Deuterium oxide (heavy water; 2H2O); D2O) when injected into the water table gives direct insight into the movement and distribution of groundwater within an aquifer. Deuterium can be detected in small amounts informing scientists on where and how much water is flowing in a specific location (groundwater hydrology). Hydrology, using deuterium as a tracer, is applied in environmental studies, water and waste-water mapping, and more recently in the monitoring of hydraulic fracturing, otherwise known as “fracking.” Deuterium is used in hydraulic fracturing in the exploration process for natural gas by oilfield services and applications, as a tracer in drilling fluids (used to determine if drilling fluid penetrated the core), and in geological research. As there are no harmful chemicals associated with deuterium oxide, and it is a stable isotope, its hydrological applications cause no environmental hazards.