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Numerical Simulations to Assess the Monitoring Zone Achieved during Low‐Flow Purging and Sampling
Author(s) -
Varljen Mark D.,
Barcelona Michael J.,
Obereiner James,
Kaminski David
Publication year - 2006
Publication title -
groundwater monitoring and remediation
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.677
H-Index - 47
eISSN - 1745-6592
pISSN - 1069-3629
DOI - 10.1111/j.1745-6592.2006.00029.x
Subject(s) - aquifer , sampling (signal processing) , flow (mathematics) , flux (metallurgy) , water well , environmental science , geology , groundwater , volumetric flow rate , soil science , hydrology (agriculture) , mechanics , petroleum engineering , geotechnical engineering , engineering , materials science , telecommunications , physics , detector , metallurgy
A detailed three‐dimensional numerical simulation of ground water flow into the screen of a monitoring well during “low‐flow” purging and sampling provides a means to investigate the actual monitoring zone (i.e., “where the water comes from”) achieved during these sampling operations. Such a simulation allows for examination of the influence of several variables such as pump placement, well screen and sand‐pack configuration, and aquifer heterogeneities on the actual part of the aquifer that is interrogated. In these simulations, ground water was sampled from the entire length of the well screen, although as could be expected, flux was higher into the well screen from zones of higher permeability, if present. Furthermore, at low flow rates, pump position within the screen had little influence on the vertical distribution of flux into the well, and in all cases where a monitoring well partially penetrates an aquifer, the zone monitored extended above and below the well screen. At the low flow rates examined (250 and 500 mL/min), there was no increase in flux (from the aquifer into the well screen) adjacent to the pump intake. The major implications of these findings are that a detection monitoring approach predicated on sampling from the entire well screen is valid when purging at low flow rates (i.e., contaminants are not “missed”), or conversely, that discrete vertical sampling may only be accomplished with very short well screens due to potential concentration averaging that would occur along the entire length of a screen. Finally, the simulation results emphasize that aquifer heterogeneities must always be recognized and considered in ground water monitoring design. Aquifer heterogeneities have the most significant influence on the actual monitoring zone (and therefore the potential to “hit” or “miss” contaminants in a detection program). Pump placement and purging rate have little influence on the vertical interval that is sampled (as compared to screen length and placement) and therefore should be selected only so as to minimize sampling artifacts and promote sampling efficiency.