An Emerging Frontier: Metal(loid) Soil Pollution Threat Under Global Climate Change.

Abstract

An important topic that has received very little attention is the impact of climate change, in particular global warming, and associated extreme events, on soil pollution and metal mobility in soils. The Commission on Pollution and Health found that all forms of pollution were responsible for approximately 9 million premature deaths, or 16% of all deaths globally in 2015. Thus, pollution is becoming the world's largest environmental cause of disease and premature death (Landrigan et al. 2017). To protect health, policy-makers and regulators must consider how global climate change may influence chemical risks to humans and develop approaches to adequately assess and manage that risk. Because legacy pollutants (e.g., mercury, lead [Pb], and chromium) persist and bioaccumulate in the environment, long-term environmental processes related to global climate change could influence their fate and transport and change ecosystem and human exposures (Balbus et al. 2013). Global warming leads to increased soil and atmospheric temperatures, more frequent and severe extreme events (e.g., flooding, torrential rains, wildfires, droughts, hurricanes) and, consequently, soil erosion that, in turn, causes redistribution of metal(loid)s. Wildfires increase the atmospheric loadings of chemicals that create acid rain and also remobilize metals. Flooding disperses metal(loid)s from mines and industrial areas to residential and agricultural lands (Balbus et al. 2013). Accelerated melting of arctic ice causes increased mobilization of, for instance, Pb, leading to metal bioaccumulation in marine and terrestrial animals (Balbus et al. 2013). With excessive rainfalls, more contaminants will be redistributed laterally over larger areas. Existing mobile metal(loid)s will translocate downward in a soil profile, moving to lower horizons, which might be beneficial for healthy gardening and farming in topsoil. However, groundwater may become contaminated, causing other environmental problems, for example, pollution of drinking water. It is important to model the prediction of rainfall events to identify major areas of concern as well as to monitor changes in groundwater levels. Increased precipitation may potentially mobilize, translocate, and transform soil contaminants. Metal(loid)s are more extractable in soils with low pH. Excess rainfall may result in soil acidification, which will make metal(loid)s more soluble and available for plant or human uptake. Furthermore, fertilizers and amendments applied in farm fields and gardens will be washed away or leached out, and as a result, contaminants will not be bound to phosphates to form stable (less bioavailable) minerals in soils. The development of methods that can rapidly and reliably assess metal bioaccessibility and phytoavailability in a constantly changing environment is vital to prevent human exposure. Improvement of our basic understanding of soil physics and chemistry, especially in cities with their heterogenous pedosphere and land management, is necessary for the successful development of such methods. Changes in climate may also impact the amounts of time humans spend indoors and outdoors, influencing exposure to both indoor and outdoor contaminants (Balbus et al. 2013). Many studies indicate that urban soils contaminated with Pb become suspended in the atmosphere in the summer and autumn when evapotranspiration is at a maximum and soils are dry. These particles then settle down elsewhere, potentially contaminating clean soils. Correlational studies show increases in blood Pb concentrations during droughty periods when soil is dry and dusty and decreases in blood Pb concentrations during rainy periods when soil is wet and dust is settled (Laidlaw et al. 2017). Rapid assessment of soil and dust pollution by metal(loid)s using portable X-ray fluorescence (XRF) is recommended to determine soil metal concentrations, to prevent future human exposures. These XRF determinations combined with atmospheric patterns (e.g., wind direction) can be used to predict potentially contaminated areas where dust will settle. Droughts will lead to reduced soil microbial survival, colonization, diversity, and function. When plants are stunted under drought conditions, concentrations of trace metals can be elevated in the tissues (Fritioff et al. 2005). In metal-polluted soils, elevated CO2 levels result in increased plant-associated microbial populations protecting these microbes from metal stress. Thus, at higher levels of CO2 in metal-polluted soils, plant-associated microbes can improve plant growth and metal uptake, leading, for example, to potentially contaminated agricultural produce or enhanced metal uptake by plants. Crop concentrations of metals tend to increase when soil temperatures are higher (Antoniadis and Alloway 2001); however, it is not clear whether this is due to accelerated evapotranspiration, more rapid organic matter breakdown in soil, faster release from soil particles and diffusion to roots, or some combination of several factors. Urban areas may be an excellent venue for simulation experiments because of the urban “heat island effect,” which may be viewed as a natural long-term climate manipulation experiment. In the case of dry seasons with a limited amount of precipitation, metals will stay in topsoil or translocate from lower horizons upward to the rhizosphere due to increased evaporation rates, thus becoming more readily available for plant uptake. The addition of phosphate fertilizers, composts, and soil amendment will not form stable (and thus less bioavailable) forms of metal(loid)s due to the lack of water content required for chemical reactions to occur. Additional research is needed to determine the effect of metal contamination on microbial biodiversity and survival rates under drought conditions in different landscapes and along an urban–rural gradient. Modeling rates of decomposition of organic matter used to form organo–mineral complexes as a remediation practice is also necessary. The effects of climate change on soil pollution with metals (i.e., their concentrations, mobility, and distribution) have been understudied and require immediate attention to prevent future human and ecosystem damage. We recommend developing metal(loid) fate and transport models encompassing factors such as atmospheric and soil temperature, precipitation, atmospheric circulation patterns, soil chemistry, groundwater levels, and degradation pathways integrating GIS tools for visualization. We encourage studies that explicitly relate the effects of temperature on metal behavior in soils due to global warming/climate change. The present study was supported by the RUDN University “5-100” project and the Russian Science Foundation project 19-77-30012. Data, associated metadata, and calculation tools are available from the corresponding author (apaltseva@gradcenter.cuny.edu).