Controlled Laboratory Investigations of Wellbore Concentration Response to Pumping

Abstract

The relationship between the ground water sample and the surrounding ground water environment is controlled by wellborn flow and mixing that carry ground water to the sampling device. Controlled laboratory conditions and uniform inflow and inflowing concentration allowed detailed investigations of wellbore concentration responses to pump-induced flow and mixing independent of external complicating influences. During pumping from the top of the screen, the results agreed with wellbore flow theory except for high-frequency, high-amplitude concentration fluctuations and mixing with the blank casing due to slight density contrasts (∼0.005%). Concentration fluctuations indicated partial mixing of ground water with well water and declining concentrations in the blank casing indicated that convection due to the slight density contrast overcame pump-induced velocities. During pumping from the bottom of the well, opposing forces of buoyancy and pumping produced small-scale cumulative mixing as the ground water from different portions of the screen approached the pump. Even during low-flow pumping (i.e., 220 mL/min), 5% of the water was prepurge well water after five well volumes were pumped. In the field the density contrasts that resulted in these mixing processes might be caused by a slight cooling of the wellhead or a slight increase in temperature with depth (>0.2°C/m). If the temperature gradient was more pronounced, the convective mixing of screen water with the potentially altered casing water would be more thorough and sampling without mixing would not be possible. If the temperature gradient is stable or if the water in the blank casing is not significantly altered, casing water mixing will not present a problem.