Understanding and measuring groundwater gases

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Dissolved groundwater gases have been under-utilized as a tool to understand groundwater geochemistry partly because their sampling and analysis can be challenging. Groundwater gases are particularly important in the natural attenuation of groundwater contaminants, since many of the desired end products are gases (e.g. CO2, CH4, and N2). True dissolved gas concentration estimates rely on accurate in situ measurement of total dissolved gas pressure (TDGP). The measurement of TDGP, and its incorporation into 'true' groundwater gas concentration estimation is relatively new, and accurate field measurement approaches are still in development. The goal of this proposal will be to improve accurate groundwater gas pressure measurement, and to use these improved techniques to better understand controls on groundwater gas ebullition and transport. This will permit more robust mass balances of geochemical reactions (including natural attenuation of groundwater contaminants). The research has five related investigations. 1. Development of downhole instrumentation to trap and measure the volume of gas bubbles released from gas-charged groundwater; 2. This 'ebullition trap' will be one of the tools used to investigate the reason that groundwater in open wells degasses to concentrations that are well below solubilities (which increase with depth below the groundwater table). 3. In situ TDGPs will be measured in gas-charged wells to evaluate the common assumption that TDGP increase linearly with depth (and solubility). 4. Seasonal changes in groundwater temperatures at shallow, contaminated and groundwater gas-charged sites will provide an opportunity to measure varying rates of gas production, and incorporate them into estimates of contaminant degradation rates in the context of Monitored Natural Attenuation at contaminated sites. 5. The accrued experience on measuring and understanding groundwater gas concentrations will be brought to bear to solve a hydrogeologic 'mystery' of the source of oxygen poor gas that has been responsible for six deaths and three near-deaths in central Alberta well pits.