The average concentration of SF6 in the atmosphere of the Northern Hemisphere has been rising since the 1970s. By comparing the SF6 concentration of groundwater and the historical record of atmospheric concentration, groundwater can be dated from about the 1970s to the present. We therefore need to collect samples using an SF6-free pump or similar device, which prevents contact of the sample with the atmosphere, thus ensuring that atmospheric SF6 does not mix with the sample. Reduction of both the time and effort for water sampling caused by this constraint will result in more efficient investigation.
We have proposed a simple manually operated water sampling method using a well water sampler (well bailer bucket) for SF6-based dating of groundwater. During the process of groundwater sampling using a well bailer bucket, the sample comes into contact with the atmosphere. The calculation of SF6 transfer from gas to liquid phase indicates that during this process, the theoretical increase in the SF6 concentration of groundwater is less than 1%, which corresponds to a residence time of 0.2 years and is within the range of analysis accuracy of 3%.
This simplified method using a well bailer bucket (the simple method) can obtain the same results as the ordinarily used sampling method (the conventional method) in which water samples are collected without contact with the atmosphere. The difference between the SF6 concentration of the conventional method and that of the simple method is about 2% of the average value of both, which corresponds to about 0.5 years of apparent residence time. Even if sampled water comes into contact with the atmosphere during the sampling process by using the simple method, the dissolution of atmospheric SF6 into the sampled water is extremely small.
Depending on the sampling depth and survey equipment, the simple method can reduce the sampling time (for example, a reduction of 60 to 70%, including preparation for sampling). The simple method, which does not use a pump, has no sampling depth restriction due to the pump head. Furthermore, the simple method can lessen the weight of equipment, reducing the burden on survey participants.