In-Situ, Inc. Technical Note October 2011
A dense non-aqueous phase liquid, or DNAPL, is a substance that is denser than water and only slightly soluble. Because of these properties, DNAPLs introduced into the environment generally sink below the water table and may migrate down through an aquifer. This can pose risks to the environment and human health and may necessitate remediation. Site remediation is a complex process that often requires a custom approach, and depending upon site characteristics, different treatment techniques will be appropriate. During the various phases of remediation such as site characterization, clean-up, and long-term monitoring, it is important to evaluate the DNAPL layer in a water column. One way to measure water and DNAPL levels is to collect real-time data with Level TROLL Instruments. By installing one Level TROLL in the DNAPL layer and a second Level TROLL in the water column, it is possible to calculate the thickness of the DNAPL layer based on the difference in density between the two fluids. This automated method of collecting data helps avoid the difficulties and labor associated with manual measurement of water and DNAPL levels. The equation for calculating the thickness of DNAPL within the well at any point in time given two flids with different densities is shown below. This calculation may be applied during the test or after data collection.
bd = the DNAPL thickness (feet)
H1 = the pressure reading at Level TROLL 1, installed in water column (feet)
H2 = the pressure reading at Level TROLL 2, installed in DNAPL (feet)
ΔZ = the elevation difference between Level TROLL 1 and Level TROLL 2
SGw = the specific gravity of water (dimensionless)
SGd = specific gravity of DNAPL (dimensionless)
You will need:
- Two Level TROLL 500 Instruments with RuggedCable Systems
- TROLL Com Communication Device
- Laptop computer or RuggedReader Handheld PC
- Rugged Oil/Water Interface Meter (Do not use this tape to measure the total depth of the well.)
- Rugged Water Level Tape (or some other method of determining the total depth of the well)
The following procedure should be used only as a reference. Customers are responsible for their own applications and data.
- Sample the DNAPL material to determine the specific gravity (SG). You will use the SG value when applying the calculation to your data.
- Measure the total depth of the well. The drill log will not account for material that has accumulated in the bottom of the well (e.g., sanding-in). Do not use the Level TROLL Instruments or the Oil/Water Interface Meter to determine total depth because it may damage the instruments.
- Program both Level TROLL Instruments to record Depth, in linear mode, with specific gravity set to the default value of 0.99. Set both instrument logs to start at the same time.
- Determine the water level.
- Determine the DNAPL/water interface using the Rugged Oil/Water Interface Meter.
- Deploy Level TROLL 1 in the water column.
- Deploy Level TROLL 2 in DNAPL layer several inches above the measured total depth of the well. Take note of the distance of the probe from the bottom of the well.
- Logging will start at the time set in step 2.
- Allow the instruments to record data for the desired period of time.
- Using the data from both Level TROLL Instruments, apply the DNAPL level calculation to determine the height of the DNAPL layer.
Note: Remember to add the distance from the bottom of the well to probe 2 to the DNAPL layer bd.
Bhandari, et al. 2007, Remediation Technologies for Soils and Groundwater. American Society of Civil Engineers.