Surface Water and Groundwater Mixing in Alluvial Floodplains: A Significant Issue for Understanding Contaminant Transport at Coal Ash Facilities Adjacent to Rivers

摘要

Many coal ash storage facilities are located in the floodplain of major waterways. Investigation of the interaction between groundwater and surface water in these areas is critical to understanding hydrogeology, proper design of groundwater monitoring networks, groundwater chemistry, potential contaminant transport issues, and design of remedial alternatives. Considerations include transient interaction between bank storage, upland groundwater, and out-of-bank inundation. In this presentation, we discuss a two-dimensional numerical flow and transport model used to simulate the short-term fluctuations of river stage on mixing of surface and groundwater in the floodplain. We apply the model to a transect through the floodplain of a major river using transient river stage data and aquifer properties for both in-bank and out-of-bank events. The model is then used to simulate movement of hypothetical conservative tracers to confirm and enhance our understanding of floodplain hydrogeology. Short-term fluctuations in river stage (e.g., during river flood events) can significantly influence the extent of groundwater and surface water mixing within the floodplain. Our simulations show that river flood events can trigger influx of surface water to groundwater far inland from the normal stream bank. The amount of groundwater/surface water mixing is linked to the duration and amount of flooding, as well as the history of flooding events. Our results suggest that interactions between surface and groundwater flows in floodplain settings is complex. We conclude that both dynamic groundwater/surface water interaction and unsaturated zone processes need to be considered when monitoring groundwater and potential contaminant transport related to coal combustions residuals (CCR) Units.