Radon as a natural radiotracer to investigate infiltration from surface water to nearby aquifers: a case study from the Barada riverbank, Syria

Abstract

Radon (222Rn) is an inert radioactive gas that is constantly emanated from soils, rocks and groundwater due to alpha decay of radium ( 226Ra). The radioactivity, inertness and gaseous nature of radon make it a significant environmental tracer for examining some hydrogeological issues that involve interactions between surface and ground waters. The main objective of the present study is to investigate possible infiltration from the surface water of the Barada River, located in the Damascus basin, to an adjacent aquifer, which includes some wells frequently exploited to support the drinking water network of Damascus city. The technique applied here is based on the principle that as low-radon river water flows through the aquifer matrix, it continuously absorbs radon emanated from clayey sediments present in the aquifer, with equilibrium between radon uptake and radioactive decay being reached. The distribution of spatial groundwater radon variation has been monitored in four pre-existing wells drilled in a fluvial aquifer system along the margin of the Barada riverbank. The results revealed clear ingrowth of radon concentration by three to four orders of magnitude, reaching a steady equilibrium state after about four halflives. Based on the standard law of radioactivity, the residence time of infiltrated river water was estimated. Hydrochemical data in addition to geological investigations and radon activities revealed primarily evidences of infiltration from the Barada river water to the nearby aquifers system, which may become vulnerable to a risk of environmental contamination. Finally, the results also showed that ingrowth of radon activity in groundwater is positively correlated with the concentration of EC (R2 = 0.89) and to a less extent with TDS values (R2 = 0.67). This evident association is most likely due to the presence of some organic matter and clayey minerals in the sediments of the aquifer matrix.