4D dispersion of total gaseous mercury derived from a mining source: identification of criteria to assess risks related to high concentrations of atmospheric mercury

Abstract

Mercury (Hg) is a global pollutant that can be transported long distances after its emission from primary sources. The most common problem of gaseous Hg in the vicinity of anthropogenic sources is its presence in inorganic forms and in the gaseous state in the atmosphere. Risk assessments related to the presence of gaseous Hg in the atmosphere at contaminated sites are often based on episodic and incomplete data, which do not properly characterize the Hg cycle in the area of interest or consider spatial or temporal terms. The aim of this work was to identify criteria to obtain the minimum amount of data with the maximum meaning and representativeness in order to delimit risk areas, both in a spatial and temporal respect. Data were acquired from May 2014 to August 2015 and included vertical and horizontal Hg measurements. A statistical analysis was carried out, and this included the construction of a model of vertical Hg movements that could be used to predict the location and timing of Hg inhalation risk. A monitoring strategy was designed in order to identify the relevant criteria, and this involved the measurement of gaseous Hg in a vertical section at low altitude (i.e. where humans are present) and in horizontal transects to appropriately characterize the transport cycle of gaseous Hg in the lower layers of the atmosphere. The measurements were carried out over time in order to obtain information on daily and seasonal variability. The study site selected was Almadenejos (Ciudad Real, Spain), a village polluted with mercury related to decommissioned mining and metallurgical facilities belonging to the Almadén mercury mining district. The vertical profiles revealed that higher total gaseous mercury concentrations are present at lower altitude during nocturnal hours and at higher altitude at dawn and dusk. On a daily basis the most important process involved in gaseous mercury movements is the mixing layer. Vertical transferences are predominant when this process is active, i.e. in all seasons except winter, while major sources act as constant suppliers of gaseous Hg to the mixing cell, thus producing Hg deposition at dusk. Conversely, horizontal transferences prevail during the hours of darkness and the main factors are major and minor sources, solar radiation, wind speed, and topography. The study has shown that it is important (i) to identify the sources, (ii) to get data about Hg movements in vertical and horizontal directions, (iii) to extend the measurements over time in a sufficiently representative way both daily and seasonally, and (iv) to determine the different populations of data to establish the background levels; this work proposes the use of Lepeltier graphs to do so. In terms of risk assessment, the nights carry greater risk than the days in all seasons except autumn. The main factors involved in the creation of high-risk periods are those related to dilution (or its absence), namely wind speed and solar radiation at null levels. The results of this study highlight the possible importance of relieving the distribution of gaseous mercury in proximity to discrete sources. Furthermore, systematic monitoring strategies can offer significant information for the Minamata Convention emission reduction scenario. Further studies, including a detailed topographic model of the area, are required in order to make precise estimations of the influence of this parameter, which appears in this study to be less important than the other factors but is still appreciable.