Gamma-ray spectrometry and geological maps as tools for cadmium risk assessment in arable soils

Farmers in some parts of Sweden have problems achieving quality assurance certification for their products due to frequently excessive cadmium (Cd) concentrations in wheat grain. Such problems are most frequent in areas where soils are affected by sedimentary rock containing alum shale. In this study we used aerial and ground-based gamma radiometry to assess Cd risk in food production at regional and field scale in one such area. A regional risk map for Cd in arable soils in the south-eastern tip of Sweden was created using aerial gamma-ray spectrometry measurements (238U, 232Th, and 40K) combined with maps on bedrock geology and Quaternary soil deposits. The map had 5 hypothetical risk categories. Category 1 areas (highest risk) had > 5 ppm 238U, category 2 had 3–5 ppm 238U, category 3 till on Cambrian sandstone, category 4 fluvial deposits downstream from risk categories 1–3, and category 5 other arable soils (low risk). The high uranium (U) levels were caused by inclusion of alum shale fragments, which may also contain high amounts of Cd. The Cambrian sandstone is a possible source of high soil Cd concentrations (but low in U) since it in this area contains sphalerite (ZnS), which is high in Cd. Directed sampling revealed very high Cd concentrations in both soil and winter wheat grain in the higher risk categories. The R2 value for the positive correlation between Cd and estimated 238U in topsoil was 0.59. The risk categories based on soil properties were also useful in predicting Cd concentration in wheat but there were no differences between categories 2, 3 and 4, so for wheat three risk classes appeared more relevant. The spatial variation in Cd in the study area is considerable according to the risk classification and many farms include both high and low Cd areas. Use of a ground-based gamma-ray sensor for detailed mapping of fields at two farms in high Cd risk areas gave a fairly strong correlation between 238U and soil Cd (R2 = 0.68). When 232Th/238U ratio was used instead the relationship was even stronger (R2 = 0.75). The Th/U ratio can remove some of the noise in gamma recordings and may be useful in depicting high U areas. Airborne gamma-ray scanning also outlined the Cd risk areas fairly well (R2 = 0.39) at the field level. The results show that geological maps and gamma radiation mapping, calibrated with a few analyses of Cd concentrations in soils and crops, can be used for risk classification of soils at the regional scale in this type of landscape. Using gamma-ray data, many areas that would have been overlooked when using geological maps alone were classified as high risk. Ground-based gamma-ray scanning could be useful for farmers to identify fields and parts of fields where special consideration is needed to reduce the risk of high Cd in crops.

► Cadmium in crops is a food safety factor.
► A regional risk map was created for Cd in arable soils.
► High risk was correlated to gamma radiation and bedrock geology.
► The spatial variation in Cd in the study area was considerable.
► The risk map pin-points parts of the farm needing special attention regarding crop Cd.