Final report: selenium removal from coal mine valley ; fill effluents using Chara ARIES subtask 2.2.3

Abstract

Potential Alternate Treatment Systems. Final Report: Se Removal from Coal Mine Valley Fill Effluents: ARIES Subtask 2.2.3 The Appalachian Research Initiative for Environmental Science (ARIES), under the direction of the Virginia Center for Coal and Energy Research at Virginia Tech was funded to coordinate a project to look into finding alternative treatment systems for selenium removal from coal mine valley fill ponds. In 2012, Boojum Research was was engaged to carry out a pre-feasibility study to assess, if biological polishing, one of the ecological engineering processes, would lower selenium concentrations in coal mine valley fill ponds. The University of Kentucky and 4 coal mining companies participated in the study. The main objective was to determine if, and how much, selenium could be accumulated by the aquatic vegetation found in the valley fill ponds. Periphytic or attached algae are known to be effective ‘polishers’ which adsorb and absorb contaminants from waste water. Selenium is one such waste water contaminant, although its structure is more complicated than most. Periphytic algae with associated ad/absorbed contaminants become buried in the sediment upon death. Phytoplankton (free-floating algae) generally leave the pond or sink to the sediment surface and decay, releasing selenium again to the water. Rooted vegetation can also sequester selenium, but mostly from the sediment and not from the water. The most effective algal group for biological polishing in alkaline water is the Characeae. This family of attached algae is one of the first invaders of freshly- dug ponds and ditches. These algae form dense perennial underwater meadows. These characteristics are especially suited for valley fill ponds, where mining companies have to remove the sediments periodically to maintain a certain water volume. Hence colonization by Characeae is likely. Many of the ponds visited and sampled had some Characeae growth. Only one pond had an abundant underwater meadow. Some ponds were ‘choked’ with rooted emergent vegetation. In one pond with the extensive underwater meadow of Characeae selenium concentrations were lower by 7 µg L‐1 when outflow concentrations were compared to inflow concentrations. Concentrations of in the biomass ranged from 2.2 to 8.0 mg.kg‐1. It was essential that part of the feasibility study determine whether selenium was enriched in the sediment in ponds colonized by the Characeae. This was assessed by collecting sediment cores and dividing them into vertical horizons. The surface sample (0‐2 cm) contained 17 µg. g‐1, decreasing to 7 µg.g‐1 at a depth of 2‐4 cm and further to 1.37 µg.g‐1. The deeper portions and those below in 6 cm, concentrations of around 0.6 µg. g‐1 were reported. These selenium concentration decreases with depth are a strong indication that the use of characean algae should be further studied. The challenge will be to determine the conditions needed to support the growth of an underwater meadow of the algae. Seeding the ponds after dredging with biomass or oospores (a type of seed) would likely be enough to ensure a healthy population. Using this relatively cost-effective approach would likely lead to reductions in the selenium concentration of the effluents leaving valley fill ponds.