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Book Review

Rachel Carson had the right idea: DDT persists in unexpected ways in dolphins

DOI: 10.1021/acs.est.6b03150

Rachel Carson published her seminal book, Silent Spring, in 1962; the book was one of the first pieces of popular media to highlight the toxic effects of some human created chemicals, particularly the pesticide DDT (dichlorodiphenyltrichloroethane). Carson emphasized that DDT was impacting bird populations where it was sprayed, causing egg shell thinning and other maladies. The book became hugely popular; it is credited with inspiring the modern day environmental movement and getting the public to pay more attention to persistent organic pollutants (POPs) like DDT. Today, we know DDT is persistent and accumulates up the food chain; it is banned in a number of countries around the world. However, Susan Mackintosh and other researchers from California recently found that DDT persists even to this day, over forty years after its ban in the US.

Mackintosh and her colleagues tested blubber from male common bottlenose dolphins (Tursiops truncatus) stranded within the Paldo Verde Shelf; the shelf is part of the Southern California Bight, the curved portion of the coastline from Point Conception to San Diego.

Blubber from stranded common bottlenose dolphins was used in this research. Common bottlenose dolphins are globally distributed and are considered an apex marine predator. Credit: NASA - http://mediaarchive.ksc.nasa.gov/detail.cfm?mediaid=21807, Public Domain, https://commons.wikimedia.org/w/index.php?curid=37679800

Blubber from stranded common bottlenose dolphins was used in this research. Common bottlenose dolphins are globally distributed and are considered an apex marine predator. Credit: NASA via Wikimedia Commons

This area was home to the Montrose Chemical Plant, a chemical production company that essentially flushed DDT down the drain from the 1950s to 1970s, leading to high regional concentrations of DDT in sediments and animals. The area is considered a Superfund site because of the historical contaminant burden.  Superfund sites are areas that have been flagged by the Environmental Protection Agency for further investigation, monitoring, or action to address historical or ongoing hazardous environmental pollution concerns.

These maps give an idea of the study area; on the left, the dashed line shows the area of the Palo Verde Shelf; stranded dolphins were collected from around this area onshore. Credit: epa.gov

This map gives an idea of the study area; the dashed line approximately shows the area of the Palo Verde Shelf Superfund area; stranded dolphins were collected from onshore locations near this area. Credit: epa.gov

Today, DDT levels in the area are relatively lower and the area has been evaluated extensively to meet Superfund program monitoring requirements. However, Mackintosh and colleagues decided to look at DDT concentrations in a new way. They used a technique called two-dimensional gas chromatography with time of flight mass spectrometry (GCxGC/TOF-MS). Although it sounds complicated, it is basically a method that allows researchers to separate chemicals out according to their size and structure with really high resolution.

A signal in chromatography can (very simply) be described as a response; if a pollutant is present in a sample, a peak a.k.a. a signal appears that is proportional to the concentration of the pollutant in the sample. A sample with a ton of DDT would have a larger signal compared to a sample with low DDT levels.

A signal in analytical chemistry basically gives an idea of how much of a compound is present; here, the larger peak/signal on the left likely corresponds to a higher concentration of the chemical being looked at, compared to the smaller peak/signal seen on the right. Credit: By Klaas1978 - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1134993

A signal in chromatography is essentially a peak saying how much of a compound is present; here, the larger peak/signal on the left likely corresponds to a higher concentration of the chemical being looked at, compared to the smaller peak/signal seen on the right. Credit: By Klaas1978, own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1134993

Typically, chromatography requires researchers to plan ahead and look for compounds that are already known. Concentrations of a chemical in a sample are figured out according to how the sample signal compares to the signal from an analytical standard that has a known concentration of the compound under investigation. With this traditional approach, you can’t accurately figure out the identity or concentration of any signals that don’t also appear in the standard; if you’re left with any unexplained signals, you’re somewhat up a creek without a paddle in terms of identifying them or quantifying them.

The GCxGC/TOF-MS technique allows researchers to look beyond known compounds to unknown compounds that may also be present in the sample; they essentially get well-resolved signals from the entire spectrum of compounds present, and then deduce what the unknowns are and their tentative concentrations by comparison and structural reasoning.

This study found some really interesting results with this novel technique. They found 45 compounds that are potentially capable of bioaccumulation, 80% of which are not typically monitored with routine DDT monitoring. The researchers believe that the unknowns could be compounds that are formed by the metabolism or transformation of the original DDT, or impurities from technical mixtures of DDT used commercially.


Researchers found that a large portion of the DDT load in dolphins was due to compounds that aren’t typically evaluated in routine contaminant monitoring. Image adapted with permission from Mackintosh et al. 2016. Copyright 2016 American Chemical Society.

The implications of this are pretty disconcerting. This essentially means that current contaminant screening in the region and perhaps elsewhere could be missing the bulk of any remaining DDT by focusing on only the typical set of known DDT-related compounds, leaving breakdown products or impurities that are harmful in their own right totally unaccounted for….*insert surprised gasp here*

This study reiterates the need for constant improvement of methods and instrumentation, along with continued questioning of routine practices and monitoring procedures; pollutants in our environment aren’t stable and predictable, so our ways of looking for them should also be flexible and evolving.


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