Groundwater age is defined as the time between recharge at the water table to the time when groundwater was sampled. Groundwater age estimates are based on concentrations of environmental tracers (i.e., tracers derived from the atmosphere, as opposed to artificial tracers) in groundwater. Because no tracer is perfect, these age estimates are typically referred to as "apparent" ages.
Groundwater transit time is the time between recharge and discharge from the aquifer. So groundwater transit time is equal to groundwater age at the point of exit from the aquifer, such as the point of discharge from an irrigation well, or discharge to a stream. Groundwater transit times are strongly linked to the ratio of groundwater recharge rate and groundwater storage capacity (commonly represented as saturated thickness times porosity).
Rapidly recharged groundwater is less likely to be depleted over time though it can be more vulnerable to surface activities leading to nitrate or pesticide contamination. Because water quality is a concern over the entire state, many groundwater age studies in Nebraska have utilized dating methods that focus on groundwater <70 years old. However, groundwater with apparent age of >45,000 years has been observed. The range of groundwater ages, and links to each groundwater age study, can be viewed by clicking on the location symbols in the map above. Note that some locations are generalized (samples may have been collected over a range of locations), and some studies are represented by multiple symbols because samples were collected at locations that were relatively far apart.
It is helpful to remember that total transit time is equal to groundwater transit time plus the time it takes water to move through the unsaturated zone above the water table. Where unsaturated zones are greater than a few meters, the unsaturated zone transit time may be a significant portion of total transit time.
Map revised 2/4/2019
Have you ever wondered why groundwater nitrate concentrations vary in aquifers? One reason is that there are substantial lag times between groundwater quality changes and the human activities that cause them. These lags can cause distinct patterns of groundwater age and quality in aquifers. This module introduces the concept of groundwater age and how age information can be used to understand groundwater variables, including nitrate and recharge.