Gamma dose equivalent (0.18 μSv/h) is higher than background of Salamanca province, Guarda district and worldwide average. The concentration of water-soluble U (or bioavailable) in soils regionally distributed with no influence of mining works, ranges from 1 to 137 μg/L with a median of 9 μg/l. The U content in shallow well waters ranges 0.1 -79.5 μg/L with a median of 0.4 μg/l. In river and stream samples the concentration are lower (median < 0.1 μg/L). The association of U, As and pH in water samples indicates an increase of U and As solubility with increasing pH linked to the formation of uranyl-carbonate and uranyl-arsenate complexes.

The application of different organic residues as a soil amendment is an agricultural practice used to improve soil fertility by increasing the soil organic matter (OM). However, the OM from these residues can influence the behavior of pesticides applied jointly to the soil. Modification of the pesticide bioavailability in soils is of special interest since it can affect the activity and/or functioning of soil microbial community. Accordingly, the dissipation kinetics of mesotrione in unamended soil (S) and soils amended with sewage sludge (S+SS), green compost (S+C) and commercial pellets (S+P) and its possible effects on the soil microbial communities were studied. Soil biological parameters were determined as indicators of the soil microbial activity, functioning and structure: microbial biomass, dehydrogenase activity, respiration, and analysis of the phospholipid fatty acid (PLFA) profile extracted from the soil. Dissipation was more rapid in unamended soil than in amended soils and half-life (DT₅₀) values followed the order S+SS &gt; S+C ≥ S+P &gt; S. The biomass values increased in the amended soils with the exception of the P-amended soil. However, mesotrione had different effects on this parameter depending on the soil treatment. In general, dehydrogenase activity was stimulated by the addition of the amendment and herbicide to soil. Initially, respiration was higher in the unamended soil (control and treated soils) than the amended soils and mesotrione did not have any effect on this parameter. PLFAs analysis indicated that the overall structure of active microbial communities as well as the relative abundance of certain groups of microorganisms clearly changed according to the type of amendment and the incubation time, but remained unaffected by the application of mesotrione.