A diagram of the natural gradient tracer test used to detect anammox

A diagram of the natural gradient tracer test used to detect anammox in groundwater. The tracer solution was injected in the well on the left, and wells on the right were used to monitor the subsurface plume caused by the tracer injection. Analytical testing for the isotopic ratios of nitrogen gas (³⁰N₂ or ²⁹N₂) helped determine rates for the denitrification of nitrate (NO₃^-) to nitrogen gas (N₂) and for anammox of ammonium (NH₄⁺) to N₂. From Smith and others, 2015.