Can vegetative filter strips efficiently trap trace elements during water erosion events? A flume experiment with contaminated sediments

Abstract

Soils polluted with trace elements can become sources of pollution for surrounding soil and water resources as a result of dispersion of contaminated soil particles by water erosion. Vegetative filter strips (VFS) implemented downstream of the source of pollution may be used to trap sediments and their associated contaminants. An experimental flume was designed in order to test the potential of Lolium perenne and Trifolium repens VFS to retain sediment contaminated by As and Pb (Ath) or Zn and Cd (Prayon). T. repens with its creeping posture and large leaves had a higher sediment retention efficiency (SRE=47%) than L. perenne with an erect posture (SRE=26%). Sedimentation upstream of and in the strip led to the selective retention of coarse particles and an enrichment in fine soil particles at the outlet of the flume. This effect was most marked for the T. repens VFS compared to L. perenne and bare soil. Consequently, for the Prayon soil, the retention efficiency of Cd in T. repens VFS (53%) was higher than the SRE (48%) because Cd concentrations were 3 times higher in the sand fraction than in the clay fraction. On the contrary, the retention of Zn (48%), which was distributed more homogeneously across particle size fractions, was similar to the SRE. For Ath, the retention of both As (26%) and Pb (11%) in the T. repens VFS was lower than the SRE (46%) because both trace elements were preferentially associated with clay size particles. Trace element retention efficiency of VFS may thus be higher or lower than the SRE depending on the distribution of trace elements among particle size classes. This must be taken into account in the proper design of VFS aimed at controlling off-site pollution by trace element-contaminated sediments.