Cross‐Shelf Transport of Submarine Groundwater Discharge Tracers: A Sensitivity Analysis

Abstract

AbstractThe coastal mass balance for environmental tracers can be used to evaluate submarine groundwater discharge (SGD) and is often estimated using advective‐dispersive cross‐shelf transport models. Here we evaluate the sensitivity of different types of SGD tracers to key variables in this transport model under a wide range of environmental conditions. Salinity and long‐lived radionuclides (here represented by 226Ra; half‐life = 1,600 years) were transported by a combination of advection and dispersion under the scenarios considered, but short‐lived radionuclides (here represented by 224Ra; half‐life = 3.66 days) were mostly dispersed unless the offshore coefficient of solute dispersivity (Kx) was very low (<1 m2/s). Sensitivity to variations in key transport variables was further evaluated by reinterpreting the cross‐shelf trends in Ra activity in the semiarid Gulf St Vincent of South Australia (GSV). In GSV, the trends in most Ra isotopes (223Ra, 228Ra, and 226Ra) in seawater could be explained similarly by a large number of parameter combinations, implying these tracers would provide highly uncertain SGD estimates in this environment. However, the range of possible parameter combinations was relatively smaller for 224Ra, suggesting that SGD estimates derived from this tracer would be most reliable for GSV. In both the sensitivity analysis and the reevaluation of the GSV data, Kx was the most sensitive parameter in the transport equation. Because Kx is an empirical parameter potentially encompassing different dispersion processes in time and space (tidal currents, storms, etc.), it is likely to be tracer, time, and site specific.