Quantifying diffuse contamination: Comparing silver and mercury in organogenic and minerogenic soil

Area of Science:

• Geochemistry
• Environmental Science
• Soil Science

Background:

• Silver (Ag) and mercury (Hg) are trace elements with significant environmental implications.
• Continental-scale geochemical surveys provide baseline data for understanding element distribution in soils.

Purpose of the Study:

• To determine and compare the background concentrations of Ag and Hg in minerogenic and organogenic topsoils.
• To investigate the influence of natural factors (geology, climate, topography) and anthropogenic sources on Ag and Hg distribution.
• To assess the potential for diffuse contamination of Ag and Hg in organogenic topsoils.

Main Methods:

• Analysis of soil samples from continental-scale geochemical surveys in Australia, China, and Europe.
• Aqua regia extraction to determine total Ag and Hg concentrations.
• Comparison of element concentrations in different soil horizons (O horizon vs. C horizon) and soil types (minerogenic vs. organogenic).

Main Results:

• Median concentrations of Ag and Hg in minerogenic top- and subsoils are approximately 0.02 mg/kg.
• Organogenic topsoils (O horizon) exhibit median Ag and Hg concentrations around 0.2 mg/kg, about tenfold higher than minerogenic soils.
• Geochemical maps reveal regional patterns primarily driven by geology, climate, and topography, with anthropogenic sources causing only local anomalies.
• Estimated maximum diffuse contamination for Ag and Hg in organogenic topsoil is 0.02 mg/kg and 0.03 mg/kg, respectively.
• Hg concentrations show less variability in the O horizon compared to the C horizon.
• Elevated Hg/Ag ratios suggest substantial Hg soil contamination.

Conclusions:

• Soil type significantly influences Ag and Hg concentrations, with organogenic topsoils acting as sinks.
• Natural factors are the dominant drivers of large-scale Ag and Hg distribution in soils.
• Anthropogenic inputs of Ag and Hg are generally localized, but diffuse contamination can occur.
• Hg/Ag ratios can serve as an indicator for mercury contamination in soils.