Fate and Transport of Metals From Biosolids Entrenched for Reclaiming Mineland with Hybrid Poplar.

High rates of entrenched biosolids have been used as a nutrient source for hybrid poplar without nitrate-N pollution of groundwater due to the maintenance of nitrification rate-limiting, low redox conditions in the biosolids and the presence of a fine-textured stratum below the trench. We initiated a study to investigate the fate and transport of entrenched biosolids-borne metals in coarse-textured soils used to grow high nutrient-assimilating hybrid poplars by comparing two rates each of anaerobically digested and lime-stabilized biosolids with fertilized and unfertilized treatments at a heavy mineral mine in the Virginia coastal plain. Treatment rates (replicated 4x in a CRB) were set by filling trenches of pre-determined volume (45 cm wide x 75 cm deep and 90 cm wide x 75 cm deep) with biosolids in summer 2006 and covering with 30 cm of mine soil. Zero tension lysimeters place below and suction lysimeters adjacent to the trenches were used to sample leachate at least monthly during the 15-month following entrenching. Biosolids were sampled and analyzed at application for chemical composition and bulk density to calculate the constituent loading rates. Application rates were 328 and 656 Mg ha^-1 for the lime-stabilized material and 213 and 426 Mg ha^-1 for the anaerobically digested biosolids.  Hybrid poplar cuttings were planted over the trenches at a spacing of 3 m in March 2007. Analysis of leachate showed that transport of metals from the biosolids was no greater than from the control or very low for nearly all the metals studied. Silver, Cd, Pb, and Sn did not move vertically or laterally to any significant extent. Lime-stabilized biosolids produced higher cumulative metal mass transport for Cu (967 g ha^-1), Ni (171 g ha^-1), and Zn (1027 g ha^-1) than the anaerobically digested biosolids and control. Where detected, metals were largely transported in particulate phase.