Ammonium Concentration in Stream Sediments Produced by Decades of Discharge of a Wastewater Treatment Plant

Abstract

The study of ammonium pollution in the sediments of a stream that collected wastewater treatment plant (WWTP) discharges has been carried out. To understand the behaviour of this ion, introduced by a historical (50 years) contamination process, vertical boreholes in the stream banks were drilled to depths of between 30 and 120 cm. Moisture, pH, ammonium (soluble and exchangeable), and the clay fraction content were analysed. The profile variation of these parameters was evaluated as a function of depth to determine factors related to the distribution of the ammonium in several locations of the stream sediment banks.The ammonium concentration is asymmetrically distributed comparing samples collected in near surface locations. It usually concentrated at depths of 30-40 cm, in the sediments with the highest clay fraction content. The contamination does not affect more than 60-70 cm depth, controlled by its retention in a clayey sediment natural barrier.The clays in our study area (identified previously as smectite 2:1 sheet silicates) were able to control the contamination by retaining the contaminant in the interlayer, which retarded nitrification. It is suggested that clay serves as a geo-indicator for ammonium pollution evolution.

Keywords: ammonium, geo-indicator, WWTP, wastewater, clay sediment