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.

Testing the In‐Well Horizontal Laminar Flow Assumption with a Sand‐Tank Well Model

“…No‐purge techniques are also being explored and are being adopted where applicable (Vroblesky 2001; Parker and Clark 2002, 2004; Parsons Engineering Science 2003; Interstate Technology and Regulatory Council 2004). These alternative techniques can solve some problems like elevated turbidity and volatile organic compound loss caused by bailer agitation or high pump rates, but they do not solve problems with vertical flow (Elci et al 2001, 2003) or pumping‐induced variability (Martin‐Hayden 2000a, 2000b; Gibs et al 2000). Other techniques involve installing multichannel tubing wells (Einarson and Cherry 2002), short‐screen direct‐push wells (Kram et al 2001), or devices such as the discrete multilevel sampler (DMLS) within existing longer screen wells (Puls and Paul 1997).…”

An Assessment of Aquifer/Well Flow Dynamics: Identification of Parameters Key to Passive Sampling and Application of Downhole Sensor Technologies

“…Analytical models also allow the results to be generalized in order to provide design criteria for pumping and sampling (Martin-Hayden 2000b). Previous models assumed inviscid flow, i.e., Darcian flow, within the wellbore.…”

“…No-purge techniques are also being explored and are being adopted where applicable (Vroblesky, 2001, Parker andClark, 2002;Mulherin, 2007, Parsons Engineering Science, 2003;ITRC, 2004ITRC, , 2006ITRC, , 2007. These alternative techniques can solve some problems like elevated turbidity and VOC loss caused by bailer agitation or high pump rates, but they do not solve problems with vertical flow (Elci, et al, 2001 and, pumping-induced variability (Martin-Hayden, 2000a and2000b;Gibs, et al, 2000, Martin-Hayden and Britt 2006), or well convection (Martin-Hayden and Britt 2006Vroblesky et al 2007). …”

Measuring Salinity within Shallow Piezometers:Comparison of Two Field Methods

“…Watertable level data were measured with a standard water-level meter (resolution: 1 cm). The new method (method B) uses a different approach; in order to reduce the bias due to stagnant water above the screens, we decided to flush away from the piezometer a water quantity equivalent to 3 wet borehole volumes [24] and to measure the electric conductivity and temperature at the top and bottom of the piezometer. Pumping has been conducted from the top of the watertable (see Figure 1) at an approximate rate of 4 liters per minute.…”

“…The salinity measured after pumping was interpreted using the work described in [24] on wellbore responses to pump-induced flow and mixing in controlled laboratory conditions. Our idea was to interpret field salinity profile variations in wells due to pumping in controlled conditions for different well screen lengths in a way to relate these variations to different well geometries.…”