The recharge illusion: How seasonal surface loading can hide continuing groundwater resource declines in confined deltaic aquifers

• Novel method extension uses ocean and river tides in passive aquifer parametrization. • Observed groundwater heads in the VMD rise due to seasonal surface water loading. • Loading-corrected groundwater heads reveal ongoing depletion during rainy season. • Surface loading must be considered in deltaic groundwater recharge assessments. Reliable estimates of groundwater recharge are critical for managing water resources and preventing groundwater depletion. However, groundwater recharge estimates can be distorted when loading signals are mistakenly interpreted as signs of groundwater recharge. This study introduces a novel extension to existing frequency-domain methods to quantify loading imprints in groundwater measurements by disentangling superposing signals from Earth, atmospheric, ocean, and river tides, using the K 1 tidal constituent to remove the river tide influence. This approach enables the application of frequency-domain methods for calculating the loading efficiency in deltaic groundwater systems, where superposing multiple forcings govern tidal constituents in groundwater heads. Using multi-factor regression deconvolution analysis, surface loading signals are removed from groundwater head time series. Applying this method to groundwater head data from the southern Vietnamese Mekong Delta strongly suggests that the observed seasonal rise in groundwater heads does not reflect recharge. Instead, the apparent upward trend in uncorrected data is attributed to mechanical surface loading from seasonal surface water accumulation linked to expanding aquaculture. The loading-corrected time series show a consistent decline during both dry and rainy seasons, aligning with observed annual depletion rates. This indicates a deficiency of net effective groundwater recharge throughout the year, reflecting unsustainable groundwater exploitation practices. The methods and findings of this study offer a scientific framework for the analysis of multiple superimposed surface loading processes in deltaic aquifer systems, thereby enhancing groundwater recharge assessments in such hydrogeological environments.