Technical Note: The Vertical Distribution of Beryllium-7 Activity in Soil Profiles Determined by a Designed Device

Abstract

Beryllium-7 (7Be) is an environmental radionuclide that is continuously deposited onto the surface of the earth as fallout. After depositing onto the earth, 7Be is rapidly and strongly adsorbed by soil particles. Beryllium-7 has a short half-life (53.3 days) and thus is primarily distributed in the upper 10 mm of the soil profile; it is therefore considered a useful tracer for documenting soil interrill erosion. The use of 7Be to document erosion is based on a comparison of the 7Be inventory at a sampling point with a reference value determined for a nearby reference site. The reference site is often very difficult to find; as a result, determining the reference value is usually the bottleneck for successfully using 7Be to document soil erosion. The objective of this study was to develop a device that can easily measure a 7Be reference inventory. A raingauge-like device was designed and used to determine the reference 7Be inventory and its vertical distribution with depth in three types of soil. The results show that the vertical distribution of 7Be in soil can be described by an exponentially decreasing function. The majority of 7Be was distributed in the first 10 mm of the soil surface. These results indicated that the device is valid to determine 7Be inventory as a reference, as well as its vertical distribution. According to the vertical distribution of 7Be, samples from the upper 4 mm of the soil surface should be less than 2 mm thick, while samples 2 to 3 mm thick can be used in the soil profile below 4 mm. The 7Be collector can be used to determine both the total 7Be inventory as a reference value and the distribution of 7Be at depths in a disturbed soil profile, which is important for estimating short-term interrill soil erosion.