Could you be swimming in hydrocarbons at the beach?

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Australians love the beach but we all know there can be many dangers lurking beneath the waves. We all know about sharks and blue bottles, but what about the stuff that fuels just about all of our taxis? Or the fuel that is in that BBQ near the beach? New research is showing that there could be a chance that you are swimming in natural gas at the beach.

Before you panic and lock away your towels and cozzies for the summer, let’s be clear up front and note that you are not stepping into a pool of potentially flammable or lethal liquid gas when you go to the beach. The major component of natural gas is methane (CH4) and this compound is naturally found in our environment and is most probably found in very small quantities at your local beach.

Much like carbon dioxide (CO2), CH4 is developing a bad name as it is known to be a very potent greenhouse gas (GHG). You probably know of CH4 as a GHG as it is famously produced by the flatulence of cows. But, like CO2, CH4 occurs naturally in the atmosphere and you are probably breathing in CH4 right now. It is also produced naturally by a number of sources, particularly by microbes that decompose organic materials in the absence of oxygen (for example in wetlands, estuaries and tidal flats).

CH4 also occurs naturally in the oceans and there are areas of especially high concentrations of CH4 known as ocean seepages. Oil and gas companies explore the oceans with high tech equipment, such as the LSM Laser Methane Sensor and sonar, to try and detect areas of high dissolved CH4 seepage. It is possible that around such areas there is a potential new gas field. Another method to measure methane in the ocean is to have a fixed mooring with sensors, such as the METS Methane Sensor, continuously monitoring CH4 at one particular location.

Scientific research with CH4 sensors attached to moorings have recently been finding consistent results. A study in Germany and another study in New Zealand have both discovered that at low tide there is a distinct increase in CH4 in the ocean (Grunwald et al 2009; Krabbenhoeft et al 2009). The following figure is from the Grunwald et al. study which was conducted in the German Bight near various tidal flats. The dashed line in the top portion of the figures shows tidal water level. The bottom portion of the figure with the solid line shows dissolved CH4 values. CH4 increases when there is a low tide and there is a very consistent pattern over many days.

Quick Link to Figure 5a of Grunwald et al 2009

The exact causes of this pattern are unknown and probably differ depending on the site. The authors of the German study suggest it could be caused by a dilution effect with freshwater, low in CH4 concentration, diluting CH4 at high tides. In the New Zealand study, which was conducted offshore and in the deep ocean, the authors hypothesised that at low tides there was reduced hydrostatic pressure on the seabed that led to higher rates of CH4 seepage into the ocean.

What can we learn from these studies? Firstly, there is more CH4 during low tide than high tides; secondly, there is more CH4 around tidal estuaries than freshwater river mouths or beaches. Thirdly, when you go down to the beach there is not much to worry about in terms of pools of natural gas and methane ruining your swim. Those sharks and blue bottles, on the other hand, could be more of an issue!

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references

Grunwald, M. et al 2009. Methane in the southern North Sea: Sources, spatial distribution and budgets. Estuarine, Coastal and Shelf Science, 81: 445–456. doi:10.1016/j.ecss.2008.11.021

Krabbenhoef, A. et al 2009. Episodic methane concentrations at seep sites on the upper slope Opouawe Bank,
southern Hikurangi Margin, New Zealand. Marine Geology, doi:10.1016/j.margeo.2009.08.001