The mise a la masse technique is an electrical resistivity method that has been used in the mining industry since the 1920s for delineating electrically conductive subsurface ore bodies.  In that application, a current electrode is placed in the conductive body (either at a surface exposure or in a drill hole), with a second current electrode (to complete the circuit) in the ground at electrical infinity (actually just a distance greater than 5 times the largest expected dimension of the subsurface body).  This causes the entire body to radiate electric current.  The equipotential lines from this current flow pattern can be mapped with a voltmeter and two mobile probes.  The shape of these equipotentials typically mimic to some degree the footprint of the conductive body. Enviroscan has employed this method to map (among other targets) conductive (ionic) leachate plumes, flooded mine workings, and mineralized fault zones.

In the mid 1990s, Enviroscan conducted a series of tests and pilot projects of a modified mise a la masse method on sites with known light non-aqueous phase liquid (LNAPL) plumes.  These tests were performed in conjunction with site investigations under an EPA general technical assistance contract (GTAC), and led to development of an effective means for delineating the footprint of LNAPL plumes - which are typically highly electrically resistive.  In this application, one current electrode is placed beneath a hydrocarbon layer (e.g. in a monitoring well beneath known LNAPL), with the second at the ground surface above the body (typically near the same well head). For the circuit to complete, current must flow around the edges of a resistive body.

Since the physics of electric current and water flow are identical, picture the following: Hold a dinner plate above the drain of a sink or tub. If you try to pour water from a pitcher into the drain, it will spill over the edges of the plate, and cascade into the sink before running to the drain. If you are an ant with no ability to look up and see the plate, you could find the edges of the plate by running around and finding the extent of the water running towards the drain. Now substitute one current electrode (the “source” electrode for the pitcher, another current electrode for the drain (the “sink” electrode - no pun intended), an electrical resistor for the plate, and a voltmeter for the ant.  Flip the mental picture over, and you’ve got the mise a la masse technique for delineation of LNAPL.

Of course, it is not perfect.  Here are some of the difficulties:

• Conductors (e.g. metal well casings, USTs, etc. penetrating or spanning the plume will allow current to “leak” through - creating apparent holes in the plume.
   
• A thin plume with a wide smear zone may be difficult to delineate since the plume may reside as a coating on grains, with interstitial water locally capable of allowing diffuse current “leakage”.
   
• Some very old plumes with highly weathered product have recently been shown to have an electrically conductive corona (probably due to alteration of local water chemistry by the products of hydrocarbon breakdown).  In this case, a very old plume will still produce a mise a la masse anomaly, but with an apparently conductive footprint. Note that in this case, the method is simply the old original mise a la masse technique from the mining industry.

Since mise a la masse studies provide only footprints of electrical anomalies, Enviroscan recommends that they be employed for reconnaissance in advance of drilling or geoprobing, or to provide extrapolation of the boundaries of known plumes.  The mise a la masse technique for LNAPL delineation has proven particularly useful for siting of recovery wells within known plumes to allow maximum recovery of product.

Using the standard mise a la masse method, Enviroscan has delineated the traces of the water-producing fractures in wells - facilitating placement of other production and/or monitoring wells in the same feature.