Unstable Water Flow in a Layered Soil I. Effects of a Stable Water‐Repellent Layer

Abstract

The development of preferential water flow in a soil profile can cause accelerated movement of pollutants to the groundwater thus reducing groundwater quality. This study investigated the effects of a stable water‐repellent soil layer on the development of unstable water flow in a homogenous profile. Stable water‐repellent soil is defined as one whose degree of water repellency does not change with time after contact with water. The effects of water entry pressure (hp), water‐repellent layer depth (L) and depth of ponded water at the soil surface (ho) on the development of unstable flow were investigated using homogenous coarse sand packed into a specially built rectangular chamber. The hydraulic conductivity of the water repellent soil was also measured as a function of hp and ho in a separate experiment using the constant head method. The hydraulic conductivity and the water content of the water repellent soil increased as ho/hp increased. No water penetrated the water repellent layer for values of (ho + L)/hp < 1, unstable flow developed for values between 1 and 1.5 and a stable front developed for values > 1.5. The conclusion is that stable flow occurred when the water flux through the water repellent layer exceeded the saturated hydraulic conductivity of the underlying wettable layer. The water flux through the water repellent layer was a function of the hydraulic conductivity of the water repellent layer which increased as ho/hp increased.