Ghosts of the past appear when walking through the Luquillo forest. Take the cable bridge across the Quebrada Sonadora and hike uphill half a kilometer, and you will soon find the remains of a stout metal fence. It is decaying now, but once served a purpose: keeping people out of a highly radioactive area.
In the 1960s, this forest hosted an experiment on the effects of radiation on ecosystems – then a controversial question, made relevant by the imminent threat of nuclear war. The project, sponsored by the US Atomic Energy Commission, brought together several agencies and dozens of scientists in an effort that is rarely matched now. Today this experiment seems wild and negligent toward inhabitants of the island, but it was once a political and scientific reality.
What actually happened? The forest was clear-cut in two areas. In one, the control, nothing further was done; in the other was placed a 10,000 curie radioactive Cesium-137 source (i.e. approximately 300 trillion radioactive decays per second). Cesium undergoes beta decay to become a barium isotope, in the process giving off gamma radiation (high-energy photons). The forest was exposed to this radiation for three continuous months, after which scientists were able to assess the impacts on the forest.
The logistics of this experiment are interesting to consider. Safety was a consideration, so the radioactive source was chosen to not make a permanent impact on the site – no long half-life isotopes were used. And the source was contained in an interesting fashion – suspended from metal poles, connected to a series of motors and tripwires. If anyone were to have gotten too close to the source, they would trigger a tripwire and the source would be dropped into a lead containment device – loss of power or control would hopefully have the same outcome. If anything were to seriously go wrong, the source could be picked up by helicopter using a built-in metal loop and then flown to another location. Apparently the plan was to simply drop it in the ocean (and hope for the best) if necessary. Researchers controlled the source from a ‘safe’ distance, but one method for determining the status of the source was to simply open the field station door and see if a Geiger counter registered an above-background level of radiation. I don’t think you could do that today! Below are some historical photographs of the experiment, taken from the book published after the completion of these experiments.
The effects of this radiation were mixed, but described at length in dozens of publications. Most importantly, it was found that many plants are damaged by high levels of radiation – perhaps unsurprising, but a novel finding in the 1960s. The effects came in the form of tree death, shifts in phenology, loss of seedling growth, widespread defoliation, and abnormal growth. Below is one photograph from the forest canopy near the radiation source.
However many aspects of the forest were apparently unaffected by high levels of radiation, even close to the source. Some species grew entirely unaffected. Interpreting these results is difficult, because of an inherent limitation in this experiment: with only a single replicate radiation treatment and control, it is difficult (impossible) to determine if the observed effects are due to true differences between sites or random effects of chance at two equivalent sites. But imagine the difficulty in replicating this experiment, or consider the ecological and human impact (and risk) of doing so. Even at the height of the Cold War and the era of above-ground nuclear tests this was not considered.
Despite the ambiguous radiation results, this study did have many other impacts. The Luquillo forest became one of the earliest and best-studied tropical forests, simultaneously assessed by dozens of scientists from multiple perspectives. Much of our foundational understanding of ecology comes from this experiment. I suspect that the project director, HT Odum, understood the long-term implications of this experiment: perhaps he chose to facilitate the radiation experiment in order to obtain the financial and human resources to conduct a range of other ecological studies in parallel.
How do the ethics of this experiment feel to you?