Following the Current: An Introduction to Mise à la Masse
First used in the mining industry in the 1920s, mise à la masse (MALM) – which means “excitation at the source” – is a technique typically used to map conductive mineral deposits. However, RETTEW has also adapted the MALM process to delineate resistive bodies, such as light non-aqueous phase liquid (LNAPL) plumes floating on groundwater surfaces. Using MALM to delineate these features helps us to site recovery wells within the LNAPL plume to maximize product recovery.
Since MALM studies provide only footprints (not cross sections) of electrical anomalies, they are generally employed for reconnaissance in advance of drilling or geoprobing, or to estimate the boundaries of known plumes.
Out of the Shadows: Shedding Light on a Hidden Plume
Gasoline or heating oil that spills on a pervious surface or leaks from an underground storage tank (UST) will infiltrate downward until it encounters the groundwater table. Since these products are less dense than water, they float as an LNAPL plume. RETTEW has developed a variation of traditional MALM surveys to map the footprint of such plumes. If a plume has a measurable thickness and a non-conductive-cased well is installed to penetrate its footprint, a current electrode can be lowered into the well and below the plume. Another current electrode is used at the ground surface to take spaced measurements across the area of concern. The electrical field lines between the two electrodes cannot pass through the LNAPL (an electrical resistor) and must travel beyond its edge to complete the circuit; thus, MALM surveys map the “shadow” of the plume as seen in the schematic above.
A residential UST experienced a catastrophic leak in the 2000s, contaminating the property and surrounding land. Environmental remediators installed several wells around the property to characterize the plume extent; some wells showed product, yet others did not. So, RETTEW was hired to detect and delineate the extent of LNAPL using MALM (Figure 1, upper right). Remediators cleaned up the spill; however, tests in the 2010s revealed residual product, so the MALM survey was repeated (Figure 1, lower right). Continued monitoring with another MALM survey in the 2020s (Figure 1, left) indicated that the plume, while still present surrounding the original leak, had been localized.
Introducing Tyler Howe
Geophysicist
Tyler Howe spends his days uncovering what lies beneath the surface, but there’s much more to his story than geophysics. Learn how a Massachusetts native with a talent for juggling found his way to Lancaster and the world of subsurface exploration.
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