Paper No. 6
Presentation Time: 10:15 AM
CHOOSING THE RIGHT FIELD METHODS TO OBTAIN TRUE IN SITU TDGP
RANDELL, J., Geoscience, University of Calgary, 2500 University Dr. N.W, Calgary, AB T2N 1N4, Canada, ROY, James W., National Water Research Institute, Environment Canada, Burlington, ON L7R 4A6, Canada and
RYAN, M.C., Department of Geoscience, University of Calgary, 2500 University Drive Northwest, Calgary, AB T2N 1N4, Canada, cryan@ucalgary.ca
True groundwater gas concentrations require
in situ total dissolved gas pressure (TDGP) estimation in addition to gas composition. Although total dissolved gas pressure is being rapidly adopted as a valuable tool for a wide variety of applications, obtaining in situ values and selecting a gas pressure probe remains a challenge. For both snapshot measurements and temporal data collection,
in situ TDGP needs to be measured in ‘fresh’ (i.e. not degassed) groundwater at the well screen depth, under a hydrostatic pressure that is close to static level. Groundwater in monitoring wells that have not been purged is typically degassed, and drawdown can result in in-well ebullition in gas-charged groundwater.
There are currently two approaches to estimating in situ TDGP. 1. Productive wells can be purged without significant drawdown. In this case TDGP is measured at the screen during low-flow purging until an equilibrium value is reached. 2. TDGP measurement in wells that cannot be purged without significant drawdown (common on the prairies) is more tractably obtained by isolating groundwater at the well screen and measuring downhole TDGP until an equilibrium value is obtained. Both of these approaches can be adopted for temporal data collection. The choice of field method can also depend on water table depth and the pumping approaches available, and whether one wants to avoid pumping contaminated groundwater.
Since equilibration times can vary depending on the type of groundwater gas(es) and between TDGP probes, data showing the variation in calibration times are presented.
