A sensitivity analysis of tide-induced head fluctuations in coastal aquifers

The response of coastal aquifers to sea-level fluctuations, notably tides, is known to contain much information about hydraulic parameters. We performed sensitivity analyses to assess how much, about what and where this information can be best obtained. It is well known that the response to harmonic fluctuations (and many harmonics can be superimposed to describe sea-level fluctuations) decreases exponentially with distance inland. The characteristic length of this decay is image, where D is hydraulic diffusivity and P is period. Maximum sensitivity is obtained for distances equal to Lc, which is where maximum information would be obtained if the aquifer is treated as homogeneous. However, sensitivity depends not only on the problem dynamics, but also on parameterization. In fact, if heterogeneity is acknowledged by finely discretizing hydraulic conductivity, we find that connection to the sea (i.e. K near the coast) is what can be characterized best, while the most informative measurements are located at around 0.5 Lc. Thin low conductivity zones near the coast lead to a stepwise decrease in the amplitude of groundwater head fluctuations. We find that the fluctuations are independent of buoyancy effects, so that they can be simulated by constant density codes. High information content and ease of use suggest that they should be helpful in characterizing the aquifer–sea connection, which is important for coastal aquifer protection against seawater intrusion.