Cross-shore transport pathways between coastal aquifers and coastal seas: Archetype of a nearshore variable-density contaminant plume

On permeable beaches, tides and waves lead to an upper saline plume underneath the beach face in addition to the saltwater wedge. The transport and fate of land-derived chemicals depend on process dynamics at the land-sea interface, which can be examined using a nearshore variable-density contaminant plume. Such a plume can be taken as an archetype, since it includes tracer transport as a special case, or can become unstable if a suitable density contrast is imposed. Using this archetype, we consider three features of coastal aquifers: (i) Dense plumes interact with groundwater to produce features that resemble instabilities but are, instead, distinct features that depend on the flow regime and aquifer geometry. While evident in the laboratory, such features are unlikely to occur in the field. (ii) In oceanic settings, coastal aquifers respond and interact with tides and waves. Simulations confirm that beach morphology and in/exfiltration across the beach face are both linked to wave breaking on short time scales, i.e., linkages between beach morphology and in/exfiltration both reflect their relationship with coastal waves. On longer time scales, tide and wave effects are roughly additive, with tides usually being more important. (iii) The density of inland contaminant plume and the coastal aquifer depth both impact on the distribution of travel times from the contaminant site to the beachface. Tracer plume breakthrough curves resemble a Gaussian distribution, with increasing skewness as the plume density increases.