Experimental investigation of upward and downward cycling of salt contaminants in the vadose zone

Capillary-driven upward movement of contaminated groundwater and subsequent surface evaporation can lead to salt accumulation in the vadose zone. Conversely, infiltration events such as rainfall or flooding may remobilize these salts and transport them downward toward the water table. While previous studies have often addressed these processes in isolation, a comprehensive understanding of their coupled behavior remains limited.

This study presents a controlled laboratory investigation of salt mobility in the vadose zone using three types of granular porous media: beach sand, silica sand, and fine gravel. Each experiment was conducted in three sequential stages, referred to as stress periods: (1) capillary wicking of clean water into a dry porous column, simulating saturation by groundwater rise; (2) evaporation-induced upward transport and salt crystallization, where saltwater replaced the clean water and evaporation concentrated salts near the surface; and (3) dissolution and flushing of salts by infiltrating water, simulating rainfall events that remobilize and leach surface salts.

Salt concentrations in the effluent were measured after each flush cycle to assess the flushing efficiency under different conditions, including varying evaporation durations (brief, moderate, and extended) and conditions (natural vs. forced evaporation). Results showed that sands, due to shallow evaporation fronts, facilitated surface crust formation, while gravel exhibited subsurface salt deposition. Fine gravel achieved faster initial flushing due to larger pores, while finer sands retained salts near the surface, leading to slower but more sustained removal. Forced evaporation further impeded flushing by forming dense salt crusts.

The findings provide new insights into how pore structure, grain size, and evaporation conditions influence salt retention and mobilization. These mechanisms are relevant to sites like the arid and semi-arid regions affected by salt contamination, where similar cycling occurs between evaporation-induced salt accumulation and flushing during recharge. The study informs improved groundwater remediation strategies in flood-prone or arid environments.