Because water quality is a concern over the entire state, many groundwater age studies in Nebraska have utilized dating methods that focus on groundwater 45, 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.
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. Using the noble gas isotope 81 Kr solved this problem. Krypton is found throughout the troposphere, where it mixes with raindrops.
Yokochi said that 81 Kr, with its half-life of about , years, allowed the team to measure the age of aquifers that are up to 1.
She was not involved in the research. In the past, Yokochi said that amassing enough krypton gas in a water sample required sampling about — liters. The team used atom trap trace analysis ATTA methods to measure krypton isotopes in the lab. The researchers found that 81 Kr data revealed two distinct groundwater recharge dates: one less than 38, years ago and a much older wet period about , years ago.
Pairing this information with hydrogen and oxygen isotope data from the samples, the team found that the younger aquifer was recharged with water sources from the Mediterranean, whereas the older wet period brought precipitation from the tropical Atlantic.
McIntosh said she anticipates—and is excited for—krypton isotopes to be widely used in future studies. Derouin, S. Virtually all aquifers are dynamic, and flow is constantly occurring although rates of flow will vary within the aquifer with deeper water tending to move more slowly.
The direction of flow within the aquifer is generally from the point of recharge to the point of discharge. These places may be some distance apart and ground water discharging into a wetland or well may have received most of its recharge some distance away.
With slow travel times, and long travel distances, the age of ground water may be tens of years or more between recharge and discharge. Some deep confined ground water is thousands of years old and yet is still slowly on the move.
Most ground water has curved flow paths. We hope you enjoyed our articles.
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