Thallium in French agrosystems—I. Thallium contents in arable soils

The thallium (Tl) content of the upper horizons of 244 French soils was determined as the first step towards the creation of a reference data bank for total Tl content of arable soils. Forty soil samples were collected in the vicinity of potential anthropogenic sources of Tl, but the remainder came from rural areas. The distribution of Tl concentrations in soils was characterized by a median value of 0.29 mg Tl kg −1 and a 90th percentile value of 1.54 mg Tl kg −1. Very high pedogeochemical contents were found (up to 55 mg Tl kg −1) but none could be attributed to obvious anthropogenic pollution. Areas of very high Tl concentration belong to an epihercynian transgression zone with a contact between a sedimentary basin and a crystalline massif. This contact is associated with stratified mineralizations (Zn, Pb, F, Sb, Ba, Tl and pyrites). High Tl concentrations were common in limestone, marl or granite derived soils, and the Tl in limestones or marls is probably concentrated in the sulfides contained in these rocks because Tl has a high affinity to S. In granites, Tl may be in the micas and feldspars because Tl + can replace K + in these minerals. Silty or clay-silty soils showed the highest concentrations. These granulometric fractions contain the majority of the minerals, which are supposed to be the major hosts of Tl in soils, i.e. clay minerals, oxides and micas. Tl in the soils was positively correlated with Ba, V, Pb, Fe, Ni, Cd, Zn, Co, As and especially Mn. A significant proportion of Tl may be in the Mn oxides: in oxidizing conditions, Tl(III) could enter the Mn oxides by sorption, or Tl(I) could replace K(I) in the oxide.