Precipitation input and antecedent soil moisture effects on mercury mobility in soillaboratory experiments with an enriched stable isotope tracer

Terrestrial soils are net mercury (Hg) sinks, but leaching of Hg from upland environments constitutes an important source to downstream wetlands and water bodies. Broadly, hydrology is instrumental in facilitating Hg transport within, and export from watersheds but the relative influences of specific hydrological factors such as antecedent soil moisture and precipitation in controlling the transport of Hg through upland soils are not well understood. The purpose of this research was to elucidate the relative controls of these hydrological factors using a full factorial laboratory experiment involving the application of an enriched stable Hg isotope tracer to intact soil cores. Antecedent soil moisture and precipitation input depth were statistically significant, mutually exclusive controls on tracer Hg mobility. Neither factor however had a strongly significant influence on the mobility of ambient Hg. Tracer Hg mobility was enhanced with larger precipitation events as well as from initially drier soils and appeared to move via simple piston flow. The majority (>99.5%) of added tracer Hg was sorbed to soil organic matter in the surface 3cm, regardless of the hydrological treatment combinations. Overall, these results suggest that tracer and ambient Hg are differentially controlled by hydrological conditions. Changes in hydrology may have little impact on ambient Hg mobilization in sandy loam soils. If tracer Hg is broadly representative of contemporary soil Hg stocks, extreme precipitation events among otherwise drier conditions could enhance the export of contemporary Hg from upland systems. Copyright (c) 2015 John Wiley & Sons, Ltd.