and is eventuallyprecipitated in a reducing environment(Schumann and Gundersen, 1996). 226 Raoccurrence and distribution in groundwateris guided by its production from its immediateparent isotope thorium ( 230 Th) and itsremoval from solution is governed by theadsorption or cation exchange properties(Herczeg et al., 1988). The re-deposition of238 U and 226 Ra along the walls of a fracture isknown as secondary mineral enrichment andcontributes to significantly increasing theconcentration of222 Rn in groundwater(Åkerblom and Lindgren, 1997).The above-mentioned microscopic processescombined with the heterogeneity offractures in a rock matrix often make itdifficult to quantify transport of radioactiveelements in a bedrock aquifer. Not all fracturesare hydraulically significant - only a feware hydraulic conductors and facilitate fluidflow (NRC, 1996; Olofsson et al., 2001).However, despite the complexity of thebedrock aquifer system, there is still a needto predict radon potential in groundwater. InSweden and other countries, a large numberof people depend on groundwater extractedfrom bedrock aquifers for their daily waterneeds, implying long-term exposure to radioactive222 Rn in some cases. At municipallevel, prediction of radon potential ingroundwater is important for the planningof new housing areas, as well as the considerationof remediation measures for existinghouses. In order to make realistic predictionof 222 Rn levels in groundwater, influencingfactors should be studied in a holistic way.Many studies conducted on radon in thenatural environment are principally surveysand involve sampling and measurements ofradon in groundwater. Such studies areperformed for different reasons, includingsearching for uranium deposits, searchingfor thrust and faults, searching for seismicrelatedchanges and also for better knowledgeof the spatial distribution of radon(Monnin and Seidel, 1992; Choubey et al.2001; Porsani et al., 2005; Schubert et al.,2005; Ishikawa et al., 2005). Such surveys arenecessary steps in the reconnaissance of anarea prior to more detailed analyses regardinginfluencing factors. Other studies focusmainly on geological aspects, such as bedrockcomposition and uranium occurrenceand distribution in bedrock. Åkerblom andLindgren (1997) investigated bedrock andsoil data in Sweden with the aim of mappingthe groundwater radon potential. Choubeyet al. (2000) investigated hydrogeology andfound that the groundwater flow characteristicsinfluence the concentration of radon insprings. Other studies related to bedrockand distribution of uranium include thework of Lidén et al. (1995), Ståhl (1997),Choubey and Ramola (1997) and Salih et al.(2002). Studies on temporal variations inradon in groundwater are few. Knutsson(1977) studied seasonal fluctuations for dugwells and found large variations dependingon type of soil aquifer. Nilssen (2001) observedlarge fluctuations in 222 Rn concentrationin drilled wells from granite rocks.Regarding radon and groundwater chemistry,only a few correlation studies have beenperformed (Lewin Pihblad, 1998; Lewin andSimeonidis, 1998; Salih et al., 2004). Most ofthe research work related to groundwaterchemistry focuses principally on the parentelements of 238 U and 226 Ra (Waite et al.,1994; Schumann and Gundersen, 1996;2