The Influence of Evaporation and Seasonal Effects on the Water Content in the Unsaturated Zone: A Multi-Year Laboratory Study

Abstract

The unsaturated zone of the subsurface plays a critical role in the environmental fate and transport of a wide range of environmental contaminants, and is also important for plant growth and agriculture. Quantitative prediction of processes in the unsaturated zone requires knowledge of how water content varies with elevation above the water table, a relationship known as the capillary pressure (Pc)–saturation (S) relationship. While the Pc–S relationship is conventionally thought of as being primarily a property of the porous medium and fluids, previous work found evidence suggesting that it actually results from a dynamic equilibrium between capillary forces and phenomena driven by evaporation. The focus of this work was on gaining a further understanding of the role of evaporation on the Pc–S relationship. The work made use of a tall instrumented laboratory column connected to an external reservoir for maintaining water table height. Following an initial imbibition experiment, the column was saturated and allowed to drain, and then water content was monitored in the column as a function of height over 1207 days (3.31 years). While initial imbibition and drainage were rapid, on the order of hours, redistribution and evaporation effects became apparent over longer time scales (hundreds of hours). A drying front moving downward in the column was apparent from changes in the slopes of saturation vs. time curves as it passed individual sensors; unlike previous experiments with a more fully-vented column, the evaporation front appeared to stall, balanced by the capillary-driven upward flow of water. Over the full multi-year duration of the experiment, seasonal trends in water saturation were apparent, with significant, reversible variations observed that closely followed atmospheric conditions. Specifically, saturation above the water table appeared to increase during the spring and summer months and decrease during the fall and winter months, despite the constant water table location, consistent with a changing driving force for evaporation. This result may suggest the likelihood of seasonal effects in the long-term transport and fate of contaminants in the unsaturated zone.