Patterns and variability of groundwater flow and radium activity at the coast: A case study from Waquoit Bay, Massachusetts

Radium is widely used as a natural tracer of groundwater discharge to the ocean. To better understand how radium activities vary within the groundwater systems that contribute this discharge to coastal waters, we characterized the spatial patterns of porewater activities of the four radium isotopes in the context of the groundwater flow system of Waquoit Bay, Massachusetts. Radium activities were highly heterogeneous across locations, though qualitatively consistent with inferred groundwater flowpaths and simple models of radium evolution. A comparison of radium activity measurements in discharging groundwater, porewater, and baywater indicates that saline submarine groundwater discharge is likely a mixture of water with long and short subsurface residence times. Flow is driven by forcing mechanisms acting on a range of timescales, from waves, to tides, to seasons and longer. Estimates of radium flux along transects extending offshore are disproportionate to groundwater discharge, and individual isotopes have distinct patterns with peak activity occurring at different locations. Modeled sensitivity of porewater radium activities to production rates and sorptive properties of the subsurface indicates that variability in radium activity is caused by aquifer property heterogeneity, variations in groundwater flowpaths, and hydrologic transience. These results suggest that, in many cases, the complexity of coastal groundwater flow and its influence on radium activity present a challenge for using radium activity measured in seawater to quantify groundwater inputs. However, characterization of the spatial pattern of radium activities within the groundwater system may provide valuable qualitative understanding of the subsurface flow processes that drive groundwater discharge.