Seawater Intrusion Mechanisms in Heterogeneous Coastal Aquifers Subject to Pumping

Seawater intrusion (SWI) in coastal aquifers is influenced by both spatial distributions of hydraulic properties and pumping rates. The heterogeneity of parameters such as hydraulic conductivity (K) may have a significant impact on the vulnerability of these systems to SWI and the sustainability of groundwater supply.

In this work vulnerability is assessed in relation to indicators such as: (a) the salt concentration of extracted groundwater (C); and (b) the total dissolved mass (TDM) of salt in the aquifer. This study delves into a modified version of the classical two-dimensional Henry's problem, featuring a single pumping well situated in proximity of the coastline. The well pumping rates are expressed as fractions of the total flow rate entering the aquifer from the inland boundary. The fraction range from a minimum of zero (no pumping) to a maximum of five.

To investigate the impact of aquifer heterogeneity a series of stochastic simulations are conducted using the popular variable density flow model SEAWAT. Heterogeneity in the K spatial distribution is modelled as a geostatistical log-normal process characterized by an exponential covariance function, with variance values ranging from 0.25 to 1.0, horizontal correlation scales from 10 m up to 2000 m, and a single vertical correlation scale of 10 m. This approach allows for investigating the effects of the K heterogeneity, the layering features of the aquifer system, and the intensity of groundwater pumping on C and TDM.

A detailed examination of model results reveals several interesting outcomes. Most notably, the TDM indicator is rather sensitive to the K heterogeneity, whereas the C indicator is much less affected by it. In terms of TDM, selected cases are singled out and discussed. In general, lower K values exhibit minimal SWI even with large pumpage, whereas high K values lead to pronounced SWI even at low pumping. Increased variance and horizontal correlation correspond to varied SWI behaviour. The TDM spread remains generally low due to relatively low variance. With an increase in horizontal correlation, the spread becomes more pronounced for low pumping rates and less pronounced for large pumping rates.