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 thes e 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 Bra wley, California. One field contained a nonswelling sandy loam soil an d the other contained a high shrink-swell clay soil. Both fields were planted to alfalfa on shallow beds. Soil samples were taken prior to b romide treatment, after treatment, and after several subsequent irriga tions. Three experiments were conducted on each soil: (1) initial brom ide spike injection; (2) midstream bromide spike injection; and (3) in filtration box bromide injection. Surface water was sampled during bro mide application and during subsequent irrigations. On the noncracking soil the water and salt moved to a depth of 60-75 cm. During subseque nt irrigations salt was leached to deeper depths and surface waters ha d only small amounts of bromide. In the clay soil, water and salt rapi dly filled and flowed in the soil cracks. Significant quantities of br omide were found at the 90-120 cm depths. Surface waters were laden wi th bromide during subsequent irrigations, and bromide was transported down the furrow by irrigation water. Traditional water and salt transp ort concepts for porous media are not applicable to cracking clay soil s.