NATURAL ATTENUATION OF AMMONIUM AT A FORMER COAL CARBONISATION PLANT (MANSFIELD, UK): CONCEPTUAL MODEL FOR BIODEGRADATION PROCESSES

The cost-advantage of Monitored Natural Attenuation over active remediation strategies has put the natural processes involved more into the focus of research. However, large uncertainties concerning quantification of governing processes (e.g. biodegradation) remain. These uncertainties often inhibit the implementation of MNA due to cost-intensive investigation campaigns necessary to provide sufficient plume control.

In order to strengthen the scientific basis for plume prediction, the “Rexco” site near Mansfield (UK) serves as a field site for investigation with high-resolution groundwater samplers of biodegradation of ammonium. Earlier investigations located a strongly retarded ammonium-plume in the Sherwood Sandstone aquifer down gradient of a former drain, with outwash of residual ammonium from the unsaturated zone as ongoing source.

This paper presents the conceptual model, prior to installing multilevel samplers. The following processes have been identified as predominating after review of data from the site:

Nitrification

Oxidation of NH₄⁺ to NO₃^- occurs only in presence of abundant oxygen, restricting nitrification to the source zone, the top and the up gradient fringe of the plume. Since the O₂-supply is limited by dispersivity, the zone of nitrification in the saturated zone is believed to be few decimetres thick at the most.

Denitrification

Reduction of nitrate to N₂ is restricted to O₂-depleted zones of the plume and is likely to prevail throughout most of the plume, as O₂ decreases rapidly from the fringes inside. The process is rather slow due to limited availability of electron donors.

Anaerobic Ammonium Oxidation (Anammox)

If anammox, a process in which ammonium combines with nitrate or nitrite to form mainly N₂ and water, occurs at the site, it is hoped that it can be verified by the intended high-resolution groundwater sampling. Anammox is most likely to occur below the source zone with low dissolved O₂ and high NH₄⁺ and NO₃^-. Degradation is expected to be rather slow due to low growth rates of bacteria involved.

Other processes, such as nitrate reduction or ammonification are of minor importance. The conceptual model at hand will be adjusted and refined according to field data from multilevel groundwater sampling with a higher spatial resolution (spacing of ports in the order of 1-2dm) than has been achieved with methods to date.