Professor Avner Vengosh of Duke University in the United States recently visited South Africa.
His research group (link) recently published several influential and award-winning papers,
which systematically document the environmental impacts of hydraulic fracturing (fracking)
and unconventional shale gas exploitation in the United States.
While in South Africa,
Vengosh and gave lectures and workshops at
universities around the country. In his lecture at UCT
on 25 June, he discussed the main findings of their recent review paper on the risks posed to water resources in the US by
fracking and shale gas exploitation (link).
In short, there are risks throughout the life-cycle of shale gas development and exploitation, related to:
See below for more details:
In short, there are risks throughout the life-cycle of shale gas development and exploitation, related to:
- Water acquisition
- Chemical additives
- Disposal of flowback and produced water
- Contamination of groundwater by stray gas
- Contamination of groundwater by flowback and produced water
See below for more details:
Impacts on water
resources
- Water acquisition: large volumes of water are used for high-pressure injection into the target formation to induce hydrofracturing. Data from North America shows that between 8000 and 100 000 cubic metres of water are needed, per well. The availability of water is a key issue for fracking, and Vengosh refers to studies from North America showing that the use of both groundwater and surface water by shale gas developments has led to local water shortages. The disposal of the water, once it has been used for hydrofracturing, and has been recovered from the well (flowback), is the next problem. The water now contains chemical additives and is mixed with formation water – see below.
- Chemical additives: This has been a particularly controversial issue. One problem has been that the industry not been forthcoming about additive composition of fracking fluid. But based on what is known (from e.g. www.fracfocus.org), water injected into wells for fracking has various additives, including:
- proppants – sand or other substances to “prop” open incipient fractures,
- acids, generally hydrochloric acid,
- additives to adjust the viscosity of the fluid – e.g. guar gum or borate compounds and ammonium persulfate,
- corrosion inhibitors (isopropanol, acetaldehyde), citric acid – used to prevent iron precipitation,
- biocides – typically glutaraldehyde,
- oxygen scavengers (e.g. ammonium bisulfite),
- scale inhibitors (e.g. acrylic and carboxylic polymers), and
- friction reducers (surfactants, ethylene glycol, polyacrylamide).
- Flowback: Another impact of shale gas production by fracking is that it produces large volumes of water, along with gas. Initially, this is called “flowback”, which is a mixture of injected hydrofracturing fluid (water plus chemical additives) and natural groundwater from the shale gas formation. Apart from the additives, and depending on the quality of the deep groundwater, flowback this water may also be saline, and contain hydrocarbons, metals and, radioactive isotopes - like produced water (below).
- Produced water: During gas production, this deep groundwater or 'produced water' is a continuous by-product. Produced water is water which had been trapped along with gas in the deep formations targeted by fracking, and naturally tends to have very high salt (typically chloride and bromide) concentrations, and so is often referred to as brine. It may also contain high levels of metals and radioactive elements, as well as hydrocarbons.For example, in a recent paper (link) Vengosh and his colleagues show how, even after treatment, the disposal of produced water into rivers can cause the accumulation of very high levels of radioactive isotopes in river sediments.
Another concern is the effect of drinking water treatment, by chlorination, on river water that has been contaminated by produced water. When water with elevated levels of bromide or chloride is treated with chlorine, trihalomethane compounds, such as chloroform and bromoform , are produced. Both are known carcinogens. - Stray gas: one of the more spectacular problems caused by fracking in the US has been the contamination of shallow groundwater by natural gas leaking from gas production wells. Methane concentrations in groundwater may be so high that it is possible to set the groundwater alight. There have been numerous cases where methane levels in groundwater from domestic supply wells exceeds the legal limit. The issue here is not the safety of the water for drinking, but rather the risk of explosion. Vengosh's group has published two recent papers on the topic, with almost opposite findings:
- One paper showed that homeowners within 1 km of gas production wells in the Marcellus Shale of north-eastern Pennsylvania have a far higher risk of stray gas contamination in their groundwater wells, and that, based on the gas composition (ethane, propane and carbon isotope composition), it is highly likely to be coming from leaking gas well casings (link).
- The other paper, based on similar research in Arkansas, found no evidence of stray gas contamination of water wells (link).
- Groundwater contamination: if stray gas can migrate from leaky gas wells into shallow groundwater, surely produced water could too? Deep groundwater might also move upwards if fracking has opened new flow pathways from deeper formations into shallow aquifers. So far, Vengosh and his co-researchers have found no evidence that this is happening, but natural cross-formation flow of deep brines into shallow aquifers does happen, in suitable geological settings (giving rise, for example, to hot springs along fault zones).
Possible solutions
Prof. Vengosh makes a
number of suggestions for reducing the
environmental risks of fracking, including:
- Impose a zero-discharge policy for untreated flowback and produced water, and ensure that treatment and disposal methods are safe (2, 3).
- Enforcing a safe zone of at least 1 km between water wells and and gas wells, to avoid stray gas contamination.
- Reducing the quantities of fresh water used in shale gas development, by recycling of flowback and produced water, and the use of marginal water sources.
Notes:
- Prof. Vengosh was in South Africa to participate in a Water Research Commission-funded project on deep groundwater in the Karoo. The local project, with participants including hydrogeologist Dr. Ricky Murray, and veteran groundwater geochemists Dr. Gideon Tredoux and Dr. Siep Talma, aims to understand the chemistry and quality of the deep groundwater in the Karoo – water which could be released into the environment as produced water from fracking.
- In the 2014 review paper, and in the UCT lecture, there was some discussion of deep well injection as a disposal option for fracking wastewater. One significant concern associated with deep well injection is triggered seismicity, in which the injection of fluids into deep formations can disturb the equilibrium of stresses along geological fault zones and cause earth tremors. There are other potential operational problems, too, including the need to ensure that injection well casings do not leak, and allow injected wastewater to escape into shallow aquifers.
- One fascinating experimental treatment method currently under investigation is blending high-sulphate acid mine drainage with produced water containing excessive, and radioactive, barium and radium, resulting in the precipitation of strontium barite and significant reductions in the concentrations of radium and barium levels in the wastewater (link).
Website with links to
the papers and related information:
sites.nicholas.duke.edu/avnervengosh
sites.nicholas.duke.edu/avnervengosh
