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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.

Previous Blog: Antarctica Travels

August 3, 2010

While you all wait with bated breath for my state-of-the-science DNAPL post (which I am working on, promise), I thought I would introduce my old blog. In 2006, I was fortunate enough to be invited by one of my Middlebury College professors to accompany her on a NSF-funded research cruise in the Weddell Sea. We spent nearly 6 weeks on the RV Nathaniel B. Palmer and it was the experience of a lifetime. The landscape in Antarctica is unlike any I have ever seen; the lack of flora and fauna was noticeable and unsettling, even when replaced by the regal beauty of ice, rock and sea. This blog is in reverse chronological order, which is inconvenient for review, but it starts out with pretty pictures. I posted to the blog by emailing my parents, who then copied and pasted (and perhaps edited grammar). We did not have “internet” on the ship but instead sent and received email several times during a 24-hour period.

http://www.katharinetoantarctica.blogspot.com/

Thanks for stopping by!

My letter opposing SB 624: Serpentin(it)e

July 29, 2010

I wrote this letter to Mariko Yamada, the Assemblymember whose district covers Davis, CA.  I tried to address the multiple issues that bother me about the bill and included information that she might feel was relevant to decision-making. I clearly didn’t focus on tectonic origins (I’m a hydrogeologist). Some of you will see information that you (you!) posted on your blogs.

Dear Assemblymember Yamada,

I am writing to express my opposition to SB 624, the bill to remove serpentine as the official state rock. I was surprised to learn recently that this bill had already passed the State Senate because I initially believed it to be a joke. With the serious issues that California state legislators face at this time, not the least of which is another unresolved budget, I am appalled that representatives are spending time and energy on legislation that not only has no tangible benefit to the residents of California but also contains misinformation. California is known nationwide for its public education, but SB 624 flies in the face of scientific knowledge, and has become an embarrassing sidebar in national media outlets (e.g., The New York Times, “California May Drop Its Official State Rock,“ July 13, 2010).

SB 624 is patently inaccurate in promoting the notion that simply being in contact with serpentine, known to geologists as serpentinite, will result in cancer: “Serpentine contains the deadly mineral chrysotile asbestos, a known carcinogen, exposure to which increases the risk of the cancer mesothelioma.” In truth, the serpentine mineral group contains 20 different minerals; only one of these minerals, chrysotile, occurs in the asbestos form.

Let me be clear: in no way does my opposition to SB 624 deny the pain and suffering of mesothelioma victims. I understand that mesothelioma is caused by asbestos; however, I also understand that a direct link between chrysotile asbestos and mesothelioma has not been established (Yarborough, C.M., 2006, Chrysotile as a cause of mesothelioma: an assessment based on epidemiology, Critical Reviews in Toxicology, Vol. 36, No. 2, p 165-187). No matter the specific cause, asbestos fibers can only cause cancer if they are extracted from their natural environment, ground and inhaled. Instead of taking the opportunity to educate the public about asbestos, serpentine, mining and public health, SB 624 promotes scientifically inaccurate and indefensible statements about serpentine.

Through my opposition to this bill, I have discussed serpentine with several geologists and educators around the state and beyond. I have learned that California has serpentine grasslands that support unique ecosystems, which some of my fellow UC Davis graduate students are studying. I have learned that serpentine is present in 42 of the state’s 58 counties. I have learned about hiking trails, polished serpentine jewelry and mining geology. I believe that these qualities are valuable, not “toxic,” to California.

I am also concerned, perhaps unnecessarily, that the original language of the bill appears to address anaerobic digestion, or composting.  That language was wholly scrapped after the State Senate passed the bill. This action seems underhanded.

Serpentine represents the history of California: geological, industrial and ecological.  It is an appropriate symbol precisely because all of these aspect tell a story unique to our state. Please oppose SB 624 and encourage your colleagues to join you. Thank you.

Sincerely,
Katharine North
Davis, California

Thanks to Garry Hayes (@geotripper), Andrew Alden(@aboutgeology), Jon Christensen (@westcenter),  @sfoxx, and Chris Rowan (@allochthonous). Hopefully I didn’t miss anyone; apologies if I did.

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