Phytoremediation management of selenium-laden drainage sediments in the San Luis Drain: a greenhouse feasibility study.

An estimated 100,000m(3) selenium (Se)-laden drainage sediment resides in the San Luis Drain (SLD) of Central California. This greenhouse study was undertaken to evaluate the feasibility of growing salt- and boron-tolerant plant species in sediment for reduction of Se content by plant extraction. Drainage sediment was collected from the SLD and mixed with control soil (i.e., uncontaminated soil) to the following ratios (sediment:control soil) by volume: 0:3 (i.e., control soil only), 1:2 (i.e., 1/3 sediment and 2/3 control soil), 2:1 (i.e., 2/3 sediment and 1/3 control soil), and 3:0 (i.e., sediment only). Salt-tolerant plant species consisted of canola (Brassica napus var. Hyola 420), tall fescue (Festuca arundinacea var. Au Triumph), salado grass (Sporobulus airoides), and cordgrass (Spartina patens var. Flageo). Increased ratios of sediment:soil resulted in decreased dry matter production for all tested plant species; especially at ratios of sediment:soil greater than 1:2. Plant Se concentrations (mgkg(-1) DM) ranged as follows for plant species at all ratios of sediment:soil: canola (51-72), tall fescue (16-36), and cordgrass and salado grass (9-14). Total Se concentrations in the soil were at least 20% lower at postharvest compared to preplant concentrations for all plant species at each ratio of sediment:soil. In contrast, water-extractable Se concentrations in the soil were at least three times higher at postharvest than at preplant for all plant species, irrespective of the ratio of sediment:soil. Leaching of Se occurred in irrigated bare pots from each respective ratio of sediment:soil over a duration of 60 days. Based upon the downward movement of Se in bare pots of sediment:soil, it may be more prudent to leave the drainage sediment in the SLD, incorporate clean soil, and then grow low maintenance salt-tolerant plants (e.g., cordgrass, salado grass) in the concrete-lined canal. By this means, possible contamination of groundwater with soluble Se will be eliminated, while phytoremediation slowly reduces Se content in the drainage sediment.