Thermonuclear chlorine-36 in arid soil

Abstract

Thermonuclear bomb testing in the 1950's and 1960's produced a significant amount of chlorine-36 (C1-36). This anthropogenic C1-36 pulse has been proposed as a tool in dating ground water and as an environmental tracer. The feasibility of bomb C1-36 as a hydrologic tool in the unsaturated zone was studied. Soil samples were collected from a vertical auger hole drilled to 5 m near Socorro, New Mexico. A distinct C1-36 pulse was located at 1.125 m depth indicating a mean vertical moisture velocity of 4.5 cm/yr. The calculated mean seepage rate of 2.8 mm/yr indicates only 1.2% of the annual rainfall percolates to 1 meter. Below 2 m depth the C1-36/C1 ratio is relatively constant, averaging 717 x 10⁻¹⁵ and represents the prebomb production rate of subsurface and cosmogenic C1-36. Total bomb C1-36 fallout in the soil profile was 7.4 x 10¹¹ C1-36 atoms/m². The author feels C1-36 will be a useful tool in recharge studies of the unsaturated zone.