But researchers say naturally occurring chemicals, some radioactive, coming out of the wells may be more of an issue. Avner Vengosh, a professor of geochemistry and water quality at Duke University, has studied the brine - salty water from rock layers such as the Fayette Shale in Arkansas that are fracked for natural gas. He says brine-containing things such as bromide and naturally radioactive radium could become a problem - even if people have focused on the artificial compounds that drillers put down the holes. "But the natural occurring constituents are as problematic and sometimes even more - could be very harmful to the environment and human health, but are naturally occurring," he explains. Vengosh says the oil and gas industries are exempted from the Clean Water Act, otherwise they'd have to clean up the brine before it gets into surface or groundwater. The industry says it is recycling more of the fracking fluids it uses, and says it is not damaging water quality. Vengosh agrees that the industry is recycling more. But he says bromide from the brine is still a cause for concern if it gets into the water treated for drinking. He says bromide combines in a dangerous way with chlorine. "This combination can generate a very toxic organic compound, could be very toxic if consumed by the people that use the water from this utility," he stresses. Vengosh adds natural radium in the brine puts out a low level of radioactivity. He says that might not be a concern, but he has found it can accumulate in the sediment at water treatment facilities. And he says it may also bio-accumulate - build up as organisms feed on each other, perhaps even fish that humans eat. "It could move from bugs in the sediments into higher organism, higher-order organism and eventually end up with fish," he explains. He says the good news is that the brine can be treated. "And it's doable," he explains. "There's no need for technological breakthroughs. All those technologies are available. The only question is the cost."