Glossary

³H/³He ³H/³He refers to the combined use of ³H (tritium) and its decay product, ³He (helium-3). Radioactive decay of ³H to ³He allows elapsed time to be calculated by comparing the tritiogenic 3He (that is, the amount of ³He that is attributed to decay of ³H) to the original amount of 3H, where the original amount of ³H is equal to the measured ³H plus the tritiogenic ³He.

Δ⁴He The amount of ⁴He (helium-4) in a water sample that is in excess to that attributed to solubility equilibrium with air, expressed as a percentage of the ⁴He in water at solubility equilibrium. Δ⁴He usually is from excess air and from elemental radioactive decay such as decay of uranium and thorium in rocks.

ΔNe  The amount of Ne (neon) in a water sample that is in excess to that attributed to solubility equilibrium with air, expressed as a percentage of the Ne in water at solubility equilibrium. ΔNe usually is from excess air.

Anthropogenic Resulting from or pertaining to human activities.

CFCs CFCs, or chlorofluorocarbons, are anthropogenic compounds that have been commercially produced since the 1930s, volatilize into the atmosphere, and subsequently partition into water that is in contact with the atmosphere. Concentrations of CFCs have varied over time, facilitating their use in age-dating. In this report, CFCs refer to the CFCs that commonly were used for age-dating purposes during the time period 1992–2007: CFC-11 (CFCl₃), CFC-12 (CF₂Cl₂), and CFC-113 (C₂F₃Cl₃).

Contaminated Where used in the context of CFC or SF₆ dating, indicates concentrations greater than those that would be found in groundwater that was at equilibrium with peak atmospheric CFC or SF₆ concentrations at the assumed recharge elevation and temperature.

Data Warehouse (DWH)  The DWH is a publically available database of NAWQA data http://infotrek.er.usgs.gov/traverse/f?p=NAWQA:HOME:0:.

Environmental Tracer For the purposes of this report, an environmental tracer (or “tracer”) is considered to be a widespread element, compound or isotope that is used to infer groundwater time-of-travel. 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.

Excess Air Atmospheric air (gases), beyond the amount that can be attributed to air/water solubility, that is incorporated into shallow groundwater during or following recharge. Excess air is added to groundwater by air entrainment during infiltration and (or) by water-table fluctuations.

Land-Use Study (LUS) A focused investigation of water-quality conditions associated with an individual land-use setting. LUSs generally are composed of shallow wells (usually monitoring wells) in recharge areas of regionally important land-use settings. Variability from hydrogeologic factors is reduced by restricting an individual LUS to a single aquifer.

Major-Aquifer Study (MAS)  A broad assessment of water-quality conditions in groundwater of a Study Unit, generally focusing on the shallow used resource. MASs typically are composed of networks of domestic supply wells located in principal aquifers. Major-Aquifer Studies were formerly called Study Unit Surveys.

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

Modern Where used in the context of tracer-based CFC-based piston-flow ages, slight enrichment above concentrations that would be found in groundwater that was at equilibrium with peak atmospheric CFC concentrations may be referred to as “modern”, to differentiate minor enrichment from greater enrichment. Minor enrichment is enrichment that could reasonably be attributed to uncertainties in the methodology, for example: young groundwater recharged under conditions of a local atmospheric CFC anomaly, or minor bias in estimated recharge temperature or recharge elevation. Greater enrichment frequently indicates the presence of CFCs from non-atmospheric sources. This term is used in this report only in appendix A.

Piston Flow A simplified and idealized concept of groundwater flow in which groundwater moves in discrete packets by advection only, without hydrodynamic dispersion or mixing.

Reference (REF) Well  A well installed with the intent of providing samples that represent groundwater that has been minimally impacted by anthropogenic activities, that is, background or natural groundwater.

SF₆ SF₆, or sulfur hexafluoride, is a compound that has both natural and anthropogenic sources. Like CFCs, 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.

Study Unit A major hydrologic system of the United States in which the NAWQA Program has focused water-quality studies. Study Units are combinations of ground- and surface-water systems. Most NAWQA Study Units are greater than 4,000 square miles (10,000 square kilometers) in area.

Study-Unit Survey (SUS)  See Major-Aquifer Study (MAS).

Tracer-based piston-flow age The time-of-travel of groundwater, assuming that piston-flow conditions and assuming the time-of-travel implied by the tracers reflects the time-of-travel of the groundwater.

 

First posted July 31, 2012