Long-term mercury dynamics in UK soils

Mercury accumulation in soils remains poorly quantified over long timescales. Prior research has shown that mercury can persist in soils for centuries, but the relative roles of local versus global deposition are unclear. No prior work had resolved the extent to which historical mercury inputs from human activities influence current soil mercury levels. This gap motivated a study to model mercury dynamics in UK soils from 1850 to the present. Existing models often assume uniform deposition rates, which may not reflect regional differences in industrial activity. That uncertainty drove the need for a spatially resolved mercury budget. Understanding mercury retention in soils is essential for predicting future atmospheric and aquatic mercury levels. This paper's contribution is a detailed assessment of mercury sources and sinks across UK soils.

The study aimed to evaluate mercury dynamics in UK soils using a first-order loss model. It sought to distinguish between mercury from local and global sources over the past 170 years. The researchers focused on quantifying mercury deposition patterns and their spatial variability. By integrating historical data with inverse modeling, they aimed to estimate mercury retention and fluxes. The specific problem addressed was the lack of a comprehensive mercury budget for UK soils. Motivation came from the need to understand mercury persistence in soils after industrial emissions declined. The study also aimed to assess how mercury in soils affects atmospheric and water mercury levels. This approach allows for a more accurate prediction of mercury behavior over long timescales.

The researchers used a first-order loss model to simulate mercury dynamics in UK soils. They constructed temporal deposition patterns from literature sources spanning 1850 to the present. Inverse modeling was applied to estimate mercury inputs and retention across 898 rural sites. The model assumed mercury losses through evasion and leaching as primary removal mechanisms. Spatial variability in mercury deposition was assessed by comparing local and global sources. Topsoil and lower soil layers were analyzed separately to evaluate mercury retention. The study combined historical emission data with soil geochemical properties to estimate mercury fluxes. Model outputs included estimates of mercury deposition rates and current soil mercury stocks.

The model estimated an average mercury deposition rate of 16 μg Hg m(-2) a(-1) to rural soils. For rural and non-rural soils combined, the average was 19 μg Hg m(-2) a(-1). Inverse modeling showed that 30% of rural sites receive mercury only from global circulation. Local deposition exceeded global inputs in 51% of rural sites. UK soils currently hold 2490 tonnes of reactive mercury, with 2140 tonnes from anthropogenic sources. Topsoil releases 5.1 tonnes of Hg(0) per year to the atmosphere, 50% more than anthropogenic fluxes. Sorptive retention in lower soil layers strongly influences surface water mercury concentrations. Following reduced inputs, soil mercury is predicted to decline over hundreds of years.

The study found that UK soils retain a large mercury stock, mostly from local anthropogenic sources. Mercury evasion from topsoil exceeds historical anthropogenic emissions by 50%. Lower soil layers control mercury availability to surface waters through sorption. Declines in mercury inputs will not rapidly reduce soil mercury concentrations. The model suggests mercury persistence in soils for centuries after emission reductions. Local and global deposition patterns differ significantly across UK soils. These findings support the need for long-term monitoring of mercury in soils and water. The study provides a framework for predicting mercury behavior in soils under changing emission scenarios.