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DNAPL History and Transport: A Summary

August 8, 2010

This post was prompted by interest in a lecture I attended as part of the Princeton Groundwater Pollution and Hydrogeology course (recommended!). North America’s most prominent DNAPL expert, John Cherry, spoke for approximately 5 hours about the topics I have summarized here. Cherry’s notes are copyrighted, as is the whole course, so I have paraphrased the take-home messages. An annotated list of references will follow in a subsequent post (soon)!

Background

DNAPL is Dense Non-Aqueous Phase Liquid. The term refers to the group of groundwater contaminants that have a density greater than water (commonly cited as 1.0 g/cm3). These are a tricky group of compounds for contaminant hydrogeologists because the source tends to sink and can be difficult to find. DNAPLs include chlorinated solvents (tetrachloroethylene (PCE), trichloroethylene (TCE), trichloroethane (TCA), cis-1,2-dichloroethylene (DCE), vinyl chloride(VC)), creosote, coal tar, polychlorinated biphenyls (PCBs) and undiluted pesticides.  The most widespread DNAPLs are chlorinated solvents because PCE is the main chemical used for dry cleaning, TCE is a heavily-used industrial solvent, and DCE and VC are the sequential daughter products. (VC degrades further into ethene and ethane, which are the desired end-products of in situ degradation of a chlorinated solvent.) Chlorinated solvents were reportedly developed in Germany in the late 19th Century; their use increased drastically during WWII. Until 1979, MSDSs for chlorinated solvents recommended disposal in dry soil because they were thought to volatilize.

Dissolved plumes caused by DNAPLs were discovered in the 1970s but DNAPL (the free phase, not dissolved phase) was not discovered until the mid-1980s. This is partially because monitoring wells are a poor method to detect DNAPL; it is rarely found in wells. Discovery was precipitated by legislation introduced during the previous decade: Safe Drinking Water Act (1974), Resource Conservation and Recovery Act (RCRA, 1976) and the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA, commonly known as Superfund, 1980).  This legislation required sampling of municipal wells specifically for chlorinated solvents, which were summarily discovered in some drinking water systems. Unlike some other contaminants, such as my favorite, methyl tert-butyl ether (MTBE), chlorinated solvents have high taste and odor thresholds, meaning that people don’t taste or smell the compounds in water until a relatively high concentration. Chlorinated solvents have taste thresholds around several hundred ug/L; MTBE is at least one if not two orders of magnitude less. Taste thresholds are highly dependent on the individual.

Fate and Transport

DNAPL transport has been studied in three categories, in order of decreasing scientific understanding: sand and gravel, fractured clay and fractured rock. Research on DNAPL fate and transport in fractured rock is nascent and appears to be dominated by Beth Parker and John Cherry at the University of Guelph (but of course I learned this at a course taught by Cherry).

Sand and Gravel

A series of field experiments were conducted in the fine sand aquifer of Canadian Forces Base Borden (Ontario), starting in 1989, to explore the transport of different chlorinated DNAPLs with different release scenarios. This work isn’t news anymore, so I won’t go into more than to say that the DNAPL traveled deeper than expected and along very thin, slightly coarser-grained beds in an fairly homogeneous sand, forming multiple layers of free phase product at discrete depths.

Fractured Clay

The extent of the fractures in an aquitard are the most relevant bit of information: do fractures extend through the unit and connect to an underlying aquifer or not? The importance of aquitard fracture extent was discovered by accident during a controlled release experiment of PCE at Base Borden in 1991 (I can’t find a publication of this specific experiment, though I believe it became part of Parker’s PhD dissertation). A falling head hydraulic test indicated that the aquitard was a sufficient barrier to flow; however, the hydraulic test failed to indicate slight leakage, which DNAPL is happy to exploit. It’s easy to forget that aquitards are defined in terms of water supply, not contaminant transport, and methods to test the aquitard’s hydraulic properties are likely insufficient to determine contaminant transport properties. How to determine that an aquitard has continuous fractures? Carefully inspect cores.

Fractured Rock

There is still very little literature on DNAPL in fractured rock. There are several references addressing fluid flow in fractured rock (conference proceedings, National Resource Council “Rock Fractures and Fluid Flow” and a guide to regional groundwater flow available as a PDF here). Modeling demonstrated, before any field experiments were conducted, that the “orderly” and interconnected fracture network in sedimentary rock would generate a dispersion-dominated plume. In the 1997, a major field effort was initiated at Santa Susana (Simi, California), to study the fate of large volumes of TCE, which had been used to clean rocket test components on top of a sizeable shale/sandstone hill.  Cherry spent a lot of time on the methods (message: don’t open a hole that becomes a transport pathway) but the surprising discovery at this site was that despite the high water table beneath the source area, TCE was not appearing at discharge locations. Analysis of numerous rock cores indicated that transport of DNAPL was dominated by matrix diffusion, not advection, and diffusion had sufficiently retarded TCE such that it never appeared at discharge points in the valley below. After four decades, no DNAPL source zone remained. (Parker presented this at the Battelle Conference on Chlorinated and Recalcitrant Compounds, Monterey, CA, May 24-27, 2010.)

If you made it this far, thank you! This is my first real blog post. I encourage you to leave comments or questions. Were you hoping that some nugget of DNAPL properties would be addressed here that wasn’t? Let me know and I’ll do my best.  If you have suggestions on how I could write better, I would really appreciate those comments. If you don’t want to leave a comment in this public forum, feel free to email me or DM me on Twitter. Cheers.

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10 Comments leave one →
  1. annejefferson permalink
    August 8, 2010 6:02 pm

    Interesting post! One of my PhD students is working on stable isotope signatures of TCE and its daughter products, but I actually know very little about the contaminant transport and remediation aspects of the stuff, so I’d love to know more. One question: Isn’t it true that DNAPLs do volatilize, just not so fast/effectively as to prevent percolation down below the water table?

  2. August 9, 2010 12:54 am

    It’s great that one of your students is working on TCE-specific isotopes. I don’t have my Battelle notes with me now, but Ramon Aravena (U. Waterloo) gave a talk about it. I have used CSIA with MTBE and TBA (thanks to Tomasz Kuder at U. Tulsa) and I know that the chlorinated solvents are much more confusing (because of the chlorine?).

    So, with the disclaimer that I don’t research DNAPL myself, I will say that yes, they generally are somewhat volatile. Chlorinated solvents are reportedly very volatile (from J. Cherry); I have often heard them referred to as chlorinated volatile organic compounds – CVOCs. Most spills of chlorinated solvents are either large volume or long duration – or both – such that contaminant surface area is too small for volatilization to be an effective attenuation mechanism. In contrast, petroleum hydrocarbons are both volatile and readily biodegraded such that they attenuate in the vadose zone if enough oxygen is present. My reductive dechlorination knowledge is poor, but it’s not an aerobic process.

    CVOCs also have properties other than high density that make them especially good at getting into groundwater and remaining a persistent problem: low viscosity, low surface tension, solubility that’s relatively low but high with respect to MCLs. I took home the impression that CVOCs basically form oily blobs that stick in pores.

  3. Vadrosaul permalink
    August 10, 2010 3:22 am

    One thing I’ve never been able to find out is if there is a solubility boundary between NAPL’s and APL’s at atm. Do you know of such a number?

    • August 10, 2010 3:02 pm

      Good question. Do you mean a single solubility above which chemicals act as APLs and below which they act as NAPLs? I don’t know this but will look into it. I would suspect it would be difficult to have one solubility dividing NAPL and APL because of other properties like viscosity, sorption affinity, surface tension, etc.

      • Vadrosaul permalink
        November 29, 2010 8:43 pm

        I recently asked my Hydro prof as this question came up. He seems to think its either 10mg/L or 1mg/L

  4. August 12, 2010 4:26 am

    This is a fascinating post – I’ve not known what DNAPL meant before, just thought it was someone’s handle on Twitter!

  5. December 9, 2010 7:01 pm

    Vadrosaul, I can’t seem to reply to your comment directly, but the 1-10 mg/L range seems reasonable to me. Again, I think it’s hard to come up with a single number because of the various other properties of molecules (polarity, etc.). Thanks for following up!

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