Temporal and spatial evolution of residual saltwater contamination in coastal subterranean reservoirs

This study numerically investigated the dynamic process of residual saltwater contamination in subterranean reservoirs (aquifer upstream of subsurface physical barrier) for the first time. The results indicated that the groundwater level elevation resulting from subsurface physical barriers induced a small hydraulic gradient in the subterranean reservoir, with residual saltwater convection driven by the density contrast between saltwater and freshwater. This led to persistent inland intrusion of residual saltwater, causing a gradual expansion of the contamination area (i.e., concentration higher than 0.25 g/L) and a decreased area of high-salinity zone. The area of low concentration residual saltwater (0.25 g/L) initially increased and then decreased over time. For large-scale aquifers, the proportion of the maximum contaminated area to the total aquifer area increased from 0.05 to 0.17. Subsequently, the low-concentration saltwater was considered as a relative high-concentration for ambient freshwater, which released salts into the surroundings and the contamination area reduced to 0.0003 at 160000 d. In small-scale aquifers, residual saltwater transport was affected by inland boundary, and the low-concentration saltwater reached inland boundary at approximately 2000 d, which limited its horizontal expansion and resulted in the polluted area reaching an inflection point early relative to large-scale aquifers. The salt transfer from high-concentration residual saltwater to low-concentration saltwater was closely related to aquifer parameters of hydraulic conductivity, dispersivity, and molecular diffusion coefficient. These findings suggest that residual saltwater contamination should not be overlooked when assessing the efficiency of subterranean reservoirs.