To crack open shale and free the trapped gas, "typically, several millions of gallons of water, sand, and chemicals" are pumped down "at pressures you might see in the deepest point of the oceans," says Robert Jackson, a professor of Global Environmental Change at Duke University, US, who has extensively studied fracking. Both the gas and the drilling fluid are captured back at the surface. From 2005 to 2009, three million cubic metres of fracking products – not including the water – were used in the US alone, he said.

Jackson addressed participants in a science policy conference organised by the American Geophysical Union (AGU) in Washington, DC, this month.

Fracking fluids contain some 2,500 substances and most, but not all, of them are known publicly. Most are benign, Jackson says (including coffee and walnut shells), but they also include carcinogens (benzene and naphthalene), air pollutants (toluene and hydrochloric acid), and such other chemicals as ethylene glycol and lead.

Do these chemicals actually affect wells used for drinking water and air quality at the surface, given the great depths at which they are injected into shale and the presence, typically, or one or more layers of impervious rock above the shale? Jackson thinks it is possible, but that the issue primarily concerns the disposal of the fracking water after it has done its job. A secondary issue might be leaks in the concrete casing of boreholes, which allow the escape of toxic water at shallow depths on the way down.

The returned fluids, mixed with naturally occurring highly-toxic deep water, are extremely saline, contain high concentrations of dangerous elements, and are 1,000 times more radioactive than the safe limit for drinking water. This produced water, as it is called, should not be released back into the environment, Jackson said, so its disposition is a major health issue. Among positive developments, used by some drillers, according to Jackson, are disposal deep underground, treatment in a specialised industrial wastewater plant (not a municipal wastewater plant), and recycling for use in a future fracturing job. Bad ideas, he said, include spraying the produced water on land, which some US states permit.

Even greater transparency regarding chemicals used would probably lead to the phaseout of the more toxic ones, Jackson says, noting that one firm in Ireland has promised to eliminate chemicals altogether from its fracking operations. He added that this firm operates at much shallower depths than is typical in the US. Another step forward, increasingly employed, is storage of produced water in enclosed, rather than open, tanks.

Jackson's own research, concentrated in northeast Pennsylvania, has involved testing private water wells in fracking areas. The state now holds drillers presumptively guilty for bad drinking water discovered within six months of drilling. The companies pay homeowners to have their water tested prior to drilling, to establish a baseline.

Jackson favours economic incentives to encourage producers to do the "right thing", both for their bottom lines and the environment. Nevertheless, he concluded, "The technology and the production has outstripped our scientific knowledge of this. We are playing catch-up, in terms of some of the potential consequences and understanding where problems occur, why they occur … . There is a strong role for science in providing positive solutions."

This month, the Obama administration issued rules that require disclosure of chemicals used in fracking on federal lands, but only after drilling is complete. An earlier draft would have required advance disclosure, but was lobbied by the energy industry.

• AGU officials said they were pleased with the 400-plus turnout for the two-day science policy conference and plan to make it an annual event.