Linking the Surface and Subsurface in River Deltas—Part 2: Relating Subsurface Geometry to Groundwater Flow Behavior

Understanding subsurface structure and groundwater flow in deltaic aquifers is essential for evaluating the vulnerability of groundwater resources in delta systems. Deltaic aquifers contain coarse-grained paleochannels that preserve a record of former surface river channels as well as fine-grained floodplain deposits. The distribution of these deposits and how they are interconnected control groundwater flow and contaminant transport. In this work, we link depositional environments of deltaic aquifers to stratigraphic (static) and flow and transport (dynamic) connectivity metrics. Numerical models of deltaic stratigraphy were generated using a reduced-complexity numerical model (DeltaRCM) with different input sand fractions (ISF) and rates of sea-level rise (SLR). The groundwater flow and advective transport behavior of these deltas were simulated using MODFLOW and MODPATH. By comparing the static and dynamic metrics calculated from these numerical models, we show that groundwater behavior can be predicted by particular aspects of the subsurface architecture, and that horizontal and vertical connectivity display different characteristics. We also evaluate relationships between connectivity metrics and two environmental controls on delta evolution: ISF and SLR rate. The results show that geologic setting strongly influences both static and dynamic connectivity in different directions. These results provide insights into quantitatively differentiated subsurface hydraulic behavior between deltas formed under different external forcing (ISF and SLR rate) and they are a potential link in using information from delta surface networks and depositional history to predict vulnerability to aquifer contamination. Plain Language Summary: Geologic structure and groundwater flow behaviors influence groundwater resources in delta plains. In deltaic aquifers, channel structures were created by past surface rivers. These channels in the subsurface are “fast-travel” pathways for groundwater and contaminants. We created synthetic delta structures with a numerical model and then simulated groundwater flow through them in order to tie geologic structure to groundwater flow behavior. By using many different models, we investigate how structure and flow relate, and how the subsurface geology and groundwater system are affected by different sediment inputs and sea-level rise rates. The findings will help us better manage delta groundwater resources and provide an opportunity to predict groundwater contamination from surface characteristics.
This article was originally published in Water Resources Research. The version of record is available at:
Xu, Z., Hariharan, J., Passalacqua, P., Steel, E., Paola, C., & Michael, H. A. (2021). Linking the surface and subsurface in river deltas—Part 2: Relating subsurface geometry to groundwater flow behavior. Water Resources Research, 57, e2020WR029281. https://doi. org/10.1029/2020WR029281