Seawater intrusion (SI) poses a substantial threat to water security in
coastal regions, where numerical models play a pivotal role in
supporting groundwater management and protection. However, the inherent
heterogeneity of coastal aquifers introduces significant uncertainties
into SI predictions, potentially diminishing their effectiveness in
management decisions. Data assimilation (DA) offers a solution by
integrating various types of observational data with the model to
characterize heterogeneous coastal aquifers. Traditional DA techniques,
like ensemble smoother using the Kalman formula (ES)
and Markov chain Monte Carlo, face challenges when confronted with the
non-linearity, non-Gaussianity, and high-dimensionality issues commonly
encountered in aquifer characterization. In this study, we introduce a
novel DA approach rooted in deep learning (DL), referred to as
ES, aimed at effectively characterizing coastal
aquifers with varying levels of heterogeneity. We systematically
investigate a range of factors that impact the performance of
ES, including the number and types of observations,
the degree of aquifer heterogeneity, the structure and training options
of the DL models. Our findings reveal that ES excels
in characterizing heterogeneous aquifers under non-linear and
non-Gaussian conditions. Comparison between ES and
ES under different experimentation settings underscores
the robustness of ES. Conversely, in certain
scenarios, ES displays noticeable biases in the
characterization results, especially when measurement data from
non-linear and discontinuous processes are used. To optimize the
efficacy of ES, attention must be given to the design
of the DL model and the selection of observational data, which are
crucial to ensure the universal applicability of this DA method.