Movement of pollutants in soil and groundwater systems: adsorption of ammonium ions from percolating waste water used for groundwater recharge

James Eric Bawden

The feasibility of recharging Perth groundwater supplies using secondary treated sewage effluent is being investigated by the Metropolitan Water Supply Sewerage and Drainage Board. Research at Murdoch University proposes the lining of the sandy recharge basins with Pyrton Loam to remove ammonium-nitrogen from the waste water percolating down to the aquifier. The complete removal of ammonium-nitrogen from the percolating waste water requires the correct management of infiltration rates. Guidelines are required on the rates of adsorption of ammonium ions and the best requirements for bacteria utilising this form of nitrogen in the soil. This research programme focuses on the parameterisation of the dispersion and adsorption of ammonium ions in the soil profile. Preliminary research indicates the adsorption of ammonium ions to Pyrton Sandy Loam occurs both instantaneously and in a time dependent fashion. Here a theory has been devised to model the reactive dispersion of ammonium ions in this soil type. The theory has the capacity to predict the increase in concentration of a solute front moving through the soil. Its graphical representation, known as a breakthrough curve, should change shape significantly when the depth of soil increases above a critical level. Solute retention time increases with an increase in soil depth allowing a greater completion of time dependent adsorption processes. The experimental component of this research was aimed at identifying the effect of retention time on the solute breakthrough curve. However, when similar ammonium chloride solute concentrations were passed through three columns of increasing length no significant change was observed in the form of the breakthrough curve. Various batch tests were designed to compliment the soil column breakthrough data where Pyrton Sandy Loam was contacted with ammonium chloride solutions. They indicated adsorption was predominantly instantaneous and that time dependent adsorption was completed within a time value exceeded for the column experiment with the lowest retention time. It followed from the theoretical background that the form of the breakthrough curve would change very little once the column retention time exceeded the time required to reach an equilibrium adsorption state. A theoretical breakthrough curve derived from batch contact test parameters coincided poorly with the experimentally observed breakthrough data. The discrepancies were attributed primarily to lateral diffusion in soil pores in the columns and preferential adsorption of calcium ions over ammonium ions in the batch contact tests.

Movement of pollutants in soil and groundwater systems: adsorption of ammonium ions from percolating waste water used for groundwater recharge

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