A comparison of the diffusion sampler and copper tube sampling techniques for the extraction and analysis of noble gases in groundwater
Julia van der Merwe, Ian Clark
Department of Earth and Environmental Sciences, University of Ottawa
Julia van der Merwe, Ian Clark
Department of Earth and Environmental Sciences, University of Ottawa
In 2013, the Government of Canada estimated that 30% of the Canadian population relies on groundwater as their primary source of freshwater. Understanding properties about aquifers is critical to avoid problems like over pumping, which would deplete the aquifer and remove it as a source of freshwater. Noble gases allow us to determine recharge temperatures of the aquifer, which can then be used to help define recharge conditions and, in the case of old groundwater, past climate conditions during recharge. This paper will compare two methods of collecting noble gases from groundwater accessed via artesian wells. In both methods, soft copper tubes are used as the malleable nature of copper allows it to be sealed and reopened, and its non-permeable barrier stops gases from exiting the sample. The first method extracts a sample of groundwater to be stored in copper tubes for sampling at a later date. This method takes about 20 minutes in the field. The second method uses a diffusion sampler, which allows for the separation of gases from the groundwater using a semipermeable silicon membrane attached to a copper tube. This method requires equilibration times of 48 to 72 hours. Samples from the field were taken in Elmvale, ON, where groundwater is among the purest in the world. In addition to field sampling, determining a minimum time requirement for the diffusion samplers to equilibrate is necessary. By building a sampling line with both the diffusion sampler and a Residual Gas Analyser, aliquots of gas from the diffuser can be taken at anytime, and the concentration determined using the Residual Gas Analyser. By determining equilibration time of the diffuser with water in a controlled setting, sampling costs in the field can be reduced while accuracy of the results remains high.