David Kaminski – Senior Vice President
Severn Trent/QED Environmental Systems Inc.
Ann Arbor, Michigan
There is a too-common misconception that
low-flow sampling (LFS) will produce a discrete sample from a very narrow
zone within the well screen or borehole. This leads to the conclusion that
pump placement becomes a critical factor in proper application of LFS to
avoid "missing the contamination" in the well.
But contrary to this popular belief, LFS will typically produce a
composite sample of the entire well screen zone, with some effect on flow
in any portion of the screen due to geologic heterogeneities. The
published research has shown that during pumping (as opposed to under
ambient flow) in a typical short well screen (10-20 feet long), the water
column within the screen zone is effectively mixed by the movement of
water through the sand pack and within the screen during pumping,
averaging sample concentrations across the entire screen zone
(Martin-Hayden and Robbins, 1997; Martin-Hayden, 2000a and 2000b; Puls and
Paul, 1998). This mixing occurs even at relatively low flow rates of 250 -
500 mL/minute.
Earlier this year, I worked with Mark Varljen
and Mike Barcelona to model this flow in the vicinity of the well screen
to determine how pump placement might affect this mixing. Our modeling
results show that pump placement had very little effect on the flux
measured in small vertical slices of the entire screen zone.
Download David
Kaminski's presentation on this topic...
References
Martin-Hayden, J. and G.A. Robbins, 1997.
Plume distortion and apparent attenuation due to concentration averaging
in monitoring wells. Ground Water vol.35, no 2, pp. 339-346.
Martin-Hayden, J. 2000a. Sample Concentration Response to Laminar Wellbore
Flow: Implications for Groundwater Data Variability. Ground Water. V.38,
no. 1. pp. 12-19.
Martin-Hayden, J. 2000b. Controlled Laboratory Investigations of Wellbore
Concentration Response to Pumping. Groundwater. V 38 no 1 pp. 121-128.
Puls, Robert W. and Cynthia J. Paul, 1998. "Discrete-Level Ground-Water
Monitoring System for Containment and Remedial Performance Assessment
Objectives."
Journal of Environmental Engineering, June 1998, pp. 549-553.
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