Can climate change transform non-toxic sediments into toxic soils?

Abstract

Our work addresses a neglected aspect of heavy metal (HM) pollution of sediments in small floodplain reservoirs. Very little is known about this type of water bodies, in contrast to oxbow lakes or old river beds. The study examines the spatial horizontal distribution of HM and the effect of texture, organic carbon (OC) content, morphometric and location features on HM concentrations. Moreover, the data from the assessment of sediment toxicity were analysed with respect to recent years' droughts to estimate the potential toxicity of sediments as soils. The statistical analyses showed that the texture and the OC content had a significant impact on the HM concentrations. Fine-grained and OC-rich sediments exhibited higher HM pollution. Only one morphometric/location factor was shown to affect HM levels in sediments – the angle between the reservoir axis and the riverbed. The angle value affected the texture and, consequently, the HM content: with a rising angle the share of the coarse-grained fraction increased leading to a decrease in the HM concentration. The spatial horizontal HM distribution did not show statistically significant results, nonetheless, HM content was found to rise along with the distance from the initial part of reservoir. The toxicity levels were not exceeded in sediments, however, the evaluation of the material as soil showed that, according to European Union guidelines, the content of at least one HM was toxic in 80% of the samples. Contaminated floodplain reservoirs should be regarded as a double threat to riverine ecosystems. On the one hand, they are one of the main non-point sources of river valley pollution; on the other hand, given the drying up of reservoirs, sediments become soils and the soil-bound heavy metals become more toxic to the environment.