Vertical Transverse Dispersion Controls the Natural Attenuation of Spatially Variable Plumes

Many, if not most organic contaminant plumes are spatially variable. This presents a challenge for natural attenuation assessment using traditional methods that rely on monitoring along plume centrelines that are necessarily assumed to be unique. An alternative approach is to characterise the processes that control attenuation, and delineate where those processes occur within the plume. For example, carbon turnover in many organic plumes is constrained to biodegradation at the plume fringe where contaminants mix with electron acceptors. This mixing is influenced by concentration gradients of organics (out of the plume) and oxygen and nitrate (into the plume), and vertical transverse dispersion. Where plumes consist of complex mixtures of organics, an added factor is the preferential degradation of certain compounds (target or non-target organics) due to various microbiological concerns. Accurate prediction of natural attenuation of such plumes may be possible if spatially discrete carbon turnover processes are considered in the context of spatial plume variability. A transect of four highly detailed multilevel sampling wells were installed across a well-studied tar acid plume migrating within the Triassic sandstones in the UK Midlands. The goal of these wells was to locate the upper plume fringe and quantify degradation within those zones. The multilevel sample ports were 20 cm apart to characterise both electron acceptor and donor profiles in great detail. The primary contaminants within the plume are the phenolics (phenol, xylenols, cresols), but other compounds are present that impose a demand on electron acceptor supply (tar neutrals TEX, C4-C8 benzenes, benzofuran and tar bases pyridines, picoline, aniline). The bioactive zone appears to be constrained to a narrow zone less than 1 m thick wherein all dissolved oxygen and nitrate are consumed, with phenol persisting to depth, consistent with weak vertical mixing due to weak dispersion at the scale of diffusion. The vertical position of this fringe varies in space, suggesting that natural attenuation cannot be accurately estimated by extrapolating the reactive processes quantified at one location to the plume as a whole.