How To Find NAPLs (LNAPL and DNAPL) With Induced Polarization

Non-aqueous phase liquids (NAPLs) are pollutants leaked into the ground.

• Light non-aqueous phase liquids (LNAPLs)—like benzine or gasoline—are lighter than water and can disperse through the earth more easily, making them difficult to image.

• Dense non-aqueous phase liquids (DNAPLs)—like creosote—are denser than water and tend to stay together as a more coherent body, making them easier to image.They will also sink downwards until they hit an impermeable layer like clay.

Induced polarization (IP) and resistivity imaging can be used to map both LNAPLs and DNAPLs, and is often used to find environmental spills or to map large areas of pollutants.

Use Case: Finding Historic Creosote Operations Using Induced Polarization

In the past, creosote—a black tar-like substance—was used to preserve the life of wooden railroad ties and telephone poles. Today, we understand that many defunct creosote treatment plants have contaminated the groundwater and soil. Many superfund sites across the U.S. are dedicated to extrapolating this DNAPL from the ground.

Finding areas affected with creosote is very difficult with any other geophysical method than induced polarization and resistivity imaging, which shows it very strongly. This is because creosote tends to sits on top of clay since clay is less transmissive than sandy soil layers. (Interestingly, NAPL itself doesn’t hold much charges, but bacteria breaks it down and uses it as a food source, and the bacteria and the breakdown product from the bacteria does hold charges.)

How To Find NAPLs With Induced Polarization

1. First, determine how deep you need to image, and how you want to view the image.

This will dictate the size of the imaging system and the number of electrodes needed. Whether or not you do a 2D or 3D image has more to do with budget and time constraints. Even if you can’t do a 3D image, you can still get a good idea of the affected subsurface with several 2D slices.

2. Hook up the resistivity and IP instrument.

Performing both resistivity and IP testing with, say, AGI’s SuperSting instrument, is very simple. To switch from resistivity to IP, simply change a setting on the instrument itself so that it will record both resistivity and IP simultaneously.

3. Inject current and measure the voltage—automatically.

Current is automatically injected through a dipole and the resulting voltage is measured some distance away. Using Ohm’s law provides you with the apparent resistivity. To measure induced polarization, the instrument simply turns off the injected current and leave the receiving electrodes on for a few seconds. This gives you the charge decay curve—or the time during which the charges stored in the ground dissipate. The chargeability is then calculated from the decay curve. This process is automatically repeated for different electrode combinations until the survey area is covered with measurements. The chargeability will change for each new electrode combination and location.

4. View map of chargeability (from induced polarization) and a map of resistivity.

The EarthImager software is used to process the data and make data presentation. When you use resistivity and IP, you can get an idea of both the geology of the area and the contamination in the ground. This area can then be tested (or ground-truthed) through drilling to confirm the contaminated area—saving the environmental cleanup company a great deal of time, effort, and money. Resistivity and IP monitoring can also provide information on how well a certain clean-up procedure progresses with time, as contaminants are removed the indication on the IP image will disappear.