What are DNAPL & LNAPL?

gasoline
Posted: January 10, 2018
Category: Environment
Tags: dense, dense product, floating layers, hydrocarbon, interface, light, light product, liquid, measurement, non-aqueous, oil, phase, sinking layers, static levels, water

They are hazardous organic liquids such as dry cleaning chemicals, fuel oil and gasoline that do not dissolve in water.  These pollutants are always associated with human activity, cause severe environmental and health hazards, and are difficult to remove from the water supply once it has been compromised.  DNAPL & LNAPL are the two major categories of these non-aqueous Phase Liquid.

DNAPL or dense non-aqueous phase liquid is denser than water and does not dissolve in water.  When DNAPL is spilled it infiltrates the ground, contaminates the groundwater and then sinks below the water table, only stopping when it reaches impermeable bedrock. These pollutants can be so far below ground surface that they are difficult to find when investigating sites for contamination.  Chlorinated solvents, coal tar, creosote, PCB’s and extra heavy crude oil are examples of DNAPLs.

LNAPL or light non-aqueous phase liquid is lighter than water and non-soluble in water.  LNAPL is less dense than water so when it is spilled it infiltrates the ground but remains on top of the water table. Gasoline, benzene, toluene, xylene and other hydrocarbons are examples of LNAPLs.

The easiest method of determining the thickness of these pollutant layers is by using an Interface Meter.  Heron Instruments interface meter will emit a solid tone when it encounters a floating LNAPL layer and then change to an intermittent beep when it reaches the water layer meniscus.  By noting the readings on the tape when these changes occur you can determine an accurate measurement of the depth of the floating pollutant layer.  The inverse reaction will occur when the probe encounters the sinking layer of hydrocarbons allowing you to determine the depth of the DNAPL layer.

contaminants

It is almost impossible to remove all of the NAPLs once they are released into the ground and an aquifer contaminated with these substances can remain so for tens or hundreds of years. It will probably never become drinking water quality again.  These pollutants can often also move quite rapidly through the ground determined by the porosity of the ground and the chemical viscosity of the spill.

Many different treatment techniques have been developed to deal with these contaminants and new methods are still being developed.  Pump and treat, where contaminated ground water is removed, treated and then reintroduced to the aquifer is quite common in dealing with LNAPL, but with DNAPL it can be much less efficient.  A monitoring oil well skimmer can be employed to remove LNAPL from the surface of the well and also often DNAPL from the bottom of the well.

Randle Reef in Hamilton Harbour is an extreme example of this type of contamination.  This area is so polluted that remediation is not even being attempted.  A 6.3 hectare containment facility is currently being constructed in the middle of the harbour into which the surrounding sediment will be placed.  This area will then be dewatered, covered and sealed thereby removing and isolating these pollutants from the environment.

To determine the presence and abundance of these liquids, Heron Instruments manufactures the perfect tool.  Our H.0IL interface meter will indicate the presence of any NAPL as well as the depth of the contaminant level, whether LNAPL or DNAPL.  We also have the Sm.0IL, available in a 20m length only, for use in shallow wells or tanks.  A visual and audible signal will be given at the top of the layer and at each interface.  Both these meters are CSA Class 0 Certified Intrinsically Safe and boast a Kynar jacketed, high tensile steel core tape able to withstand hydrocarbons, solvents and other harsh contaminants

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