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PHYSICAL PROPERTIES OF DNAPLS AND GROUNDWATER CONTAMINATION
An On-Line Version of a Column First Published in the:
National Environmental Journal Mar./Apr. 1995 Vol. 5 No. 2 Pages 23-24
By: David B. Vance
The physical behavior of Dense Non-Aqueous Phase Liquids (DNAPLs) can be broadly divided between two classes based on melting point. Polynuclear Aromatic Hydrocarbons (PAHs) have melting points that are at the lowest 80o C (for Naphthalene). Consequently PAHs are solids at the temperatures typical in the subsurface, thus the potential for migration through the soil column or groundwater systems is limited. Cosolvents can make PAHs soluble and mobile, however, the PAH solutions (when dissolved by petroleum hydrocarbons) are typically lighter than water and do not behave as DNAPLs. The second class of DNAPLs are chlorinated solvents. These compounds are liquids at normal subsurface temperatures, making them extremely mobile in unsaturated and saturated zone soils. This column will discuss those and other properties that make contamination by chlorinated solvents one of the most ubiquitous and recalcitrant of groundwater problems. The nature of those properties and the scope of the problem may well lead to significant revision of how groundwater contamination is perceived and resolved as an environmental issue.
First though it is worthwhile to remember that in the early years of this century the introduction of chlorinated solvents was perceived as a boon rather than a bane. Prior to that introduction distilled mineral spirits were the only available solvents. Explosions and fires were a constant threat and occurrence causing death and injury. In today’s climate where risk is evaluated in terms of 1 event in a million we would do well to remember the lives saved through the use of chlorinated solvents.
The properties that make chlorinated solvents so problematic with regards to groundwater contamination include:
- surface tension, and
Chlorinated solvents are significantly denser than water, density values in g/cc (water is 1.00) are:
- 1,1-DCA 1.18;
- 1,1-DCE 1.22;
- TCA 1.34;
- TCE 1.46; and
- PERC 1.62.
Upon release into the environment they sink through the vadose zone, through the water table, and over the long term possibly even through aquitards. A release of chlorinated solvents will travel downward or laterally until it has been exposed to a volume of soil sufficient to retain the released DNAPL in pore spaces within the soil matrix.
Due to high density chlorinated solvents can flow upgradient to the groundwater flow along the down dip surface of an aquitard. The most problematic phenomena caused by high density chlorinated solvents is the dimple effect. Upon release, chlorinated solvents will travel to an aquitard and seek the lowest point on the surface of that aquitard. It is unlikely that any given stratigraphic unit acting as an aquitard is perfectly planar, it will have small to large dips, depressions and dimples. Each of these can act as a reservoir for DNAPLs. These reservoirs will be outside of an advective flow regime set up by a "pump and treat" groundwater system. The only means of transport from these reservoirs is through diffusion, which is effective enough to cause significant dissolved phase contamination, but not effective enough to offer timely remediation.
Chlorinated solvents are soluble in water, values in mg/L are:
- 1,1-DCA 5,100;
- 1,1-DCE 400;
- TCA 700;
- TCE 1,100; and
- PERC 200.
These levels of solubility represent significant potential for transport of released chlorinated solvents in the dissolved phase.
Most chlorinated solvents are less viscous that water, values in centiposes (water is 1.00) are:
- 1,1-DCA 0.51;
- 1,1-DCE 0.36;
- TCA 0.90;
- TCE 0.57; and
- PERC 0.93.
The low viscosity makes chlorinated solvents extremely mobile in the vadose zone, they can readily flow through flow channels that may be as small as a human hair.
The surface tension of chlorinated solvents with respect to water has significant impact on the manner in which chlorinated solvents penetrate the capillary fringe zone and then migrate through the saturated zone. Water has a high surface tension of 73 dynes/cm, chlorinated solvents have surface tensions in the range of 20 to 40 dynes/cm. When free phase chlorinated solvents migrate to the capillary fringe they will be held up until a head is built up sufficient to overcome the capillary retention of the water. The capillary retention is directly proportional to the interfacial tension and inversely proportional to the pore throat radius and liquid density. The smaller the pore size the greater the head required to displace entrained water. This can result in significant lateral spreading of a DNAPL release above the capillary fringe in fine grained soils.
Another effect of surface tension forces involves how surfaces in larger pore spaces are wetted. The tendency of one fluid to replace another on a surface is termed wettability.
As broad rules the following applies:
- In soil, water is the wettting fluid with respect to solvents or air.
- Solvents are wetting fluids in air, but not when in the presence of water.
- With respect to carbonaceous soil components, solvents are wetting in the presence of air or water.
Free phase chlorinated solvents have the capacity to dehydrate clays, causing cracking and further migration through what at first analysis appears to be impermeable layers. Bentonite pellets used to seal wells in areas contaminated with DNAPLs may not work, due the failure of the bentonite to swell in those conditions. DNAPLs can also migrate through previously installed and sealed wells via the same mechanism.
All of the above phenomena impact the fate and transport of DNAPLs to and through groundwater systems. Those physical properties also manifest themselves by the nature of a given release. The release of a large volume of DNAPL in a short period of time causes rapid migration laterally as well as downward through the vadose and saturated zone that overcomes any small permeability differences. This leaves a significant volume of residual contamination entrained in the zone of passage. Conversely DNAPL release that occurs slowly over a long period of time will travel through narrow channels representing the most permeable path through the subsurface. Small differences in permeability will be exploited under these conditions. Under slow release conditions, less DNAPL is entrained within the soil matrix, the overall potential for vertical migration is greater, and more DNAPL will penetrate to a greater depth.
The overall effect of the physical properties described above, particularly where the "dimple effect" is manifest, makes timely remediation of groundwater contaminated with DNAPLs impossible. In those instances the most economic method of protecting health will lie in municipal or point of use treatment systems for the recovered groundwater.
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