The influence of surface - subsurface connectivity on groundwater flow and solute transport in a deltaic aquifer
Date
2021
Authors
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Publisher
University of Delaware
Abstract
Groundwater in coastal deltas provides sustainable water resources to almost five percent of the world’s population. However, it is threatened by multiple environmental issues including dense populations, urban development, and large-scale agricultural and industrial activities. The traditional geostatistical modeling of subsurface structure neglects the details of connected features or lack evidence to consider connected features in the field. This dissertation focuses on modeling the deltaic subsurface connectivity and exploring its relationship to surface channel networks. Surface channels redistribute dynamically, depositing sediments over time and creating subsurface paleo-channels. Therefore, incorporating surface-subsurface processes in the analysis of deltaic aquifers could help us better understand groundwater flow and solute transport, enhancing groundwater management strategies for these regions. ☐ Establishing a correlation between static (stratigraphic) and dynamic (flow and transport) connectivity is a key step in reaching the research goal. In Chapter 2, we studied static and dynamic behavior using DeltaRCM, a process-based model that reproduces delta evolution processes. A series of connectivity metrics were used to quantify the subsurface structure and groundwater flow behaviors on the realizations generated by DeltaRCM. We used a Pearson correlation to link the static and dynamic connectivity metrics. The results showed the static metrics that represent sand proportion in the model highly correlate with horizontal flow behavior, while metrics related to vertical sand connections highly correlate with vertical flow behavior. Realizations with higher input sand fractions show lower horizontal and vertical normalized dynamic connectivity. Realizations with higher rates of sea-level rise have higher horizontal normalized dynamic connectivity, while sea-level rise has no noticeable effect on the vertical dynamic connectivity. ☐ After understanding the relationship between static and dynamic connectivity of deltaic aquifers, three scenarios of contaminant transport were considered in Chapter 3. Three types of contamination were considered based on common environmental problems in real deltas. Case 1 is a consistent contaminant source across the entire land surface with vertical migration into the aquifer. This scenario is based on geogenic arsenic contamination in the shallow subsurface, which is widespread in Southern Asia. Case 2 simulates a contaminant source in surface river channels, a situation derived from infiltration of saline water from coastal rivers to fresher groundwater. Case 3 is lateral contamination from one vertical side, it includes two sub-cases: transient seawater intrusion and steady density-driven groundwater circulation. The simulation results show that contaminant transport is mainly controlled by the sand fraction and paleo-channel stacking patterns. Surface rivers enhance the extent of vertical contamination relative to non-river regions because rivers are better connected with paleo-channels. The saltwater circulation and submarine groundwater discharge are predominantly controlled by the sand fraction of deltas. In addition, vertical contaminant migration varies from upstream to downstream due to different channel distribution patterns. ☐ Chapter 4 is a field application of surface to subsurface prediction where parameters of surface rivers are used to generate subsurface geobodies in geostatistical simulations and compared to field data. Three regions were selected in the Bengal Delta as study areas with different surface network characteristics, and subsurface geobodies were created for each region. Lithological metrics of the object-based models were calculated and compared with field measurements (variogram ranges, sand fraction of boreholes, maximum sand connection, and number of sand-mud shifts). The results show that the stratigraphy of straight rivers is best modeled with straight sinusoid objects, while the structure of meandering rivers is best represented with tortuous sinusoid objects or ellipsoid objects. Groundwater simulations on these object-based models indicate that different geometries and flow directions impact the flow and transport behavior. ☐ Through these projects, I have shown the significance of aquifer heterogeneity and connectivity on the evaluation of groundwater and related contamination. Work from this dissertation fills gaps in our understanding of the relationship between surface river networks, subsurface stratigraphic distribution, and groundwater flow. This research will help managers better assess groundwater security and pollution in deltaic systems and provides insights for groundwater development, such as pumping locations in different types of deltas and the relationship between pumping, rivers, and paleo-channels.
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Keywords
Connectivity, Delta, Groundwater, Surface to subsurface