Methane gas, or CH4, is an odorless gas that occurs naturally in small amounts in the atmosphere and in groundwater. Methane is also a potent greenhouse gas - far more effective at warming the atmosphere than carbon dioxide - and of serious concern in battling global warming. In low concentrations, the methane in groundwater is completely harmless, but in high enough concentrations it can pose an explosion hazard.
There are two main processes that create methane, and they have different chemical signatures, which makes methane a useful chemical tracer in the environment. Thermogenic methane comes from the breakdown of organic matter under high temperatures and pressures and often occurs deep underground. Much of the fossil fuel methane drilled by the natural gas industry is thermogenic in nature. The other source is biogenic and is created by a distinct group of microbes called methanogens. These microbes thrive in places where oxygen is not present, such as in groundwater aquifers, and they produce methane as a by-product of metabolism. Sometimes, when the conditions are right, methanogens in groundwater can produce large amounts of methane, allowing well owners to actually light their tap water on fire.
Distinguishing thermogenic from biogenic methane is critical for determining whether natural gas extraction activities could have contaminated groundwater wells. One common method relies on measuring the isotopes of the methane, since biogenic and thermogenic methane have different isotopic signatures. However, there is no “smoking gun” for distinguishing the source of methane, and typically we use multiple lines of evidence. Moreover, measuring isotopes is expensive and sampling must be conducted by an experienced person, making it difficult to use for baseline measurements over long periods of time.
Nearly 15% of U.S. homeowners rely on private groundwater wells, and many more tap into municipal systems that also use groundwater sources. Maintaining safe drinking water is critically important.
Over the past few years, a boom in shale gas extraction via hydraulic fracturing has taken place in the U.S. and globally, and has raised concerns that methane pollution will become a larger problem: In eastern Pennsylvania, for example, high methane concentrations in some people's water wells have been linked to natural gas 'fracking', raising concerns that other harmful chemicals might also be able to infiltrate groundwater supplies. This project aims at developing a tool for monitoring changes in groundwater quality that result from 'fracking' related activities so that we can assess the short-term and long-term risks to environmental and public health.
We propose taking a different approach to fingerprinting methane in groundwater by going straight to the source: the methanogens. Our goal is to determine if the presence and abundance of methanogens in groundwater can be used as a reliable indicator of biogenic methane. The advantages are that the sampling is relatively easy to conduct, samples can be stored long-term and therefore be archived, and DNA analysis is relatively cheap. In the long-run, state agencies, researchers, and individual homeowners might be able to keep track of groundwater quality more frequently and over longer time periods, all for less cost.
I am currently completing one of the first surveys of microbes in well waters near areas of intense 'fracking' in Colorado. This exploratory research suggests that methanogens are useful tracers for methane. However, the isotopes of these samples all match with a biogenic source. During this field campaign, we will sample groundwaters in eastern Pennsylvania that are known to have thermogenic methane. This will allow us to compare the methanogens across the different methane sources.
We will collect water samples from well owners in eastern Pennsylvania that have previously been shown to contain thermogenic or biogenic methane. Two main analyses will be conducted: First, the isotopes of carbon in the methane will be measured. Second, we will survey the microbes within the water using DNA sequence analysis.
We will then compare the isotopes of the methane with the types of microbes in the water to test our hypothesis that methanogens will be related to methane source.
In addition to disseminating the results in scientific journals, every homeowner whose well water is sampled will receive a report of their water quality data including our interpretation of the microbial and chemical data for their records.