Cosmogenic 22Na as a steady-state tracer of solute transport and water age in first-order catchments

- We make the first measurements of cosmogenic 22Na deposition to North America.
- Atmospheric 22Na deposition is primarily controlled by precipitation.
- 22Na can be a natural “clock” for measuring groundwater recharge age.

Naturally-occurring cosmogenic 22Na (T1/2=2.6 yr) is a potentially powerful tracer of solute and water movement in catchments. However, due to its low abundance in precipitation (∼10−20 molL−1), there are only a handful of datasets documenting cosmogenic 22Na atmospheric fluxes and concentrations in surface waters. Here we present the first record of cosmogenic 22Na fallout to North America and test its use as a radiometric tracer of water age in three small catchments in the Eastern United States. We show that 22Na deposition to southeastern Virginia, USA during 2012-2014 was 187±10 mBqm−2yr−1 and that flux is largely additive with precipitation amounts. Our measurements of fallout combined with previous 22Na deposition data from other regions indicate that approximately 77% of the variability in the annual global 22Na atmospheric flux is controlled by precipitation. Export of 22Na in drainage waters from three first-order forested catchments ranged from 12.5 to 174 mBq m−2 yr−1 and can be explained by a flux-based radioactive decay model, indicating that the watersheds are in steady-state with respect to cosmogenic 22Na on annual timescales. We conclude that in temperate climates with no systematic changes in rainfall amounts at the annual timescale, 22Na may be useful for quantifying the recharge age of relatively young (<20 yr) surface waters and groundwaters and for tracing solute transport at the watershed scale.

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