The Behavior of Groundwater in the Vicinity of the Water Table Evidenced by Specific Discharge Profiles

Detailed vertical profiles of the horizontal component of the specific discharge at the water table region of a phreatic aquifer were obtained, from two research wells, employing both a modified point dilution technique under natural flow conditions and a mathematical diffusion model. H₂ ¹⁸O‐depleted water was used as a tracer. A 2.5‐m multilayer sampler with dialysis cells at 3‐cm intervals was utilized both to introduce the tracer into groundwater and determine its temporal variations. Large variations in the profiles of the tracer concentration were observed in a relatively thin layer just below the water table. The horizontal specific discharge was found to increase with depth from 0.5 m/year in the first 50 cm below the water table to 4.5 m/year at 240 cm depth. Two theories are suggested to explain the presence of an almost stagnant zone at the water table region: (1) the existence of entrapped‐migrating air bubbles which reduce the hydraulic conductivity and (2) drag forces exerted by the capillary fringe.