Salt Transport in Cracking Soils: Bromide Tracer Study

Cracking clay soils pose unique and complex water and salt management problems. Crack volumes greatly affect water and salt movement in these soils. The purpose of our study was to determine the effect of crack flow on tail water quality and the contribution of crack flow to salt leaching. Our experiment was conducted on two farmers fields near Brawley, California. One field contained a nonswelling sandy loam soil and the other contained a high shrink-swell clay soil. Both fields were planted to alfalfa on shallow beds. Soil samples were taken prior to bromide treatment, after treatment, and after several subsequent irrigations. Three experiments were conducted on each soil: (1) initial bromide spike injection; (2) midstream bromide spike injection; and (3) infiltration box bromide injection. Surface water was sampled during bromide application and during subsequent irrigations. On the noncracking soil the water and salt moved to a depth of 60-75 cm. During subsequent irrigations salt was leached to deeper depths and surface waters had only small amounts of bromide. In the clay soil, water and salt rapidly filled and flowed in the soil cracks. Significant quantities of bromide were found at the 90-120 cm depths. Surface waters were laden with bromide during subsequent irrigations, and bromide was transported down the furrow by irrigation water. Traditional water and salt transport concepts for porous media are not applicable to cracking clay soils.