Monitoring Plastic Pollution in the Oceans—Promising Technologies

Monitoring Plastic Pollution in the Oceans—Promising Technologies

By Kristen Mitchell

Earlier this year, it was calculated that in 2010, 275 million metric tons of plastic waste was generated around the world, and as much as 12.8 million metric tons ended up in the oceans. Currently, scientists are using technology that was state of the art in the 1960s to study this massive problem. This technology is made up of a net towed by a research ship and then hand counting the thousands of plastic pieces collected. While nets and research ships will always be crucial to many oceanographic research problems, many new technologies are available and have been adopted by researchers.

Monitoring phytoplankton in surface oceans is one example of the changing use of technology. Up until the late 1970s, phytoplankton was measured much the same way that we are currently measuring plastic, with nets towed behind ships. Sensors on satellites measuring chlorophyll, the photosynthetic pigment in plankton, have replaced the net method. Remote sensing methods have many advantages over surface sampling, including surveying large areas over relatively short timescales. For oceanography, this is a great step forward because the ocean is vast and ship time is expensive. It also provides a more complete picture as collecting with nets is a slow business and generally only covers a narrow area. There also tends to be long stretches of time between sampling efforts, if a return trip is made at all.

Recently, there has been interest in applying remote sensing technologies to the plastic pollution problem. Plastics have characteristic absorbance and reflectance spectra in the near infrared (NIR) part of the optical spectrum, which is in the same range as the reflectance spectra for chlorophyll. The potential for using remote sensing techniques to monitor plastic debris exists. In fact, NIR cameras and spectrometers are used in recycling facilities to sort the different types of plastic and colors. However, a major limitation of this technique is that the NIR spectra are absorbed by water, so if the plastic is not at the surface of the water, it would not be picked up with this technique. Raman spectroscopy is another tool that is used in recycling facilities, and does not have the same limitation in water. It does, however, need an active light source, something other than the sun. The use of NIR spectroscopy has been demonstrated to be able to detect small plastic pieces mixed with beach sand.

Both of NIR and Raman techniques suffer from spatial resolution issues in this application because most of the plastic found in oceans is so small; smaller than a pencil eraser. One promising way of getting around this issue is by doing surveys using unmanned vehicles—aerial, underwater or even surface (water) vehicles. Since these vehicles are generally much smaller and more agile than a boat or a helicopter/airplane, the sensors deployed on them would be able to get closer to the survey area, enabling better imaging. In the case of Raman spectroscopy, another possibility is to create a small field deployable sensor that could be used in shipboard screening or deployed on buoys.

With a bit of ingenuity, the technology of monitoring of plastic pollution can be brought into the 21st century.

References:

1. Jambeck, J.R., et al., Plastic waste inputs from land into the ocean. Science, 2015. 347(6223): p. 768-771.

2. Blondeau-Patissier, D., et al., A review of ocean color remote sensing methods and statistical techniques for the detection, mapping and analysis of phytoplankton blooms in coastal and open oceans. Progress in Oceanography, 2014. 123: p. 123-144.

3. Mace, T.H., At-sea detection of marine debris: Overview of technologies, processes, issues, and options. Marine Pollution Bulletin, 2012. 65(1–3): p. 23-27.

4. Jefferson, H., R. Dvorak, and E. Kosior, Plastics Recycling: Challenges and Opportunities. Philosophical Transactions: Biological Sciences, 2009. 364(1526): p. 2115-2126.

Image courtesy of pixabay.com

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