Characterisation of Groundwater Flow in the Deltaic Aquifer-Aquitard System

Groundwater flow dynamics are crucial to understanding biogeochemical processes within deltaic aquifer-aquitard system, which are characterised by complicated geological and hydrogeological conditions. In this study, the permanent multilevel groundwater monitoring system was installed at two distinct sites in the Pearl River Delta: an inland freshwater site (ShunDe) and a coastal saline site (HuangPu). By monitoring groundwater level fluctuations hourly, Fourier and spectral analyses were applied to assess the temporal variability of hydraulic heads, as well as tidal and barometric influences. Using Darcy's law considering density variations, we estimated vertical groundwater flow patterns. At the inland freshwater site (ShunDe), groundwater primarily flowed from bedrock to the basal aquifer and upward into the shallow aquifer, driven by a local flow system at a higher rate with negligible density effects. Seasonal climate shifts and changes in terrestrial water storage significantly impacted groundwater flow here, reflecting limited influence from the paleo-sedimentary environment. In contrast, at the coastal saline site (HuangPu), flow dynamics were governed by density differences. During the dry season, flow was predominately downward due to limited recharge, whereas in the monsoon season, significant recharge drove upward flow. The comparison between the inland and coastal sites highlights how paleo-sedimentary environments influence groundwater flow, shifting the system from localised to more regional flow patterns. Based on these findings, we propose a conceptual model illustrating groundwater flow dynamics in deltaic aquifer-aquitard system. This study provides direct field evidence of how paleo-sedimentary conditions shape groundwater flow systems in such settings.