Uranium isotope dynamics across salinity and redox gradients in a coastal aquifer: implications for the oceanic uranium budget

Abstract:

To balance the ocean’s uranium budget it may be necessary to invoke submarine groundwater discharge as a major source for uranium. However, uranium removal from seawater has been observed in coastal aquifers where steep redox gradients at the seawater-freshwater mixing zone result in the reduction of soluble U(IV) to insoluble U(IV). We investigated uranium cycling in groundwater within a permeable sand subterranean estuary in Waquoit Bay, MA using major and trace element chemistry as well as ∂²³⁴U measurements. Groundwater and sediment samples were collected across the seawater-freshwater mixing zone. In the groundwater samples uranium does not behave conservatively. During mixing it is removed in the intermediate salinities (3–4 m; 2–12 salinity; 0.1 nM U) and enriched in higher salinities (4–6 m; 20-25 salinity; 32 nM) while in salinities >25, uranium is again removed (7–8 m; 8 nM). Geochemical modeling suggests that U is removed at the seawater-freshwater interface by adsorption to Mn-oxides (3–4 m) while in the deeper saline aquifer (7–8 m), U is removed through reduction from U(VI) to U(IV). Surprisingly, while ∂²³⁴U is above secular equilibrium in both the freshwater and seawater, within the intermediate salinities ∂²³⁴U is depleted below secular equilibrium (as much as ∂²³⁴U = -50). Sediment samples were subjected to a partial leach to extract surface-exchangeable U. This leach was analyzed for ∂²³⁴U and found to be highly depleted (∂²³⁴U -80 – -20). Based on the depleted ∂²³⁴U of the sediment leaches and groundwater, we hypothesize that the high U concentrations observed within the intermediate salinities likely have a sediment source. This also implies that U within this intermediate salinity zone must have a long residence time relative to groundwater-surface water exchange rates. This might be possible if redox boundaries and Mn-oxides act as a barrier to U in the intermediate salinities allowing U leached from sediments to accumulate. Therefore, while we observe mechanisms for U removal and addition during submarine groundwater discharge, it appears that removal may be the dominant control with regards to the marine U budget. Finally, we have collected samples for ∂²³⁸U measurements which, when complete, may provide additional information on the mechanisms for U removal within the subterranean estuary.