CHARACTERISTICS OF THERMAL INSTABILITY AND MIXING WITHIN GROUNDWATER MONITORING WELLS

In the 1960s a series of papers focused on the distortion of geothermal gradient measurements due to thermal convection within deep boreholes. These studies found that water within boreholes convects as a result of more than 0.01 deg.C increase in water temperature per meter of depth. Convection has been overlooked in groundwater monitoring wells because the process is assumed to be negligible. However, with the advent of low-flow sampling and micropurging vertical flow within the borehole can influence the stabilization and representativeness of low flow samples. One premise of low flow sampling is that altered water within the blank casing of the well does not mix with the water within the screen section of the well to a significant degree. Cool temperatures during fall winter and spring can propagate to depths of 10 meters or more. This cooling will cause thermal instability at various depths within an open column of well water at various times during the year depending on 1) the seasonal temperature fluctuations at the surface, 2) the thermal diffusivity of the soil, and 3) the position of the water column within the well relative to the temperature distribution in the soil. Due to the lag of temperature propagation, instable temperature distributions will always exist at some depth even during the summer months. Thermal instability due to cooling in the fall will propagate downward at a rate related to the thermal diffusivity of the soil and thermal convection will be come vigorous during the cold seasons. The convection is episodic and may overcome the flows of a pump within the well causing water to be carried down from the blank casing of the well and causing field parameters to fluctuate erratically during pumping. As the temperatures warm and the warmer temperatures propagate downward a zone of stability will appear at the top of the water column. The varying characteristics of thermal instability within a well suggest that the representativeness of groundwater samples and fluctuation of field parameters will vary during the year.