Determining hydraulic connectivity of the coastal aquifer system of La Plata river estuary (Argentina) to the ocean by analysis of aquifer response to low-frequency tidal components

Abstract

Seawater intrusion occurs in almost all coastal aquifers that are subject to human pressure. Its effects could be reduced by avoiding pumping in those wells better hydraulically connected to the sea. This paper presents a methodology to assess hydraulic connection to the sea of a well from its response to sea-level fluctuations. Head fluctuations in a well result from the superposition of hydraulic and hydro-mechanical responses. The hydraulic response requires a good hydraulic connection to the sea, whereas a hydro-mechanical response suggests a poor connection. Sea level fluctuates with a broad range of harmonics, which allows identification of the hydraulic and hydro-mechanical responses based on the harmonic frequency. It is shown that the response to low-frequency harmonics is essentially hydraulic and the response to high-frequency harmonics is mainly hydro-mechanical. The proposed methodology facilitates the discrimination of both responses with the aim to study qualitatively the hydraulic connection to the sea. This methodology is applied to the aquifer system of La Plata river estuary in western Argentina. Surprisingly, the hydraulic response of the deepest aquifer (Paraná) is less damped than the response of the overlaying aquifer (Puelches). This finding suggests that the Paraná aquifer is better connected to the sea than the Puelches.