Quantitative characterization of the effect of compaction rate on ammonium release from clayey sediments

Abstract

Ammonium-rich pore water will release from clayey sediments into adjacent aquifers during aquitards compaction, which can adversely affect groundwater quality. Differences in the rate of groundwater exploitation can lead to differences in the rate of compaction of aquitards, which may, to some extent, affect the release of chemical components of the pore water from the aquitard. However, the effect of the compaction rate of aquitards on the process of ammonium release from clayey sediments is poorly understood. Three groups of indoor compaction experiments were conducted to identify the effect of compaction rate on ammonium release from clayey sediments during compaction. The results showed that the compaction rate and sediment liquid limit exert a clear controlling effect on the release rate and internal change process of ammonium in clayey sediments. The increase in the ammonium release rate was approximately proportional to the increase in the compaction rate. When the moisture content of the clayey sediments exceeds the liquid limit, the ammonium concentration in pore water increases mainly due to the mineralization of organic matter containing nitrogen and the dissolution of the carbonate-bound ammonium. When the moisture content drops to the liquid limit, the ammonium concentration in pore water begins to decrease. Because the increase in the compaction rate weakened sediment mineralization and accelerated pore water release, the main forms of ammonium released from the sediment change from organic sulfide-bound ammonium and carbonate-bound ammonium to carbonate-bound ammonium and ion-exchanged ammonium. The results of this study have important significance for the management of groundwater quality in groundwater exploitation areas.