Radon as a groundwater tracer in Forsmark and Laxemar

Radon concentrations were measured in different water types in Forsmark and Laxemar during the site investigation and within this study. From these measurements it can be concluded that large differences between surface water, near surface groundwater and deep groundwater can be found in both Laxemar and Forsmark. The differences in radon concentrations between different water types are used in this study to detect interactions between surface water, near surface water and deep groundwater. From the radon measurements it can also be concluded that radon concentration in deep groundwater varies largely with depth. These variations with depth are probably caused by groundwater flow in conductive fracture zones in the bedrock. The focus of this study has been the radon concentration of near surface groundwater and the interaction between near surface groundwater and deep groundwater. Radon measurements have been done using the RAD-7 radon detector within this study. It could be concluded that RAD-7 is a good technique for radon measurements and also easy to use in field. The radon concentrations measured in near surface groundwater in Laxemar within this study were low and homogenous. The variation in radon concentration has been analyses and compared to other parameters. Since the hypothesis of this study has been that there are differences in radon concentrations between recharging and discharging groundwater, the most important parameter to consider is the recharge/discharge field classification of the wells. No correlation between the recharge/discharge classifications of wells and the radon concentrations were found. The lack of correlation between groundwater flow patterns and radon concentration means that it is not possible to detect flow patterns in near surface groundwater using radon as a tracer in the Laxemar area. The lack of correlation can be caused by the fact that there are just a few wells located in areas classified as recharge area. It can also be caused by the homogenous radon concentrations measured in the Laxemar area. The radon concentrations in near surface water measured in Forsmark showed large variability with both low and high radon concentrations. This large variability in radon concentration could not be explained by the flow pattern of the groundwater since no clear correlation between radon concentration and recharge/discharge classification was found. The radon concentration was also measured at different depths in the soil profile at three locations in the Forsmark area. The results showed large differences with increasing radon concentration with increasing depth. This gradient of radon concentration can be explained largely by the radon emanation potential of the local soil type at different depths. High radon concentrations were found in wells with higher radon emanation potential like till and bedrock. These observations showed the importance of the radon emanation potential of the local soil for the radon concentration in groundwater. The main purpose of this study has been to evaluate the use of radon as a tracer for groundwater flow patterns. The method is based on the ingrowth of radon from its progenitor radium according to the law of radioactive decay. According to this law the radon concentration in groundwater will reach equilibrium conditions after approximately 30 days in contact with the surrounding soil. The equilibrium radon concentration of the near surface groundwater was measured at several location in the Forsmark area and a range of the steady state radon concentration was calculated. The measured steady state radon concentration was then used to evaluate the radon concentrations measured in near surface groundwater in the area. A recharge/discharge classification of the wells was done based on the range of steady state radon concentration and the measured radon concentrations in groundwater. All wells with radon concentration below the steady state radon concentration were classified as recharge wells and all wells with radon concentrations above the steady stat e radon concentration were classified as discharge wells. The wells with radon concentrations within the range of steady state concentrations were classified as wells with groundwater that had been stagnant for at least 30 days. Most of the classified wells were classified as stagnant wells with concentrations within the range of steady state radon concentration. Only one well had low radon concentration and was classified as recharging well. This means that radon probably is a poor tracer for recharging groundwater in the Forsmark area since recharge is only identified at one site. Four wells had radon concentration significantly higher than the steady state radon concentration. This indicates discharge of deep groundwater with high radon concentration from conductive fracture zones in the bedrock at these sites. Three of these four wells are found in the catchment of Gaellsbotraesket where signs of discharging deep groundwater have been found in previous investigations. This shows that the results from this study can be used to support the conclusions from previous studies. In general this study showed that there were no systematic differences in radon concentrations of groundwater between recharge areas and discharge areas. Even if discharging groundwater was detected in the catchment of Gaellsbotraesket the usefulness of radon as a tracer was proven to be limited in this study since no general conclusions of the groundwater flow in Laxemar or Forsmark could be drawn from the radon measurements. This may be due to the short half-life of radon that makes the signals of recharge and discharge waters decay away before the water reaches the sampling wells. It can also be caused by the fact that signals from deep groundwater with high radon concentrations are diluted by the much larger volume of near surface groundwater.