HR: 0830h
AN: H11D-0891    [PDF]
TI: Delineating Reactive Natural Attenuation Zones: Multi-level Sampling in an Ammonium Plume
AU: * H\"{u}ttmann, A
EM: a.huettmann@shef.ac.uk
AF: Groundwater Protection and Restoration Group / University of Sheffield, Department of Civil and Structural Engineering Mappin Street, Sheffield, S1 3JD United Kingdom
AU: Wilson, R D
EM: r.d.wilson@shef.ac.uk
AF: Groundwater Protection and Restoration Group / University of Sheffield, Department of Civil and Structural Engineering Mappin Street, Sheffield, S1 3JD United Kingdom
AU: Thornton, S F
EM: s.f.thornton@shef.ac.uk
AF: Groundwater Protection and Restoration Group / University of Sheffield, Department of Civil and Structural Engineering Mappin Street, Sheffield, S1 3JD United Kingdom
AU: Lerner, D N
EM: d.n.lerner@shef.ac.uk
AF: Groundwater Protection and Restoration Group / University of Sheffield, Department of Civil and Structural Engineering Mappin Street, Sheffield, S1 3JD United Kingdom
AB: Current natural attenuation monitoring involves observing concentrations along some transect parallel to flow, often in long--screened observation wells. Vertically integrated geochemical signals essentially compresses 3D-problems into 2D ones and the loss of process resolution does not allow for robust plume transport prediction. On the other hand, a monitoring approach that is based on process quantification will yield field data necessary for current natural attenuation assessment and allow more accurate prediction to plume behaviour. To quantify these spatially discrete processes, the resolution of sampling points must be higher to delineate reactive zones, that account for the bulk of natural attenuation. These reactive zones are dependent on aquifer properties, contaminant type, redox zonation and other factors. In many cases, the plume fringe will be the most active zone, because influx of certain nutrients necessary to support bacteria will be greatest in these zones. Most importantly, consortia of degrading bacteria are dependent on supply of electron--acceptors such as dissolved $\mathrm{O_{2}}$ and nitrate from background ground water and recharge; electron--acceptors are rapidly consumed at the fringes and concentrations decrease significantly towards the centre of the plume. Thus, if a process--based approach is to be achieved, one has to focus not only on the overall extent and mass of a contaminant plume, but, more importantly, on a sound analysis of the most reactive zones. The site of a former coal processing plant in the UK was chosen as a field site where the afore--mentioned hypothesis is currently being investigated by means of two recently installed multi--level samplers. Previous investigations have located an ammonium plume at the site with outwash from the thick unsaturated zone as ongoing source since the demolition of the actual plant in 1970. The conceptual model drawn from the analysis of these data shows that certain zones exhibit higher rates of degradation than others, but the extent of these zones and rates of degradation could not be determined with the sampling methods used in previous investigations. Spacing between the sampling ports in the newly installed multi--level samplers is $25cm$ across the top fringe of the plume, and the ground water is analysed for contaminants, general redox chemistry of the aquifer, and isotopes. The analysis of groundwater samples from a first sampling round performed in late summer 2003 shows that i) nitrification is ongoing at the upper fringe in a zone about $0.5m$ thick, but concentrations of nitrate encountered at the ports across the fringe are higher that derived from the degradation of available ammonium; ii) concentrations of electron--donors and --acceptors change rapidly in the fringe--zone; iii) inside the plume, different processes are prevalent than at the upper fringe; iv) a trend in the isotopic composition appears to indicate a distinct consumption of carbon and sulphate by microbial activity, but these data are not unequivocal and should only be used as supportive evidence.
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2003 Fall Meeting