TRANSIENT DYNAMICS OF SALINISATION IN OFF SHORE AQUIFER SYSTEMS AFTER SEA-LEVEL RISE

The occurrence of groundwater with a salinity significantly lower than that of sea water in aquifers on the continental shelves is a global phenomenon. Groundwater salinity patterns, groundwater isotopic composition and groundwater age distributions suggest that a large proportion of the relatively fresh groundwater occurring offshore water was emplaced during sea level low stands when the continental shelf was exposed to meteoric recharge. When during transgression the shelf was inundated with seawater, the process of salinization of the underlying aquifers commenced. However, the global occurrence of offshore fresh and brackish groundwater shows that this process has generally not been completed. The present disequilibrium observed in coninental shelf groundwater systems exemplifies the long timescales required for regional-scale, subsurface flow and salinity patterns to adjust to changes in boundary conditions.

Conceptualized numerical simulations of variable-density groundwater flow illustrate various contrasting modes of saline intrusion in a layered aquifer system across the continental shelf, resulting from sea-level rise. For shallow aquifers, simulation results show that rapid salinization will occur where free-convection of seawater into fresh water is dominant. In deeper aquifers, a lateral migration of the salt-fresh interface accomplishes the expulsion of fresh water much more slowly than in the shallow aquifers. Our results suggest that as a consequence of the contrasting rate of salinization of shallow and deep aquifers fresh water is trapped in deeper aquifers and forced to flow upward through aquitards to reach shallower aquifers. The groundwater dynamics of the shallow system remain unstable for much longer than would be expected if this upward leakage of fresh water would not be considered. Although our models are highly conceptual and the scope for validation of our results against field observations is limited, they do allow for an extension of our understanding of the likely nature and dynamics of the process of aquifer salinization across the continental shelves after sea-level low stand.