Energy In Depth has a post on a new study done assessing the risks of hydraulic fracturing: "A new, comprehensive report from Gradient examines two potential exposure pathways for hydraulic fracturing fluids to impact human health: upward migration from the shale formation itself, and surface incidents such as spills or other releases. Even by taking a conservative approach (which by design overestimates risk) the report concludes that hydraulic fracturing fluids 'are not expected to pose an adverse risk to human health' and that, in the event of a spill, natural processes would dilute fluids to 'below levels of human health concerns.'"
For a copy of the study, see here.