Anthropogenic legacy of potassium-40 in French large rivers reconstructed from sediment cores

Environmental imprint of inorganic fertilizer uses was assessed over the last hundred years at the downstream part of large French rivers (Loire, Moselle, Rhine, Rhone, Meuse and Seine rivers) based on Potassium-40 (⁴⁰K) activity concentration data sets acquired from soil monitoring (1980-2022) and from sediment coes collected from 2020 to 2022 to reconstruct the temporal trajectories of ⁴⁰K activity concentrations since the beginning of the last century. Cultivated soils were significantly enriched in ⁴⁰K compared to non-cultivated ones in the 1980s and 1990s when they turned back to the contents of non-cultivated soils during the following decades. In riverine sediments, all the rivers displayed close ⁴⁰K temporal trajectories with peaking ⁴⁰K contents in fine grain size sediments in the 1980s. Maximum ⁴⁰K enrichment factors from this period were related to the proportion of agricultural areas in the river catchment. In the Loire and Moselle rivers, some high ⁴⁰K contents were associated with sandy sedimentary strata deposited by flood events before the end of the 1950s due to the presence of potassium enriched minerals. The comparison of ⁴⁰K activity concentration in sediments with potassic fertilizer delivery in France highlighted very similar temporal trajectories giving evidence that the uses of potassic fertilizers imprint the riverine sediments of most French large rivers. Finally, the environmental resilience face to this anthropic pressure was fast as ⁴⁰K levels decreased immediately after the decreases of the delivery in most of cases.