Radium budget of the York River estuary (VA, USA) dominated by submarine groundwater discharge with a seasonally variable groundwater end-member

• A mass balance of Ra was determined in the York River estuary.
• Submarine groundwater discharge (SGD) dominated Ra inputs.
• SGD fluxes inversely covary with isotope half-life.
• Ra was enriched in surface and pore waters during the summer.
• Varying seasonal SGD behaviors were observed for each isotope.

Brackish coastal groundwater is enriched in Ra, which is transported to surface waters via submarine groundwater discharge (SGD). The Ra activity of the SGD end-member is influenced by a variety of environmental factors including salinity, pH, and isotope half-life. In the York River estuary (YRE), 223Ra, 224Ra, and 226Ra were measured in surface water and shallow groundwater across a range of salinities and additional Ra sources quantified (desorption and diffusion from sediments, input from tidal marshes). The Ra budget of the estuary indicated a major source of Ra that could only be satisfied by SGD. The apparent Ra flux was combined with groundwater Ra end-member activity to estimate SGD volume fluxes of 5–178 L m− 2 d− 1. Each isotope exhibited a different seasonal pattern, with significantly higher 224Ra flux during summer than winter, lower 226Ra SGD flux during summer than winter, and no seasonal differences in 223Ra SGD flux. However, the SGD 224Ra end-member activity varied with seasonal pore water salinity fluctuations, indicating end-member control on seasonal 224Ra flux. Each Ra isotope suggested a different SGD volume flux, indicating that different nuclide regeneration rates may respond to and reflect different flow mechanisms in the subterranean estuary. This work indicates that volume fluxes estimated using geochemical tracers are sensitive to SGD end-member variations and end-member variability must be well-characterized for reliable SGD flux estimates.