CHARLOTTE, N.C. -- One of the enduring fears about shale gas extraction is that the millions of gallons of chemical-laden water drillers pump into the ground will, over time, migrate into aquifers and contaminate groundwater supplies. About 11 percent of the injected water returns to the surface after drilling, according to industry data from the Marcellus Shale. About 90 percent remains underground, and no one quite knows what happens to it. Some residents near wells worry the wastewater may move through fractures created during hydraulic fracturing to one day emerge in their water supply. But within the ivory towers of academia, a consensus is slowly emerging that the injected fluids will remain sequestered in the formations for what geologists would call a "long" period of time, and what the layperson would call "forever."