If the carbon in freshwater is partly acquired from aged carbon, such as rocks, then the result will be a reduction in the / ratio in the water. For example, rivers that pass over limestone, which is mostly composed of calcium carbonate, will acquire carbonate ions. Similarly, groundwater can contain carbon derived from the rocks through which it has passed. These rocks are usually so old that they no longer contain any measurable , so this carbon lowers the / ratio of the water it enters, which can lead to apparent ages of thousands of years for both the affected water and the plants and freshwater organisms that live in it. This is known as the hard water effect because it is often associated with calcium ions, which are characteristic of hard water; other sources of carbon such as humus can produce similar results, and can also reduce the apparent age if they are of more recent origin than the sample. The effect varies greatly and there is no general offset that can be applied; additional research is usually needed to determine the size of the offset, for example by comparing the radiocarbon age of deposited freshwater shells with associated organic material.
Volcanic eruptions eject large amounts of carbon into the air. The carbon is of geological origin and has no detectable , so the / ratio in the vicinity of the volcano is depressed relative to surrounding areas. Dormant volcanoes can also emit aged carbon. Plants that photosynthesize this carbon also have lower / ratios: for example, plants in the neighbourhood of the Furnas caldera in the Azores were found to have apparent ages that ranged from 250 years to 3320 years.