Evaluating the effects of sub-zero temperature cycling on mercury flux from soils

A number of mechanisms have been shown to facilitate the flux of mercury from soils, although at sub-zero temperatures, there is evidence to suggest that alternate mechanisms may exist. Field studies at sub-zero temperatures have observed spikes in flux under certain conditions however very little laboratory work has been done to characterize the specific effects or mechanisms. A Dynamic Flux Chamber (DFC) and Tekran Model 2537A were used to analyse mercury flux from a naturally enriched soil. Soil moisture contents were varied between 30%, 60%, and 75% of field capacity while temperatures were cycled between 0 and −25 °C. The results, which were compared to room temperature baseline runs, showed that, at sub-zero temperatures, the mercury flux was suppressed in general. However, during the temperature cycling runs, soil flux spikes were evident during positive temperature change or warming. A one-way ANOVA by ranks proved that statistically significant fluxes were occurring during the runs at 30% and 60% of field capacity for positive rates of temperature changes. Arrhenius plots showed that for positive soil temperatures flux and temperature correlated well with the Arrhenius relationship. At sub-zero conditions however, no relationship was present, indicating another mechanism was present. The proposed mechanism for this enhanced flux is the physical evacuation of interstitial pore space gaseous mercury by the expansion and contraction of the freeze–thaw cycle.

► Hg flux from sub-zero temperature soil was observed at varying moisture contents.
► While cycling at sub-zero temperatures Hg flux was suppressed.
► Soil warming from sub-zero temperatures produced Hg flux spikes.
► A physical mechanism is proposed to account for Hg flux spikes.
► Mechanism includes the expansion and contraction of soil during freeze–thaw cycle.