Evidence of free convection in groundwater: Field-based measurements beneath wind-tidal flats

Free convection caused by salinity differences had not been conclusively detected or measured in the field. A field experiment at wind-tidal flats on Padre Island National Seashore, Texas, documents salinity-driven free convection with both direct (head and salinity data) and indirect (time-lapse 3-D resistivity) methods. Evaporative concentration of groundwater near the water table created unstable inverted density gradients, reduced groundwater levels, and reversed hydraulic gradients. These factors allowed plumes or fingers of more saline, denser fluid to flow downward into less-dense fluid as observed in monitoring wells and 3-D surveys. The development of density inversions can overcome the dissipating forces of dispersion and diffusion to create a sufficiently large unstable gradient to induce free convection. The development of free convective flow of variable-density fluids in groundwater can be detected and monitored through field techniques.