Hydraulic fracturing leaks. Artwork of a hydraulic fracturing well with cracks in it. Hydraulic fracturing, or fracking, is usually used to extract gas from shale deposits. A mixture of sand, chemicals and water is pumped under high pressure through horizontal wells to form fissures in the shale. The sand keeps the fissures open, allowing the gas to flow into the well and be taken to the surface. If cracks form in the well the fracking fluid, and/or the gas, can leak into groundwater and potentially pollute drinking water supplies.