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