Glossary

Age Dating The age of groundwater is defined as the time of travel of a parcel of water from the water table to a downgradient measurement point such as a well. Knowledge of groundwater age is essential for constraining rates of groundwater movement and rates of contaminants transported with groundwater, and can be used to quantify recharge processes. Hydrologists can estimate groundwater time of travel (“age dating”) by measuring concentrations of environmental tracers (certain widespread elements, compounds, or isotopes) in groundwater, and relating changes in environmental tracers to time-changing atmospheric inputs or time-changing properties of those environmental tracers. The term “environmental” indicates widespread occurrence, as compared with a local-spatial-scale tracer injection used to understand tracer movement at one particular study site. In practice, the tracer (not the water) is dated. Groundwater mixing can complicate age dating.

Major Dissolved Gases Dissolved nitrogen (N₂), oxygen (O₂), argon (Ar), methane (CH₄), and carbon dioxide (CO₂). Four of these gases, N₂, O₂, Ar, and CO₂, are the volumetrically dominant gases in dry atmosphere and they are incorporated in groundwater during recharge. Non-atmospheric CO₂ is incorporated in groundwater (for example, from root respiration and from oxidation-reduction, or redox, reactions), generally at concentrations considerably greater than those from atmospheric sources. CH₄ can be incorporated in groundwater as a result of redox reactions; although CH₄ is not ubiquitously detected in groundwater, CH₄ concentrations not infrequently become a volumetrically major dissolved gas. These five gases, often analyzed together, can be used to infer recharge temperatures (N₂, Ar), oxidation-reduction state (O₂, CH₄), and excess N₂ from denitrification.

Noble Gases Helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). These six gases comprise the right-most column of the periodic chart of the elements. The first five of these gases can be measured in groundwater samples to help characterize groundwater recharge conditions.

SF₆ Sulfur hexafluoride (SF₆) is a compound that has both natural and anthropogenic sources. SF₆ volatilizes into the atmosphere, and subsequently partitions into water that is in contact with the atmosphere. SF₆ concentrations have been increasing over time, facilitating the use of SF₆ in age-dating applications where natural sources of SF₆ are absent or negligible.

First posted March 7, 2013