Applications of the Radium Quartet to Quantify Water Exchange in Salt Marshes

Export of solutes from intertidal salt marshes are driven by tidal flushing occurring across the sediment-water interface and horizontal advection via underground seepage. The radium quartet (²²³Ra, ²²⁴Ra, ²²⁶Ra, ²²⁸Ra) exhibits a large range in half-lives making for useful tracers of processes that occur over various time scales. In summer 2019, radium isotopes were used to study salt marsh flow dynamics at Sage Lot Pond, an intertidal salt marsh of Waquoit Bay in northeastern USA. The flow pathways responsible for solute exchange in this marsh were elucidated using a two-pronged approach: 1) in sediment cores and 2) an endmember mixing analysis using distinct ²²⁸Ra:²²⁶Ra ratios of tidal creek and pore water samples. At the tidal creek mouth, in-situ sensors recording continuous measurements of hydraulic parameters were installed and discrete samples were collected over two tidal cycles. Bulk sediments and pore waters were sampled at various depths and distances from the creek bank. Preliminary data indicate distinct ²²⁴Ra-²²⁸Th disequilibrium patterns in the core profiles. Disequilibrium occurs near the sediment-water interface due to shallow pore-water exchange (~0-5 cm) with tidal waters. Below this depth, equilibrium is reached and accompanied by varying trends in deficits and excesses of ²²⁴Ra relative to ²²⁸Th (~20-120 cm). A zone of equilibrium is established near the peat-sand interface (~150 cm). In agreement with the core, ²²⁴Ra_ex porewater profiles show two zones of low activities: at shallow (~0-5 cm) and mid depths (~75-100 cm). Utilizing a 1D advective transport model, the ²²⁴Ra flux for the June core with a depth range of 0-150 cm was estimated to be 0.20 ± 0.37 dpm cm^-2 d^-1. A mass balance method will incorporate the ²²⁴Ra flux to deduce water exchange. The tidal creek time-series reveal an inverse relationship between ²²⁴Ra_ex and water depth at our sampling site, suggesting a higher export of ²²⁴Ra_ex at low tide. The hydrologic budget of the marsh will be further constrained using ²²⁸Ra:²²⁶Ra ratios of our water samples. The findings of this study can inform the mobility of trace elements and other solutes in estuarine systems.