Amalgamation with mercury can be used to recover very small gold particles, and mercury is still widely used in small-scale artisanal mining across the world. Mercury forms a mercury-gold amalgam with smaller gold particles, and then the gold is concentrated by boiling away the mercury from the amalgam. This is effective in extracting very small gold particles, but the process is hazardous due to the toxicity of mercury vapour. Large-scale use of mercury stopped in the 1960s. However, mercury is still used in artisanal and small-scale gold mining (ASGM).  One mechanism by which mercury is employed in hydraulic mining is as an "undercurrent", in which the flow of smaller grains is diverted over mercury-coated copper plates.  High flow velocities associated with hydraulic mining cause flouring of mercury, the wearing down of mercury particles that contributes to mercury loss into the environment.
Over  of mercury contaminated the environment in California as a result of placer mining in the late nineteenth and early twentieth centuries.  Stamp mill mining contributed an additional  of mercury contamination.  Mercury contamination in California waterways is a major contemporary environmental issue, as is groundwater pollution, mostly by inorganic mercury.