Understanding the Flow and Mixing Dynamics of Saline Water Discharged into Coastal Freshwater Aquifers

Abstract

Typically the origin of saltwater intrusion into coastal aquifers has been attributed to the reduction of freshwater flux (or head) in the aquifer. This intrusion is the most common natural source of saltwater contamination in coastal aquifers; however, it is not the only source. Other sources of saltwater contamination that include saltwater deposition caused by tsunamis and hurricanes have been observed. The country of Sri Lanka was devastated by a catastrophic tsunami event in 2004. In this study, laboratory experiments were completed using a physical two-dimensional groundwater model to observe subsurface saltwater transport processes in a coastal aquifer after a tsunami event.

Experiments were completed to investigate the transport from different types of saltwater sources. The first experiment was conducted to determine the fate of the overland saltwater that was discharged through the vadose zone. This experiment specifically considered the infiltration of saltwater along the beach face and the injection of saltwater through an open well. The next experiment investigated an isolated beach face infiltration source. The final experiment considered the fate of the saltwater deposited into a coastal lagoon and later leached into a coastal aquifer. The experimental datasets were then modeled using the USGS code SEAWAT. The numerical solutions generated by SEAWAT matched the physical model datasets well. An analysis was performed to further investigate the sensitivity of the plume to certain several flow parameters include freshwater flux, density variations, source loading rate and dispersivity. The results of the sensitivity analysis indicate that the source loading rate is the least sensitive model parameter. In this study, a conceptual basis was established for understanding the saltwater transport processes occurring from multiple sources after a large-scale inundation (tsunami-type) event. These conceptual experiments are prototypes for future field studies, and they provide insight into the density driven transport processes that would occur in a local unconfined aquifer after a tsunami-type event.